## 4.2. General Synthetic Approaches

Nanogels can be synthesized by various methods. Actually, they are often formed as transient products or by-products in many polymerization and crosslinking procedures run in the bulk or in solution [243–246] (see Scheme 7 above and corresponding discussion), but such methods are not practical if nanogels are our main synthetic goal. The useful methods vary by kind of substrate (monomers vs polymers), phase composition of the reacting system (homogeneous vs. heterogeneous) and means of initiating the reaction (using heat, light, ultrasound, or ionizing radiation).

If we consider a mixture of monomers and crosslinkers (being typically bifunctional monomers) and initiate reactions in such a system, typically by generating free radicals, polymerization and crosslinking will occur side-by-side and the resulting product will be a crosslinked, three-dimensional polymer network. Performing such reaction in the bulk or in a homogeneous solution will usually lead to the formation of a macroscopic network (called a “wall-to-wall” gel, since typically the crosslinked product occupies the whole volume of the reaction vessel). But if our aim is to synthesize a crosslinked polymer structure of the size of a single coiled macromolecule (usually not more than 100 nm when in solution), we have to run the reaction in suitable “nanocompartments”, i.e., in micelles. Since we are mostly interested here in hydrophilic nanogels, and thus the monomers, crosslinkers and the resulting polymer structures should be soluble in water, synthesis should be performed in reverse miniemulsion or reverse microemulsion, having small droplets of the aqueous phase dispersed in the continuous oil phase [247,248].

In order to initiate crosslinking polymerization in such systems, one may use either a thermally activated polymerization initiator, or alternatively a photoinitiator, or apply ionizing radiation that allows for direct, initiator-free generation of radicals [249,250]. Such reactions can also be induced by ultrasound, with or without an initiator. Ultrasound may also be used to facilitate dispersion of components [251,252].

Such a synthetic approach, while effective, also has some drawbacks. It requires the use of many chemicals, generates waste, and is a multistep procedure; the products must be isolated from the oil/water system and purified. Unreacted monomer and crosslinker must be subsequently removed, in particular for when the product is intended for biological or medical use. Initiators and sometimes also surfactants may be embedded in the final product structure, which is often an undesired effect. Moreover, thermal decomposition of an initiator is a process difficult to be precisely controlled thus, also the product properties (e.g., crosslink density) are not easily controlled. Some of these deficiencies can be nowadays alleviated by using controlled radical polymerization techniques (CRP, [113,253,254]). In particular, the reversible addition-fragmentation chain transfer (RAFT, [255]) has been demonstrated to be very useful in controlling radiation-induced polymerization and crosslinking processes [256–258].

Another way to synthesize nanogels is by using ready linear polymer as a substrate and performing intramolecular crosslinking by utilizing the functional groups contained in the macromolecular structure. Most often this approach requires a bifunctional cross-linking agent having at least two groups capable of reacting with functional groups of the polymer (Figure 14). This reaction can be performed in homogeneous aqueous solution. An example may be internal crosslinking of poly(vinyl alcohol) chains by glutaraldehyde [259]. Care must be taken to optimize the reaction conditions, mainly the substrate and crosslinker concentrations, to maximize the yield of intramolecular crosslinking while limiting the extent of crosslinking between individual molecules. This method allows to eliminate the use of the oil phase, surfactant and initiator, as well as to obtain the product not contaminated by unreacted monomer. However, still purification steps are required to remove the rests of the crosslinking agent. Moreover, not all hydrophilic polymers of interest can be easily internally crosslinked in this way.
## 4.2. General Synthetic Approaches

Nanogels can be synthesized by various methods. Actually, they are often formed as transient products or by-products in many polymerization and crosslinking procedures run in the bulk or in solution [243–246] (see Scheme 7 above and corresponding discussion), but such methods are not practical if nanogels are our main synthetic goal. The useful methods vary by kind of substrate (monomers vs polymers), phase composition of the reacting system (homogeneous vs. heterogeneous) and means of initiating the reaction (using heat, light, ultrasound, or ionizing radiation).

If we consider a mixture of monomers and crosslinkers (being typically bifunctional monomers) and initiate reactions in such a system, typically by generating free radicals, polymerization and crosslinking will occur side-by-side and the resulting product will be a crosslinked, three-dimensional polymer network. Performing such reaction in the bulk or in a homogeneous solution will usually lead to the formation of a macroscopic network (called a “wall-to-wall” gel, since typically the crosslinked product occupies the whole volume of the reaction vessel). But if our aim is to synthesize a crosslinked polymer structure of the size of a single coiled macromolecule (usually not more than 100 nm when in solution), we have to run the reaction in suitable “nanocompartments”, i.e., in micelles. Since we are mostly interested here in hydrophilic nanogels, and thus the monomers, crosslinkers and the resulting polymer structures should be soluble in water, synthesis should be performed in reverse miniemulsion or reverse microemulsion, having small droplets of the aqueous phase dispersed in the continuous oil phase [247,248].

In order to initiate crosslinking polymerization in such systems, one may use either a thermally activated polymerization initiator, or alternatively a photoinitiator, or apply ionizing radiation that allows for direct, initiator-free generation of radicals [249,250]. Such reactions can also be induced by ultrasound, with or without an initiator. Ultrasound may also be used to facilitate dispersion of components [251,252].

Such a synthetic approach, while effective, also has some drawbacks. It requires the use of many chemicals, generates waste, and is a multistep procedure; the products must be isolated from the oil/water system and purified. Unreacted monomer and crosslinker must be subsequently removed, in particular for when the product is intended for biological or medical use. Initiators and sometimes also surfactants may be embedded in the final product structure, which is often an undesired effect. Moreover, thermal decomposition of an initiator is a process difficult to be precisely controlled thus, also the product properties (e.g., crosslink density) are not easily controlled. Some of these deficiencies can be nowadays alleviated by using controlled radical polymerization techniques (CRP, [113,253,254]). In particular, the reversible addition-fragmentation chain transfer (RAFT, [255]) has been demonstrated to be very useful in controlling radiation-induced polymerization and crosslinking processes [256–258].

Another way to synthesize nanogels is by using ready linear polymer as a substrate and performing intramolecular crosslinking by utilizing the functional groups contained in the macromolecular structure. Most often this approach requires a bifunctional cross-linking agent having at least two groups capable of reacting with functional groups of the polymer (Figure 14). This reaction can be performed in homogeneous aqueous solution. An example may be internal crosslinking of poly(vinyl alcohol) chains by glutaraldehyde [259]. Care must be taken to optimize the reaction conditions, mainly the substrate and crosslinker concentrations, to maximize the yield of intramolecular crosslinking while limiting the extent of crosslinking between individual molecules. This method allows to eliminate the use of the oil phase, surfactant and initiator, as well as to obtain the product not contaminated by unreacted monomer. However, still purification steps are required to remove the rests of the crosslinking agent. Moreover, not all hydrophilic polymers of interest can be easily internally crosslinked in this way.
<figure><img src="image_24.png" /><figcaption>**Figure 14.** Intermolecular crosslinking of a macromolecule using a chemical cross-linking agent. For details see e.g., [259].</figcaption></figure>

### 4.3. Synthesis of Nanogels by Radiation-Induced Intramolecular Crosslinking of Polymers

In order to further simplify the synthesis of nanogels and avoid the use of crosslinking agents, in the end of 1990, an alternative method utilizing ionizing radiation has been proposed, based on radiation-induced intramolecular crosslinking of macromolecules [260], for recent reviews see [236,261]. The only substrates are the hydrophilic polymer and water; neither initiators nor any other chemicals are used (except in some special cases). This method is described in some detail below. It builds upon the accumulated knowledge of radiation chemistry and in particular radiation crosslinking of polymers.

#### 4.3.1. Radiation Chemistry of Polymers in Aqueous Solution

Ionizing radiation is relatively widely used in industry. Applications range from simple devices to measure the thickness and detect potential imperfections of products in steel industry to sterilization of medical devices and technologies where radiation is used to synthesize or modify materials. Commercial, large-scale irradiation facilities operate in many countries on all inhabited continents.

Ionizing radiation is defined as any kind of radiation (either photons or beams of accelerated particles) having enough energy to ionize atoms and molecules. In radiation processing, mostly two kinds of radiation are used: gamma rays and electron beams. Most large-scale gamma irradiation facilities use a radioactive isotope of cobalt, $^{60}$Co, as the source of gamma rays. Due to its metallic form, cobalt is relatively easy to process, handle, and store, while the gamma rays emitted by $^{60}$Co are high energy (average energy of 1.25 MeV), providing high penetration depth in matter and thus allowing to irradiate bulk materials (e.g., on standard palettes). Alternatively, electron beams (EB) generated by accelerators are applied.

In describing the action of ionizing radiation with matter, three important parameters are used. The dose is the amount of energy absorbed per unit mass of matter (in J/kg, which in radiation chemistry is called a gray—Gy), the dose rate is the dose absorbed per unit time (Gy/s) and the radiation-chemical yield (abbreviated simply as yield) is the number of moles of products or reactants (molecules, radicals, cross-linking bonds, etc.) produced or consumed per unit of absorbed energy (mol/J). Basic information on radiation chemistry and technology can be found in a number of books and reviews [262–264].

Electron beam technology provides high throughput due to high dose rates and does not require handling of radioactive isotopes, but, on the other hand, due to the limited penetration depth of fast electrons in matter, is less suitable for irradiation of bulk materials, typically requiring that the goods to be irradiated are packed in flat boxes or trays. Gamma rays offer high penetration in matter, allowing large boxes or whole pallets of goods to be irradiated, but the dose rates are lower and the isotope sources require proper handling and disposal. A new technology, X-ray conversion, combining the advantages and limiting the weak sides of gamma treatment and EB, is now being launched, and, when technical and economic aspects are improved, it will probably find its place on the market [265–267].

In general, polymers can be irradiated either in the solid state (i.e., in the bulk), or in solution, either in organic or aqueous solvents. Due to the desired hydrophilic character of the nanocarriers discussed in this chapter, our discussion will be focused on water as the solvent. The mechanisms and kinetics of reactions taking place in irradiated aqueous polymer solutions depend primarily on polymer concentration. The fraction of energy of ionizing radiation that is absorbed by a given component of
<figure><img src="image_24.png" /><figcaption>**Figure 14.** Intermolecular crosslinking of a macromolecule using a chemical cross-linking agent. For details see e.g., [259].</figcaption></figure>

### 4.3. Synthesis of Nanogels by Radiation-Induced Intramolecular Crosslinking of Polymers

In order to further simplify the synthesis of nanogels and avoid the use of crosslinking agents, in the end of 1990, an alternative method utilizing ionizing radiation has been proposed, based on radiation-induced intramolecular crosslinking of macromolecules [260], for recent reviews see [236,261]. The only substrates are the hydrophilic polymer and water; neither initiators nor any other chemicals are used (except in some special cases). This method is described in some detail below. It builds upon the accumulated knowledge of radiation chemistry and in particular radiation crosslinking of polymers.

#### 4.3.1. Radiation Chemistry of Polymers in Aqueous Solution

Ionizing radiation is relatively widely used in industry. Applications range from simple devices to measure the thickness and detect potential imperfections of products in steel industry to sterilization of medical devices and technologies where radiation is used to synthesize or modify materials. Commercial, large-scale irradiation facilities operate in many countries on all inhabited continents.

Ionizing radiation is defined as any kind of radiation (either photons or beams of accelerated particles) having enough energy to ionize atoms and molecules. In radiation processing, mostly two kinds of radiation are used: gamma rays and electron beams. Most large-scale gamma irradiation facilities use a radioactive isotope of cobalt, $^{60}$Co, as the source of gamma rays. Due to its metallic form, cobalt is relatively easy to process, handle, and store, while the gamma rays emitted by $^{60}$Co are high energy (average energy of 1.25 MeV), providing high penetration depth in matter and thus allowing to irradiate bulk materials (e.g., on standard palettes). Alternatively, electron beams (EB) generated by accelerators are applied.

In describing the action of ionizing radiation with matter, three important parameters are used. The dose is the amount of energy absorbed per unit mass of matter (in J/kg, which in radiation chemistry is called a gray—Gy), the dose rate is the dose absorbed per unit time (Gy/s) and the radiation-chemical yield (abbreviated simply as yield) is the number of moles of products or reactants (molecules, radicals, cross-linking bonds, etc.) produced or consumed per unit of absorbed energy (mol/J). Basic information on radiation chemistry and technology can be found in a number of books and reviews [262–264].

Electron beam technology provides high throughput due to high dose rates and does not require handling of radioactive isotopes, but, on the other hand, due to the limited penetration depth of fast electrons in matter, is less suitable for irradiation of bulk materials, typically requiring that the goods to be irradiated are packed in flat boxes or trays. Gamma rays offer high penetration in matter, allowing large boxes or whole pallets of goods to be irradiated, but the dose rates are lower and the isotope sources require proper handling and disposal. A new technology, X-ray conversion, combining the advantages and limiting the weak sides of gamma treatment and EB, is now being launched, and, when technical and economic aspects are improved, it will probably find its place on the market [265–267].

In general, polymers can be irradiated either in the solid state (i.e., in the bulk), or in solution, either in organic or aqueous solvents. Due to the desired hydrophilic character of the nanocarriers discussed in this chapter, our discussion will be focused on water as the solvent. The mechanisms and kinetics of reactions taking place in irradiated aqueous polymer solutions depend primarily on polymer concentration. The fraction of energy of ionizing radiation that is absorbed by a given component of
the irradiated system is proportional to the electron density ratio of this component, which can be well approximated by the weight ratio. If polymer makes up a considerable fraction of a polymer/water system, a large part of the energy is directly absorbed by the polymer. While there are some important practical examples of such cases (irradiation of highly hydrophilic polymers which, for practical reasons, are not completely dry when irradiated [268] or irradiation of concentrated polysaccharide solutions, in the so-called paste state, to obtain macroscopic gels [269,270]), most of basic studies and applications are based on irradiating polymers in dilute or semi-dilute aqueous solutions. In dilute solutions, the direct action of ionizing radiation on the polymer itself is of minor importance and can be neglected in mechanistic considerations. It means that almost all energy is absorbed by water, and then the reactive products of water radiolysis can in turn attack the macromolecules, thus inducing reactions in the polymer component of the system. So, the polymer is subjected to an indirect effect of irradiation.

Radiation chemistry of water has been the subject of intense and detailed studies for over a century. Current state of knowledge on this topic can be found in relevant monographs [262–264,271], while comprehensive reviews provide detailed kinetic data on the reactions involved [272,273]. Here we only recollect the most basic facts relevant to nanogel synthesis.

Interaction of gamma rays or high-energy electrons with water leads to ionization and excitation of water molecules, and subsequently the cascade of very fast reactions results in the formation of transient radiolysis products, which can subsequently attack molecules of dissolved substrates. For millimolar concentrations of the latter (in case of polymers this refers to the concentration of monomer units), reactions with polymer chains occur on the timescale of nanoseconds and longer.

The most important transient products of water radiolysis, in the absence of oxygen, are hydroxyl radicals •OH, hydrogen atoms H•, and hydrated electrons eaq⁻ (Equation (8)), their radiation-chemical yields being ca. 2.9 × 10⁻⁷, 0.6 × 10⁻⁷, and 2.9 × 10⁻⁷ mol/J, respectively. In laboratory conditions or small-scale synthesis, hydrated electrons can be easily converted into further hydroxyl radicals in a reaction with nitrous oxide, which is highly soluble in water (Equation (9)).

$$H_2O \rightarrow e_{aq}^{-}, \bullet OH, H^{\bullet}, H_2O_2, H_2, H^{+}, OH^{-} \quad (8)$$

$$N_2O + e_{aq}^{-} + H_2O \rightarrow \bullet OH + OH^{-} + N_2 \quad (9)$$

The most common reaction of water radiolysis products with simple aliphatic polymers is hydrogen abstraction from C atoms by •OH radicals and by H• atoms. As a result, a radical is formed at the carbon atom of the polymer, usually at a random position along the polymer chain, with concomitant formation of water or hydrogen molecule (Figure 15).

<figure><img src="image_25.png" /><figcaption>**Figure 15.** Formation of randomly-located carbon-centered polymer radical as a result of hydrogen abstraction by hydroxyl radicals or hydrogen atoms from a polymer chain.</figcaption></figure>

Now we should briefly consider the typical reactions of these radicals. If oxygen is present in the solution, it reacts with the polymer alkyl radicals forming peroxyl radicals (Figure 16). This subsequently leads to complex chemistry [274,275] involving chain oxidation processes, decomposition of peroxyl radicals via unstable tetroxides, formation of carbonyl groups, etc. Since these processes usually do not lead to crosslinking, at least in the sense of formation of stable covalent bonds between polymer chains and/or chain segments, irradiation in the presence of oxygen is undesirable. However, under conditions that are favorable for nanogel synthesis (i.e., low polymer concentration and N₂O-saturated solutions), oxygen is produced during irradiation in minor amounts. (see [241,242,276]).
the irradiated system is proportional to the electron density ratio of this component, which can be well approximated by the weight ratio. If polymer makes up a considerable fraction of a polymer/water system, a large part of the energy is directly absorbed by the polymer. While there are some important practical examples of such cases (irradiation of highly hydrophilic polymers which, for practical reasons, are not completely dry when irradiated [268] or irradiation of concentrated polysaccharide solutions, in the so-called paste state, to obtain macroscopic gels [269,270]), most of basic studies and applications are based on irradiating polymers in dilute or semi-dilute aqueous solutions. In dilute solutions, the direct action of ionizing radiation on the polymer itself is of minor importance and can be neglected in mechanistic considerations. It means that almost all energy is absorbed by water, and then the reactive products of water radiolysis can in turn attack the macromolecules, thus inducing reactions in the polymer component of the system. So, the polymer is subjected to an indirect effect of irradiation.

Radiation chemistry of water has been the subject of intense and detailed studies for over a century. Current state of knowledge on this topic can be found in relevant monographs [262–264,271], while comprehensive reviews provide detailed kinetic data on the reactions involved [272,273]. Here we only recollect the most basic facts relevant to nanogel synthesis.

Interaction of gamma rays or high-energy electrons with water leads to ionization and excitation of water molecules, and subsequently the cascade of very fast reactions results in the formation of transient radiolysis products, which can subsequently attack molecules of dissolved substrates. For millimolar concentrations of the latter (in case of polymers this refers to the concentration of monomer units), reactions with polymer chains occur on the timescale of nanoseconds and longer.

The most important transient products of water radiolysis, in the absence of oxygen, are hydroxyl radicals •OH, hydrogen atoms H•, and hydrated electrons eaq⁻ (Equation (8)), their radiation-chemical yields being ca. 2.9 × 10⁻⁷, 0.6 × 10⁻⁷, and 2.9 × 10⁻⁷ mol/J, respectively. In laboratory conditions or small-scale synthesis, hydrated electrons can be easily converted into further hydroxyl radicals in a reaction with nitrous oxide, which is highly soluble in water (Equation (9)).

$$H_2O \rightarrow e_{aq}^{-}, \bullet OH, H^{\bullet}, H_2O_2, H_2, H^{+}, OH^{-} \quad (8)$$

$$N_2O + e_{aq}^{-} + H_2O \rightarrow \bullet OH + OH^{-} + N_2 \quad (9)$$

The most common reaction of water radiolysis products with simple aliphatic polymers is hydrogen abstraction from C atoms by •OH radicals and by H• atoms. As a result, a radical is formed at the carbon atom of the polymer, usually at a random position along the polymer chain, with concomitant formation of water or hydrogen molecule (Figure 15).

<figure><img src="image_25.png" /><figcaption>**Figure 15.** Formation of randomly-located carbon-centered polymer radical as a result of hydrogen abstraction by hydroxyl radicals or hydrogen atoms from a polymer chain.</figcaption></figure>

Now we should briefly consider the typical reactions of these radicals. If oxygen is present in the solution, it reacts with the polymer alkyl radicals forming peroxyl radicals (Figure 16). This subsequently leads to complex chemistry [274,275] involving chain oxidation processes, decomposition of peroxyl radicals via unstable tetroxides, formation of carbonyl groups, etc. Since these processes usually do not lead to crosslinking, at least in the sense of formation of stable covalent bonds between polymer chains and/or chain segments, irradiation in the presence of oxygen is undesirable. However, under conditions that are favorable for nanogel synthesis (i.e., low polymer concentration and N₂O-saturated solutions), oxygen is produced during irradiation in minor amounts. (see [241,242,276]).
<figure><img src="image_26.png" /><figcaption>**Figure 16.** Addition of oxygen molecule to a carbon-centered polymer radical leading to the formation of a peroxyl radical.</figcaption></figure>

Usually in radiation processing of polymers the desired reaction is crosslinking, as discussed in more detail below. However, one should be aware of the presence of other reactions that may influence the outcome of irradiation, or even dominate the radiation chemistry thus preventing the polymer from being crosslinked. In general, we may divide radiation-induced reactions in polymers into two groups, i.e., processes involving one radical (mainly H-shift, degradation and addition to a multiple bond; in these processes the number of radicals in the system does not change) or involving two radicals (crosslinking, disproportionation; as a result of these reactions radicals decay).

In principle, H-shift (Figure 17) seems to be a benign process apparently not influencing other reactions in the system, since the number of radicals on polymer molecules remains unchanged. Moreover, such reactions have been shown to be relatively slow [277,278]. However, H-shift has been demonstrated to have some impact on the kinetics of intramolecular recombination, due to its influence on spatial distribution of radicals along a polymer chain [279]. Degradation (chain breakage, Figure 18) is a common process in radiolysis of polymers. As discussed in detail above, for some macromolecules (nonmodified polysaccharides, aliphatic polymers containing tertiary carbon atoms), when irradiated at standard conditions, it can be the sole or at least the dominating reaction. Even in the case of polymers undergoing predominantly crosslinking, often some contribution of degradation can be observed [8]. Thus, it is considered as an important process that competes with crosslinking and always should be taken into account when studying radiation effects on polymers. In particular cases, chain breakage and the consequent formation of a terminal polymer radical can initiate chain process of depolymerization [280]. In case the macromolecules in question contain double carbon-carbon bonds (for instance formed as a result of previous disproportionation or degradation steps), a polymer-free radical may add to such a bond, forming an inter- or intramolecular crosslink (Figure 19).

<figure><img src="image_27.png" /><figcaption>**Figure 17.** H-transfer leading to a shift in location of the radical along a polymer chain.</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>**Figure 18.** Degradation (chain scission) of a radical-bearing macromolecule.</figcaption></figure>

The bi-radical reactions are recombination (crosslinking) and disproportionation. The ratio between the two processes is mainly governed by chemical structure of the polymer and cannot be easily influenced (for an exception where this ratio can be to some extent controlled by pH due to the presence of carboxyl groups at the radical-bearing chain, see [281]). Disproportionation (Figure 20;
<figure><img src="image_26.png" /><figcaption>**Figure 16.** Addition of oxygen molecule to a carbon-centered polymer radical leading to the formation of a peroxyl radical.</figcaption></figure>

Usually in radiation processing of polymers the desired reaction is crosslinking, as discussed in more detail below. However, one should be aware of the presence of other reactions that may influence the outcome of irradiation, or even dominate the radiation chemistry thus preventing the polymer from being crosslinked. In general, we may divide radiation-induced reactions in polymers into two groups, i.e., processes involving one radical (mainly H-shift, degradation and addition to a multiple bond; in these processes the number of radicals in the system does not change) or involving two radicals (crosslinking, disproportionation; as a result of these reactions radicals decay).

In principle, H-shift (Figure 17) seems to be a benign process apparently not influencing other reactions in the system, since the number of radicals on polymer molecules remains unchanged. Moreover, such reactions have been shown to be relatively slow [277,278]. However, H-shift has been demonstrated to have some impact on the kinetics of intramolecular recombination, due to its influence on spatial distribution of radicals along a polymer chain [279]. Degradation (chain breakage, Figure 18) is a common process in radiolysis of polymers. As discussed in detail above, for some macromolecules (nonmodified polysaccharides, aliphatic polymers containing tertiary carbon atoms), when irradiated at standard conditions, it can be the sole or at least the dominating reaction. Even in the case of polymers undergoing predominantly crosslinking, often some contribution of degradation can be observed [8]. Thus, it is considered as an important process that competes with crosslinking and always should be taken into account when studying radiation effects on polymers. In particular cases, chain breakage and the consequent formation of a terminal polymer radical can initiate chain process of depolymerization [280]. In case the macromolecules in question contain double carbon-carbon bonds (for instance formed as a result of previous disproportionation or degradation steps), a polymer-free radical may add to such a bond, forming an inter- or intramolecular crosslink (Figure 19).

<figure><img src="image_27.png" /><figcaption>**Figure 17.** H-transfer leading to a shift in location of the radical along a polymer chain.</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>**Figure 18.** Degradation (chain scission) of a radical-bearing macromolecule.</figcaption></figure>

The bi-radical reactions are recombination (crosslinking) and disproportionation. The ratio between the two processes is mainly governed by chemical structure of the polymer and cannot be easily influenced (for an exception where this ratio can be to some extent controlled by pH due to the presence of carboxyl groups at the radical-bearing chain, see [281]). Disproportionation (Figure 20;
the process can also occur intramolecularly) is often neglected as an unwanted side process in polymer crosslinking. However, it should be kept in mind that in some cases the yield of disproportionation may actually be much higher than the crosslinking yield, leading to considerable loss of radicals and limitation of the crosslinking yield; a good example is poly(vinyl alcohol) where ca. 90% of radicals disproportionate [282].

<figure><img src="image_29.png" /><figcaption>**Figure 19.** Intermolecular addition of a polymer radical to a pre-existing double bond, resulting in crosslinking and re-creation of a radical site.</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>**Figure 20.** Intermolecular disproportionation of two carbon-centered polymer radicals.</figcaption></figure>

Radical recombination, leading to polymer crosslinking, can occur in two ways, either between two radicals located at separate macromolecules (Figure 21a), or between two radicals present at two different segments of the same chain (Figure 21b). The former leads to covalent binding of the two chains and an increase in the molecular weight. Consequent acts of intermolecular crosslinking may finally lead to the formation of macroscopic, three-dimensional polymer networks, where most or all the chains present in the system are linked together. This reaction is the basis of many implemented technologies: reinforcing of tires, tubes, cable insulations and jackets, production of thermoshrinkable materials or improving wear resistance of sockets of hip joint implants when polymers are radiation-crosslinked in the solid state, or hydrogel wound dressings and drug delivery systems [283,284] when crosslinking is performed in aqueous solution. In contrast, when intramolecular crosslinking occurs within a single macromolecule, a nanogel particle is formed.

<figure><img src="image_31.png" /><figcaption>**Figure 21.** Crosslinking by recombination of two polymer radicals: (a) intermolecular, (b) intramolecular.</figcaption></figure>
the process can also occur intramolecularly) is often neglected as an unwanted side process in polymer crosslinking. However, it should be kept in mind that in some cases the yield of disproportionation may actually be much higher than the crosslinking yield, leading to considerable loss of radicals and limitation of the crosslinking yield; a good example is poly(vinyl alcohol) where ca. 90% of radicals disproportionate [282].

<figure><img src="image_29.png" /><figcaption>**Figure 19.** Intermolecular addition of a polymer radical to a pre-existing double bond, resulting in crosslinking and re-creation of a radical site.</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>**Figure 20.** Intermolecular disproportionation of two carbon-centered polymer radicals.</figcaption></figure>

Radical recombination, leading to polymer crosslinking, can occur in two ways, either between two radicals located at separate macromolecules (Figure 21a), or between two radicals present at two different segments of the same chain (Figure 21b). The former leads to covalent binding of the two chains and an increase in the molecular weight. Consequent acts of intermolecular crosslinking may finally lead to the formation of macroscopic, three-dimensional polymer networks, where most or all the chains present in the system are linked together. This reaction is the basis of many implemented technologies: reinforcing of tires, tubes, cable insulations and jackets, production of thermoshrinkable materials or improving wear resistance of sockets of hip joint implants when polymers are radiation-crosslinked in the solid state, or hydrogel wound dressings and drug delivery systems [283,284] when crosslinking is performed in aqueous solution. In contrast, when intramolecular crosslinking occurs within a single macromolecule, a nanogel particle is formed.

<figure><img src="image_31.png" /><figcaption>**Figure 21.** Crosslinking by recombination of two polymer radicals: (a) intermolecular, (b) intramolecular.</figcaption></figure>
### 4.3.2. Synthesis of Nanogels by Intramolecular Crosslinking

The competition between inter- and intramolecular recombination of polymer radicals can be controlled by reaction conditions (Figure 22). The key factors to achieve this control are the average number of radicals that are present simultaneously on each polymer chain and the concentration of radical-bearing polymer chains. When discussing this issue, one should bear in mind that radicals are very reactive species thus, except special cases (see [277,285,286]), in solution they disappear within a fraction of second after being formed. If we irradiate a moderately concentrated polymer solution (typically a few weight %) in a continuous way using a relatively low dose rate (as would be the typical conditions for gamma irradiation), the rate of radical decay will soon approach the rate of radical production, and a steady-state is established where the average number of radicals per chain may be significantly less than one. If so, the probability of intramolecular reactions is very low (since it is very improbable to find 2 or more radicals simultaneously on the same chain) and the radicals would have to find reaction partners on other macromolecules (Figure 22a). In contrast, if we use a dilute solution (typically below 1%) and deliver radiation energy in short, intense pulses (which is the typical mode of irradiation when using electron accelerators), it is easy to generate many radicals (even many tens of them) on each chain simultaneously, within the pulse duration, which may be as short as nano- or microseconds. Since, due to low polymer concentration, the next chains are far away and their diffusion is slow, and chances for intermolecular recombination are low (albeit not zero, see below), while most of the radicals would recombine with their neighbors on the same chain (Figure 22b). In this way nanogels are formed.

<figure><img src="image_32.png" /><figcaption>**Figure 22.** Controlling the dominating crosslinking process: (a) high polymer concentration and low dose rate—n average less than one radical present simultaneously on each chain—domination of intermolecular crosslinking, (b) low polymer concentration and high-dose pulse irradiation—many radicals present simultaneously on each chain—domination of intramolecular crosslinking.</figcaption></figure>
### 4.3.2. Synthesis of Nanogels by Intramolecular Crosslinking

The competition between inter- and intramolecular recombination of polymer radicals can be controlled by reaction conditions (Figure 22). The key factors to achieve this control are the average number of radicals that are present simultaneously on each polymer chain and the concentration of radical-bearing polymer chains. When discussing this issue, one should bear in mind that radicals are very reactive species thus, except special cases (see [277,285,286]), in solution they disappear within a fraction of second after being formed. If we irradiate a moderately concentrated polymer solution (typically a few weight %) in a continuous way using a relatively low dose rate (as would be the typical conditions for gamma irradiation), the rate of radical decay will soon approach the rate of radical production, and a steady-state is established where the average number of radicals per chain may be significantly less than one. If so, the probability of intramolecular reactions is very low (since it is very improbable to find 2 or more radicals simultaneously on the same chain) and the radicals would have to find reaction partners on other macromolecules (Figure 22a). In contrast, if we use a dilute solution (typically below 1%) and deliver radiation energy in short, intense pulses (which is the typical mode of irradiation when using electron accelerators), it is easy to generate many radicals (even many tens of them) on each chain simultaneously, within the pulse duration, which may be as short as nano- or microseconds. Since, due to low polymer concentration, the next chains are far away and their diffusion is slow, and chances for intermolecular recombination are low (albeit not zero, see below), while most of the radicals would recombine with their neighbors on the same chain (Figure 22b). In this way nanogels are formed.

<figure><img src="image_32.png" /><figcaption>**Figure 22.** Controlling the dominating crosslinking process: (a) high polymer concentration and low dose rate—n average less than one radical present simultaneously on each chain—domination of intermolecular crosslinking, (b) low polymer concentration and high-dose pulse irradiation—many radicals present simultaneously on each chain—domination of intramolecular crosslinking.</figcaption></figure>
The characteristic feature of radiation synthesis of nanogels is a significant decrease in coil dimensions (radius of gyration, R_g) while the average molecular weight (M_w) remains nearly constant or slightly increases (since it is difficult to totally eliminate intermolecular crosslinking). Decrease in dimensions reflects the formation of internal bonds, which limit the ability of segments to diffuse away from the center of mass of the macromolecule (for illustration, see Figure 23b below). This is also a typical feature observed in chemical intramolecular crosslinking [259]. In some cases, the pattern of R_g and M_w changes becomes more complex. For poly(acrylic acid) initially some decrease in M_w is observed, due to degradation competing with crosslinking. However, when some internal bonds are already formed, further degradation acts do not lead to detachment of chain fragments and thus they do not cause any further decrease in molecular weight. Since some intermolecular crosslinking is present, the M_w starts to increase, while the dimensions go down (Figure 23). It can be further seen that the competition between intra- and intermolecular crosslinking can be influenced by polymer concentration, since this, at a constant dose per pulse, influences the average number of radicals per chain.

<figure><img src="image_33.png" /><figcaption>**Figure 23.** Radiation synthesis of poly(acrylic acid)—PAA nanogels. Weight-average molecular weight (M~w~—left panel) and radius of gyration (R~g~—right panel) of PAA macromolecules in the course of nanogel synthesis as a function of total absorbed dose (1.15 kGy corresponds to a single pulse) for samples of various PAA concentrations: □—10 mM, ○—17.5 mM, △—25 mM, irradiated in Ar-saturated aqueous solutions, pH 2. Radii of gyration measured at 25.0 °C in aqueous 0.5 M NaClO~4~, pH 10. Reprinted with permission from [240]. Copyright (2003) American Chemical Society.</figcaption></figure>

Nanogel synthesis can also be performed on industrial electron beam installations with high throughputs, such as those used for sterilization purposes, with no modification to the regular irradiation procedures [287,288]. Solutions, contained in sealed vials, are irradiated within a box passing under the scanner on a moving conveyor. The electron pulses are modulated with two frequencies; the frequency of the accelerator and the frequency of the scanning horn at which the beam sweep across the width of the samples box. The total adsorbed dose is also controlled by speed of the conveyor belt. Within certain limits, the nanogel size and molecular weight can be fine-tuned by changing the polymer concentration. At low polymer concentration, intramolecular crosslinking is the dominating process that leads to nanogel formation. At higher polymer concentrations, intermolecular crosslinking is initially competitive with intramolecular crosslinking but, through this process, the polymer concentration decreases with dose to the point where intramolecular crosslinking becomes dominating [276]. Beyond this point, no change in molecular weight or size is observed. The increase in size and molecular weight generally occur well below 20 kGy [241,242]. Higher doses are imparted by multiple passes. When a high dose range is explored (20 kGy to 80 kGy), e.g., with N~2~O-purged poly(N-vinyl pyrrolidone) solutions (concentrations in the range 0.05–0.2 %w), the average molecular weight and hydrodynamic
The characteristic feature of radiation synthesis of nanogels is a significant decrease in coil dimensions (radius of gyration, R_g) while the average molecular weight (M_w) remains nearly constant or slightly increases (since it is difficult to totally eliminate intermolecular crosslinking). Decrease in dimensions reflects the formation of internal bonds, which limit the ability of segments to diffuse away from the center of mass of the macromolecule (for illustration, see Figure 23b below). This is also a typical feature observed in chemical intramolecular crosslinking [259]. In some cases, the pattern of R_g and M_w changes becomes more complex. For poly(acrylic acid) initially some decrease in M_w is observed, due to degradation competing with crosslinking. However, when some internal bonds are already formed, further degradation acts do not lead to detachment of chain fragments and thus they do not cause any further decrease in molecular weight. Since some intermolecular crosslinking is present, the M_w starts to increase, while the dimensions go down (Figure 23). It can be further seen that the competition between intra- and intermolecular crosslinking can be influenced by polymer concentration, since this, at a constant dose per pulse, influences the average number of radicals per chain.

<figure><img src="image_33.png" /><figcaption>**Figure 23.** Radiation synthesis of poly(acrylic acid)—PAA nanogels. Weight-average molecular weight (M~w~—left panel) and radius of gyration (R~g~—right panel) of PAA macromolecules in the course of nanogel synthesis as a function of total absorbed dose (1.15 kGy corresponds to a single pulse) for samples of various PAA concentrations: □—10 mM, ○—17.5 mM, △—25 mM, irradiated in Ar-saturated aqueous solutions, pH 2. Radii of gyration measured at 25.0 °C in aqueous 0.5 M NaClO~4~, pH 10. Reprinted with permission from [240]. Copyright (2003) American Chemical Society.</figcaption></figure>

Nanogel synthesis can also be performed on industrial electron beam installations with high throughputs, such as those used for sterilization purposes, with no modification to the regular irradiation procedures [287,288]. Solutions, contained in sealed vials, are irradiated within a box passing under the scanner on a moving conveyor. The electron pulses are modulated with two frequencies; the frequency of the accelerator and the frequency of the scanning horn at which the beam sweep across the width of the samples box. The total adsorbed dose is also controlled by speed of the conveyor belt. Within certain limits, the nanogel size and molecular weight can be fine-tuned by changing the polymer concentration. At low polymer concentration, intramolecular crosslinking is the dominating process that leads to nanogel formation. At higher polymer concentrations, intermolecular crosslinking is initially competitive with intramolecular crosslinking but, through this process, the polymer concentration decreases with dose to the point where intramolecular crosslinking becomes dominating [276]. Beyond this point, no change in molecular weight or size is observed. The increase in size and molecular weight generally occur well below 20 kGy [241,242]. Higher doses are imparted by multiple passes. When a high dose range is explored (20 kGy to 80 kGy), e.g., with N~2~O-purged poly(N-vinyl pyrrolidone) solutions (concentrations in the range 0.05–0.2 %w), the average molecular weight and hydrodynamic
size of the produced nanogels do not significantly change with increasing the dose but low molecular weight fractions appear at higher doses. New functional groups, such as primary amino groups and carboxyl groups, only to mention the most interesting ones for postirradiation reactions, are formed in a dose-dependent fashion [241,242,288]. Both experimental measurements and numerical simulations suggest that O₂ is also formed under these irradiation conditions. In fact, irradiation of a polymer in dilute aqueous solution by high-dose pulses of electrons causes some hydroxyl radicals to recombine rather than react with the polymer, and in this way substantial amounts of hydrogen peroxide are formed. Under high-dose irradiation, some of the formed hydrogen peroxide is converted in molecular oxygen. The reaction of molecular oxygen with polymer radicals is at the basis of the observed dose-dependent polymer fragmentation and network functionalization. As mentioned above, double bonds are also formed in radical-radical disproportionation reactions [242,276].

Nanogels can be visualized by AFM and SEM microscopy when adsorbed at a flat surface [240,289–291]. Figure 24 illustrates PVP nanogels grafted with acrylic acid, as scanned by AFM in the tapping mode under water, while exemplary SEM pictures of PVP nanogels cast on a silicone surface and sputtered with gold are shown in Figure 25.

<figure><img src="image_34.png" /><figcaption>**Figure 24.** Height profile from tapping mode AFM under water for radiation-synthesized PVP nanogels modified with acrylic acid. The color scale represents a maximum height of 8 nm. Reproduced from [292].</figcaption></figure>

<figure><img src="image_35.png" /><figcaption>**Figure 25.** The scanning electron micrographs of PVP nanogels at two different magnifications (A,B). Reproduced from [291]. License obtained from Elsevier.</figcaption></figure>
size of the produced nanogels do not significantly change with increasing the dose but low molecular weight fractions appear at higher doses. New functional groups, such as primary amino groups and carboxyl groups, only to mention the most interesting ones for postirradiation reactions, are formed in a dose-dependent fashion [241,242,288]. Both experimental measurements and numerical simulations suggest that O₂ is also formed under these irradiation conditions. In fact, irradiation of a polymer in dilute aqueous solution by high-dose pulses of electrons causes some hydroxyl radicals to recombine rather than react with the polymer, and in this way substantial amounts of hydrogen peroxide are formed. Under high-dose irradiation, some of the formed hydrogen peroxide is converted in molecular oxygen. The reaction of molecular oxygen with polymer radicals is at the basis of the observed dose-dependent polymer fragmentation and network functionalization. As mentioned above, double bonds are also formed in radical-radical disproportionation reactions [242,276].

Nanogels can be visualized by AFM and SEM microscopy when adsorbed at a flat surface [240,289–291]. Figure 24 illustrates PVP nanogels grafted with acrylic acid, as scanned by AFM in the tapping mode under water, while exemplary SEM pictures of PVP nanogels cast on a silicone surface and sputtered with gold are shown in Figure 25.

<figure><img src="image_34.png" /><figcaption>**Figure 24.** Height profile from tapping mode AFM under water for radiation-synthesized PVP nanogels modified with acrylic acid. The color scale represents a maximum height of 8 nm. Reproduced from [292].</figcaption></figure>

<figure><img src="image_35.png" /><figcaption>**Figure 25.** The scanning electron micrographs of PVP nanogels at two different magnifications (A,B). Reproduced from [291]. License obtained from Elsevier.</figcaption></figure>
Intramolecular recombination is an interesting process from the kinetic point of view and has been the subject of numerous experimental and simulation-based studies [240,276,279,289,293–296]. When analyzing the general decay of radicals in the system it has often been observed to deviate from the classical second-order kinetics typical for recombination of radicals located on small molecules. Both experimental data and Monte-Carlo simulations indicate that the spatial distribution of radicals along the chain is the decisive factor both for kinetics and the final structure of the nanogels [279,294]. A dispersive kinetics model developed by Plonka [297,298] is helpful both for kinetic description and mechanistic interpretation of intramolecular radical recombination. Alternatively, for every pair of radicals defined by a given combination of radical positions, a rate constant can be defined. In a recently developed modeling approach, intramolecular radical-radical reactions were treated on the basis of combinations of site-dependent rate constants. This model could reproduce the kinetics of radical decay in single-pulse experiments [296] as well as the evolution in molecular weight in nanogel synthesis using pulsed e-beams [276]. As this model took the complete set of radiation-induced reactions into account, essential information about the evolution of other species in the system could be derived. This includes the already mentioned formation of O₂ as well as double bonds due to radical-radical disproportionation [276]. Recently, new kinetic studies using the Dynamic Liquid Lattice model [299–303] have been initiated on intramolecular crosslinking of macromolecules.

The above-described nanogel synthesis method is not limited to polymers of any particular chemical structure, albeit it requires a polymer which predominantly crosslinks, and not degrades, under irradiation. It has been successfully tested on simple synthetic water-soluble polymers—(poly(N-vinylpyrrolidone) [236,287–292,304–311] and poly(vinyl alcohol) [260], on an exemplary polyelectrolyte [poly(acrylic acid)] [240,312], and on a thermosensitive polymer—(poly(vinyl methyl ether)) [313]. When working with polyelectrolytes, one should set appropriate pH where ionization is suppressed. Otherwise the charges along the chain repel each other by Coulombic forces, which makes the energy barrier for recombination (and disproportionation) so high, that the radicals can live on chain segments for hours (at R.T., in water, with no stabilizing effects other than the presence of charge) [277,285,286], and slow reactions such as degradation dominate over recombination. For instance, for poly (acrylic acid) (average pKa for high-molecular-weight polymer being ca. 6, [314,315]), crosslinking starts to prevail only when pH is lowered to 3 or less, and successful nanogel synthesis has been demonstrated at pH 2.

### 4.3.3. Controlling the Physicochemical Properties of Radiation-Synthesized Nanogels

While the feasibility of radiation-induced intramolecular crosslinking for nanogel synthesis is well established, recent studies have been focused on controlling the process, so that starting from macromolecules of given molecular weight and dimensions one could arrive at nanogels of desired $M_w$ and $R_g$ (which also determines the coil density of the final, crosslinked product).

Kadłubowski et al. proposed a two-step synthetic procedure of synthesizing tailored nanogels [305]. In the first step irradiation conditions are set to promote intermolecular recombination (relatively high polymer concentration, moderate dose rate). As a result, both average molecular weight and the nanoparticle size increase with dose. When the desired average molecular weight is reached, the solution is diluted, and irradiation mode is changed to short, intense pulses. These conditions promote intramolecular recombination; therefore, the molecular weight does not significantly change, but the size is reduced to the desired value. Figure 26 shows the test of this approach for poly(N-vinylpyrrolidone).

An extensive study on the radiation synthesis of PVP nanogels has been recently published by Sütekin and Güven [291]. The effects of various parameters as total absorbed dose, dose rate, polymer concentration, molecular weight on the sizes of nanogels were investigated. It has been proved that by careful control of these parameters, PVP nanogels of sizes varying in the range of 30–250 nm can be prepared. It has been demonstrated that, when proper conditions are chosen, nanogels can also be synthesized using gamma rays. The nanogels have been shown to maintain their parameters for 2 years of storage in solution at 4 °C.
Intramolecular recombination is an interesting process from the kinetic point of view and has been the subject of numerous experimental and simulation-based studies [240,276,279,289,293–296]. When analyzing the general decay of radicals in the system it has often been observed to deviate from the classical second-order kinetics typical for recombination of radicals located on small molecules. Both experimental data and Monte-Carlo simulations indicate that the spatial distribution of radicals along the chain is the decisive factor both for kinetics and the final structure of the nanogels [279,294]. A dispersive kinetics model developed by Plonka [297,298] is helpful both for kinetic description and mechanistic interpretation of intramolecular radical recombination. Alternatively, for every pair of radicals defined by a given combination of radical positions, a rate constant can be defined. In a recently developed modeling approach, intramolecular radical-radical reactions were treated on the basis of combinations of site-dependent rate constants. This model could reproduce the kinetics of radical decay in single-pulse experiments [296] as well as the evolution in molecular weight in nanogel synthesis using pulsed e-beams [276]. As this model took the complete set of radiation-induced reactions into account, essential information about the evolution of other species in the system could be derived. This includes the already mentioned formation of O₂ as well as double bonds due to radical-radical disproportionation [276]. Recently, new kinetic studies using the Dynamic Liquid Lattice model [299–303] have been initiated on intramolecular crosslinking of macromolecules.

The above-described nanogel synthesis method is not limited to polymers of any particular chemical structure, albeit it requires a polymer which predominantly crosslinks, and not degrades, under irradiation. It has been successfully tested on simple synthetic water-soluble polymers—(poly(*N*-vinylpyrrolidone) [236,287–292,304–311] and poly(vinyl alcohol) [260], on an exemplary polyelectrolyte [poly(acrylic acid)] [240,312], and on a thermosensitive polymer—(poly(vinyl methyl ether)) [313]. When working with polyelectrolytes, one should set appropriate pH where ionization is suppressed. Otherwise the charges along the chain repel each other by Coulombic forces, which makes the energy barrier for recombination (and disproportionation) so high, that the radicals can live on chain segments for hours (at R.T., in water, with no stabilizing effects other than the presence of charge) [277,285,286], and slow reactions such as degradation dominate over recombination. For instance, for poly (acrylic acid) (average pK<sub>a</sub> for high-molecular-weight polymer being ca. 6, [314,315]), crosslinking starts to prevail only when pH is lowered to 3 or less, and successful nanogel synthesis has been demonstrated at pH 2.

### 4.3.3. Controlling the Physicochemical Properties of Radiation-Synthesized Nanogels

While the feasibility of radiation-induced intramolecular crosslinking for nanogel synthesis is well established, recent studies have been focused on controlling the process, so that starting from macromolecules of given molecular weight and dimensions one could arrive at nanogels of desired *M*<sub>w</sub> and *R*<sub>g</sub> (which also determines the coil density of the final, crosslinked product).

Kadłubowski et al. proposed a two-step synthetic procedure of synthesizing tailored nanogels [305]. In the first step irradiation conditions are set to promote intermolecular recombination (relatively high polymer concentration, moderate dose rate). As a result, both average molecular weight and the nanoparticle size increase with dose. When the desired average molecular weight is reached, the solution is diluted, and irradiation mode is changed to short, intense pulses. These conditions promote intramolecular recombination; therefore, the molecular weight does not significantly change, but the size is reduced to the desired value. Figure 26 shows the test of this approach for poly(*N*-vinylpyrrolidone).

An extensive study on the radiation synthesis of PVP nanogels has been recently published by Sütekin and Güven [291]. The effects of various parameters as total absorbed dose, dose rate, polymer concentration, molecular weight on the sizes of nanogels were investigated. It has been proved that by careful control of these parameters, PVP nanogels of sizes varying in the range of 30–250 nm can be prepared. It has been demonstrated that, when proper conditions are chosen, nanogels can also be synthesized using gamma rays. The nanogels have been shown to maintain their parameters for 2 years of storage in solution at 4 °C.
<figure><img src="image_36.png" /><figcaption>**Figure 26.** Two-step synthesis of PVP nanogels. M<sub>w</sub> and R<sub>g</sub> as a function of total absorbed dose. Black symbols denote continuous low-dose-rate gamma irradiation at a polymer concentration of 400 mM. In the two-step procedure, at the point marked by an arrow, irradiation conditions are changed—the second step (open symbols) is pulsed EB irradiation at the PVP concentration of 15 mM. Based on [305].</figcaption></figure>

For polymers that undergo temperature-induced phase transition, for instance, those characterized by lower critical solution temperature (LCST), temperature can be used for controlling the crosslinking process. Pulsed EB irradiation of poly(vinyl methyl ether)—PVME—in dilute aqueous solution below the LCST (36 °C) leads to thermo-sensitive nanogels, which exhibit lower LCST (29 °C) than the parent linear polymer [316]. Irradiation of PVME solutions above LCST leads to the crosslinking of collapsed polymer aggregates, resulting in nanogels of different sizes and structures than those obtained by irradiation at low temperature [317].

An and co-workers have demonstrated that formation of PVP nanogels by pulse irradiation in aqueous solution can also be influenced by temperature [289]. Above a threshold temperature of 50–55 °C, due to the thermal collapse of polymer chains, probability of intramolecular recombination increases sharply and there is a strong change in the activation energy of radical decay, which leads to the formation of compact nanogels. The reaction rate constants of decay (2k₂) were measured at temperatures ranging from 28 to 77 °C, Figure 27 shows their results. These results are not unexpected, as at higher temperature ranges, the distance between the carbon-centered radicals on a single change are smaller due to the polymer taking on a collapsed chain conformation. At low temperatures, the polymer chain is a noncollapsed random coil whose dimensions are closer to those predicted by random walk statistics. The carbon-centered radical–radical distances are also longer pointing towards a lower 2k₂ values as observed.

<figure><img src="image_37.png" /><figcaption>**Figure 27.** (a) Second-order reaction decay rate constant (2k₂) as a function of temperature; (b) Arrhenius plot of 2k₂ showing how the average activation energies were derived from two different temperature regions (I and II) in N₂O-saturated PVP aqueous solutions [289].</figcaption></figure>
<figure><img src="image_36.png" /><figcaption>**Figure 26.** Two-step synthesis of PVP nanogels. M<sub>w</sub> and R<sub>g</sub> as a function of total absorbed dose. Black symbols denote continuous low-dose-rate gamma irradiation at a polymer concentration of 400 mM. In the two-step procedure, at the point marked by an arrow, irradiation conditions are changed—the second step (open symbols) is pulsed EB irradiation at the PVP concentration of 15 mM. Based on [305].</figcaption></figure>

For polymers that undergo temperature-induced phase transition, for instance, those characterized by lower critical solution temperature (LCST), temperature can be used for controlling the crosslinking process. Pulsed EB irradiation of poly(vinyl methyl ether)—PVME—in dilute aqueous solution below the LCST (36 °C) leads to thermo-sensitive nanogels, which exhibit lower LCST (29 °C) than the parent linear polymer [316]. Irradiation of PVME solutions above LCST leads to the crosslinking of collapsed polymer aggregates, resulting in nanogels of different sizes and structures than those obtained by irradiation at low temperature [317].

An and co-workers have demonstrated that formation of PVP nanogels by pulse irradiation in aqueous solution can also be influenced by temperature [289]. Above a threshold temperature of 50–55 °C, due to the thermal collapse of polymer chains, probability of intramolecular recombination increases sharply and there is a strong change in the activation energy of radical decay, which leads to the formation of compact nanogels. The reaction rate constants of decay (2k₂) were measured at temperatures ranging from 28 to 77 °C, Figure 27 shows their results. These results are not unexpected, as at higher temperature ranges, the distance between the carbon-centered radicals on a single change are smaller due to the polymer taking on a collapsed chain conformation. At low temperatures, the polymer chain is a noncollapsed random coil whose dimensions are closer to those predicted by random walk statistics. The carbon-centered radical–radical distances are also longer pointing towards a lower 2k₂ values as observed.

<figure><img src="image_37.png" /><figcaption>**Figure 27.** (a) Second-order reaction decay rate constant (2k₂) as a function of temperature; (b) Arrhenius plot of 2k₂ showing how the average activation energies were derived from two different temperature regions (I and II) in N₂O-saturated PVP aqueous solutions [289].</figcaption></figure>
In addition, An and co-workers also provide a mathematical model for determining the concentration of radicals on a chain to analyze the decay kinetics of the radicals formed. The calculations were based on a monodisperse PVP model. The spatial distribution of the polymer molecules was based on Poisson distribution of Z molecules in N cells as follows (Equation 10)

$$P(m) = \left(\frac{Z}{N}\right)^m \frac{e^{-\frac{Z}{N}}}{m} \qquad (10)$$

where P(m) is the probability that a cell has exactly m polymer molecules [289].

To study the radicals generated and their reactions, the effective concentration has to be determined. This can be accomplished by taking the volume of reactive radicals aka the hydrodynamic volume into consideration. The effective molar concentration is then (Equation 11)

$$C_r = \frac{1}{N_A V_b} \qquad (11)$$

where N_A is Avogadro number and V_h is the hydrodynamic radius.

#### 4.3.4. Controlling the Chemistry of Radiation-Synthesized Nanogels

While nanogels made of homopolymers may be themselves interesting materials for various applications, and, in some cases—such as nanogels based on poly(acrylic acid)—can be easily functionalized, it is of considerable interest to adjust the above-described synthesis method to provide ability of chemical modification of the parent polymer, ideally already in the synthesis step. For many applications it would be of interest to obtain nanogels possessing some specific chemical functions, while having the main structure based on a well-tested and fully biocompatible polymer as PVP. Such functional groups can either render the nanogels some specific properties (e.g., surface charge) or allow for easy functionalization, for instance with drugs and/or targeting moieties needed to transform a nanogel into a targeted nanocarrier for controlled drug delivery. It has already been mentioned that such functionalization effect can be achieved, in some cases, just by careful adjustment of the conditions of the radiation synthesis itself, with no chemical additives [241,242].

One can also introduce chemical modifications to the nanogel structure by adding suitable chemicals, for instance monomers, to the polymer solution before irradiation. Monomer molecules can add to the polymer-derived radicals and either remain there as single-unit branches or they can initiate graft polymerization. If the monomer concentration is low, the branched chains will remain short. Other reactions of the added monomers are also possible, such as homopolymerization terminated by recombination with polymer-derived radical or by addition to the polymer double bond resulting from disproportionation. The product will contain short grafted chains and/or just single monomer units of the added monomer, providing the required functionality. In this way, PVP nanogels containing carboxyl groups can be synthesized by pulse-irradiation of PVP solution containing acrylic acid [225,292,308,318,319]. A similar approach has been demonstrated to yield poly (N-vinyl pyrrolidone)-graft-(3-N-aminopropyl) methacrylamide nanogels [287]. This indicates the great versatility of the radiation method for synthesizing nanogels of various physical and chemical properties, adjusted to the needs of particular applications, especially in pharmacy and medicine.

Another interesting option to obtain nanogels containing different chemical functions is by radiation crosslinking of interpolymer complexes. Such complexes may be based on various interpolymer interactions. One of the most interesting classes of interpolymer complexes are those based on hydrogen bonding [320–322]. They are formed by two polymers, one being a donor and the other an acceptor of hydrogen atom. As the donor, most often polymers bearing carboxylic groups are applied, while polymers possessing ether or carbonyl groups are usually used as counterparts. Hydrogen-bonded interpolymer complexes (IPCs) not only combine the chemical properties of the parent polymers, but also form heterogeneous structures containing hydrophobic domains, which may
In addition, An and co-workers also provide a mathematical model for determining the concentration of radicals on a chain to analyze the decay kinetics of the radicals formed. The calculations were based on a monodisperse PVP model. The spatial distribution of the polymer molecules was based on Poisson distribution of Z molecules in N cells as follows (Equation 10)

$$P(m) = \left(\frac{Z}{N}\right)^m \frac{e^{-\frac{Z}{N}}}{m} \qquad (10)$$

where P(m) is the probability that a cell has exactly m polymer molecules [289].

To study the radicals generated and their reactions, the effective concentration has to be determined. This can be accomplished by taking the volume of reactive radicals aka the hydrodynamic volume into consideration. The effective molar concentration is then (Equation 11)

$$C_r = \frac{1}{N_A V_b} \qquad (11)$$

where N_A is Avogadro number and V_h is the hydrodynamic radius.

### 4.3.4. Controlling the Chemistry of Radiation-Synthesized Nanogels

While nanogels made of homopolymers may be themselves interesting materials for various applications, and, in some cases—such as nanogels based on poly(acrylic acid)—can be easily functionalized, it is of considerable interest to adjust the above-described synthesis method to provide ability of chemical modification of the parent polymer, ideally already in the synthesis step. For many applications it would be of interest to obtain nanogels possessing some specific chemical functions, while having the main structure based on a well-tested and fully biocompatible polymer as PVP. Such functional groups can either render the nanogels some specific properties (e.g., surface charge) or allow for easy functionalization, for instance with drugs and/or targeting moieties needed to transform a nanogel into a targeted nanocarrier for controlled drug delivery. It has already been mentioned that such functionalization effect can be achieved, in some cases, just by careful adjustment of the conditions of the radiation synthesis itself, with no chemical additives [241,242].

One can also introduce chemical modifications to the nanogel structure by adding suitable chemicals, for instance monomers, to the polymer solution before irradiation. Monomer molecules can add to the polymer-derived radicals and either remain there as single-unit branches or they can initiate graft polymerization. If the monomer concentration is low, the branched chains will remain short. Other reactions of the added monomers are also possible, such as homopolymerization terminated by recombination with polymer-derived radical or by addition to the polymer double bond resulting from disproportionation. The product will contain short grafted chains and/or just single monomer units of the added monomer, providing the required functionality. In this way, PVP nanogels containing carboxyl groups can be synthesized by pulse-irradiation of PVP solution containing acrylic acid [225,292,308,318,319]. A similar approach has been demonstrated to yield poly (N-vinyl pyrrolidone)-graft-(3-N-aminopropyl) methacrylamide nanogels [287]. This indicates the great versatility of the radiation method for synthesizing nanogels of various physical and chemical properties, adjusted to the needs of particular applications, especially in pharmacy and medicine.

Another interesting option to obtain nanogels containing different chemical functions is by radiation crosslinking of interpolymer complexes. Such complexes may be based on various interpolymer interactions. One of the most interesting classes of interpolymer complexes are those based on hydrogen bonding [320–322]. They are formed by two polymers, one being a donor and the other an acceptor of hydrogen atom. As the donor, most often polymers bearing carboxylic groups are applied, while polymers possessing ether or carbonyl groups are usually used as counterparts. Hydrogen-bonded interpolymer complexes (IPCs) not only combine the chemical properties of the parent polymers, but also form heterogeneous structures containing hydrophobic domains, which may
be useful for instance in solubilization and controlled delivery of hydrophobic drugs or drugs that have to be protected from low pH and enzymes in the stomach [323–325].

Hydrogen-bonded IPCs based on polymers with carboxylic groups are formed only below a specific, critical pH, $pH_{crit}$. This may seem obvious, since at high pH the dissociated carboxylate groups lack the hydrogen atom capable of forming the H-bond. However, $pH_{crit}$ is typically much lower than $pK_a$ of the parent polyacid. This is because for the stable complex to be formed, an uninterrupted sequence of several protonated carboxylic groups must be present along the chain. For instance, average $pK_a$ of poly (acrylic acid) is ca. 6 [315], while $pH_{crit}$ for its complexes with poly(N-vinylpyrrolidone) is ca. 3.7–4.0 [321,325]. As a result, the complexes exist only at low pH, and dissociate in neutral and alkaline solutions. This instability at typical physiological pH may be perceived as a disadvantage in some applications. Radiation crosslinking, performed at $pH < pH_{crit}$, in an analogous way to the intermolecular crosslinking of homopolymer chains described above, can be a simple and efficient way to “fix” the complexes and make them permanent. This, of course, does not prevent the complexed domains to become dissociated at neutral and high pH (albeit complexation in a crosslinked system may differ in some respects from complexation of two separate chains [326]), but it prevents the whole structure from disintegration. From the chemical point of view, the product is an internally crosslinked block copolymer. Crosslinking of PAA-PVP complexes, leading to the formation of permanent nanogels, by pulses of fast electrons in acidic aqueous solution has been described by Henke et al. [325,327]. This idea has been further developed by Güven et al. in their detailed studies on radiation crosslinking of PAA-PVP [328] and PAA-PEO complexes [329]. In the latter work it has been clearly demonstrated that by adjusting the solvent composition (acetone has been used as co-solvent) one can influence the size of the formed nanogels. An alternative way of synthesizing PAA-PVP nanogels has been developed by Abd El-Rehim and co-workers. Gamma irradiation has been used to initiate template polymerization and crosslinking in aqueous solutions of PVP and acrylic acid [330–333].

### 4.4. *Biomedical Applications of Radiation Engineered Nanogels*

The success of nanogels is mainly due to their promising applications in the biomedical field. Nanogels, as stand-alone nanoparticles, have been proposed as nanocarriers of chemical and biological entities for therapeutic purposes and/or contrast agents for medical imaging [229,334,335], as active components of biochips or biosensors [336], building blocks of in-situ forming scaffolds for tissue engineering [337,338], cell culture systems [339,340], and antimicrobial coatings [341,342], and as components of wound dressing formulations [343,344]. Other noticeable applications of nanogels in related fields include iron chelation therapy [345], biochemical separation and contaminant removal [346,347], and bio-catalysis [348].

In drug delivery, nanogels can offer some distinct advantages compared with other nano-constructs due to their inherent features, such as high colloidal stability in aqueous media and potentially also in blood, reduced adsorption of plasma proteins and hence prolonged circulatory half-life, flexibility, and possibility to take lateral drift velocity components when moving with the blood stream favoring extravasation, tunable adhesiveness to epithelial and endothelial cells, sizable drug loading capacity with a variety of loading and controlled release mechanisms, amenability to be combined with lipid cores or shells (nanolipogels), or with inorganic nanoparticles to enable multiple functions, e.g., for chemo-photothermal or chemo-photodynamic cancer therapy, or for image-guided therapies [349].

Nanogel features such as size, morphology, and composition all impact on nanogels circulation residence time and influence their biodistribution and clearance profiles. While early clearance is not desirable, accumulation at the disease site, and degradation or elimination after the delivery function has been accomplished are highly desirable. Several targeting strategies have been proposed and evaluated, that also set requirements on each of the above features of the nanocarrier. The impact of size, surface charge density, and surface chemistry on biodistribution, accumulation at the target site and clearance, have been the topic of several review papers [349–354]. One important consideration
be useful for instance in solubilization and controlled delivery of hydrophobic drugs or drugs that have to be protected from low pH and enzymes in the stomach [323–325].

Hydrogen-bonded IPCs based on polymers with carboxylic groups are formed only below a specific, critical pH, $pH_{crit}$. This may seem obvious, since at high pH the dissociated carboxylate groups lack the hydrogen atom capable of forming the H-bond. However, $pH_{crit}$ is typically much lower than $pK_a$ of the parent polyacid. This is because for the stable complex to be formed, an uninterrupted sequence of several protonated carboxylic groups must be present along the chain. For instance, average $pK_a$ of poly (acrylic acid) is ca. 6 [315], while $pH_{crit}$ for its complexes with poly(N-vinylpyrrolidone) is ca. 3.7–4.0 [321,325]. As a result, the complexes exist only at low pH, and dissociate in neutral and alkaline solutions. This instability at typical physiological pH may be perceived as a disadvantage in some applications. Radiation crosslinking, performed at $pH < pH_{crit}$, in an analogous way to the intermolecular crosslinking of homopolymer chains described above, can be a simple and efficient way to “fix” the complexes and make them permanent. This, of course, does not prevent the complexed domains to become dissociated at neutral and high pH (albeit complexation in a crosslinked system may differ in some respects from complexation of two separate chains [326]), but it prevents the whole structure from disintegration. From the chemical point of view, the product is an internally crosslinked block copolymer. Crosslinking of PAA-PVP complexes, leading to the formation of permanent nanogels, by pulses of fast electrons in acidic aqueous solution has been described by Henke et al. [325,327]. This idea has been further developed by Güven et al. in their detailed studies on radiation crosslinking of PAA-PVP [328] and PAA-PEO complexes [329]. In the latter work it has been clearly demonstrated that by adjusting the solvent composition (acetone has been used as co-solvent) one can influence the size of the formed nanogels. An alternative way of synthesizing PAA-PVP nanogels has been developed by Abd El-Rehim and co-workers. Gamma irradiation has been used to initiate template polymerization and crosslinking in aqueous solutions of PVP and acrylic acid [330–333].

### 4.4. *Biomedical Applications of Radiation Engineered Nanogels*

The success of nanogels is mainly due to their promising applications in the biomedical field. Nanogels, as stand-alone nanoparticles, have been proposed as nanocarriers of chemical and biological entities for therapeutic purposes and/or contrast agents for medical imaging [229,334,335], as active components of biochips or biosensors [336], building blocks of in-situ forming scaffolds for tissue engineering [337,338], cell culture systems [339,340], and antimicrobial coatings [341,342], and as components of wound dressing formulations [343,344]. Other noticeable applications of nanogels in related fields include iron chelation therapy [345], biochemical separation and contaminant removal [346,347], and bio-catalysis [348].

In drug delivery, nanogels can offer some distinct advantages compared with other nano-constructs due to their inherent features, such as high colloidal stability in aqueous media and potentially also in blood, reduced adsorption of plasma proteins and hence prolonged circulatory half-life, flexibility, and possibility to take lateral drift velocity components when moving with the blood stream favoring extravasation, tunable adhesiveness to epithelial and endothelial cells, sizable drug loading capacity with a variety of loading and controlled release mechanisms, amenability to be combined with lipid cores or shells (nanolipogels), or with inorganic nanoparticles to enable multiple functions, e.g., for chemo-photothermal or chemo-photodynamic cancer therapy, or for image-guided therapies [349].

Nanogel features such as size, morphology, and composition all impact on nanogels circulation residence time and influence their biodistribution and clearance profiles. While early clearance is not desirable, accumulation at the disease site, and degradation or elimination after the delivery function has been accomplished are highly desirable. Several targeting strategies have been proposed and evaluated, that also set requirements on each of the above features of the nanocarrier. The impact of size, surface charge density, and surface chemistry on biodistribution, accumulation at the target site and clearance, have been the topic of several review papers [349–354]. One important consideration
that emerges is that even subtle changes in the nanogel network can impact the biodistribution and tumor accumulation [355] as well as their cellular internalization mechanism, degree of uptake and potential for toxicity [225,356].

Although more and more scientific papers describing new nanogel formulations, synthesis methodologies, and potential applications in the biomedical field are appearing, only very few nanogels have already been introduced in clinical trials [357]. Today's research in nanogel design and development for biomedical applications mainly focuses on how to overcome of the restrictions imposed by cost for commercial scale production and on some medical requirements and technological issues for their full exploitation as new platforms in biomedicine.

As already discussed before, the main advantages of using high-energy irradiation to synthetize hydrogel nanoparticles for biomedical applications include minimal recourse to potentially toxic chemicals, simple production schemes, possibility of fine-tuning nanogel size, crosslinking density and functionality by a proper selection of irradiation conditions, polymer concentration, and composition of the atmosphere (N₂, N₂O, air), and the possibility to obtain simultaneous sterilization when the absorbed doses are within the sterilization dose range. One major limitation of nanogel radiation-synthesis is related to the fact that crosslinking cannot be the dominant process for a few classes of potentially interesting hydrophilic polymers, such as polysaccharides and polypeptides, that mainly degrade under irradiation, but would undergo appropriate biodegradation if used as main components of nanogels. Crosslinking for these polymers may become competitive only upon chemical modification to introduce short alkyl chains and/or by achieving significant local increases of their concentration [358].

# 5. Radiation Chemistry of Natural Polymers

There are a large number of natural polymers and to review the radiation chemistry of all of them is beyond the scope of this review. This review will focus on five classes of natural polymers, polysaccharides, lignin, natural rubber (cis-1,4 polyisoprene), proteins, and RNA/DNA.

## 5.1. Radiation Chemistry of Polysaccharides

Polysaccharides are the most abundant natural polymers. They are long chains of carbohydrate (sugar) molecules, specifically polymeric carbohydrates are composed of monosaccharide units bound together by glycosidic linkages, as seen in Figure 28. Polysaccharides are widely distributed and are found in plant cell walls, seeds, and roots; algae; animals; bacteria; and fungi [359]. The most abundant monosaccharide in nature is the six-carbon sugar glucose and its derivatives as seen in cellulose and chitin. Polysaccharides can be amorphous as in hemicelluloses; crystalline such as cellulose and chitin and semicrystalline including amylopectin. Amorphous polysaccharides are most often highly branched polymers with linear polymers being more crystalline. The amount of crystallinity has been shown to affect the radiation chemistry of polysaccharides [360]. The type of glycosidic linkage can have a major effect on the physical and chemical properties of polysaccharides. Cellulose and amylose are both polymers of glucose, (1→4) linked D-glucose units. The difference is that cellulose is β (1→4) linked D-glucose units and amylose is α (1→4) linked D-glucose units. While both cellulose and amylose are insoluble in cold water amylose is soluble in hot water and cellulose is not [359]. This is attributed to the higher amount of hydrogen bonding, both inter and intra, in cellulose, Figure 29. Polysaccharides can also be made up of more than one saccharide. Alginate is a linear copolymer of β-(1–4) linked d-mannuronic acid and β-(1–4)-linked l-guluronic acid units [359]. There are many review articles and books on polysaccharides for additional information see [361–363].

## 5.2. Sold State Irradiation

When polysaccharides are irradiated in the solid state the energy is absorbed directly by the polymer and radicals are formed. This type of interaction is called a direct effect. When a polymer is irradiated two reactions can take place chain scission and crosslinking, with one predominating. In the case of polysaccharides chain scission predominates. In polysaccharides the absorbed energy causes
that emerges is that even subtle changes in the nanogel network can impact the biodistribution and tumor accumulation [355] as well as their cellular internalization mechanism, degree of uptake and potential for toxicity [225,356].

Although more and more scientific papers describing new nanogel formulations, synthesis methodologies, and potential applications in the biomedical field are appearing, only very few nanogels have already been introduced in clinical trials [357]. Today's research in nanogel design and development for biomedical applications mainly focuses on how to overcome of the restrictions imposed by cost for commercial scale production and on some medical requirements and technological issues for their full exploitation as new platforms in biomedicine.

As already discussed before, the main advantages of using high-energy irradiation to synthetize hydrogel nanoparticles for biomedical applications include minimal recourse to potentially toxic chemicals, simple production schemes, possibility of fine-tuning nanogel size, crosslinking density and functionality by a proper selection of irradiation conditions, polymer concentration, and composition of the atmosphere (N₂, N₂O, air), and the possibility to obtain simultaneous sterilization when the absorbed doses are within the sterilization dose range. One major limitation of nanogel radiation-synthesis is related to the fact that crosslinking cannot be the dominant process for a few classes of potentially interesting hydrophilic polymers, such as polysaccharides and polypeptides, that mainly degrade under irradiation, but would undergo appropriate biodegradation if used as main components of nanogels. Crosslinking for these polymers may become competitive only upon chemical modification to introduce short alkyl chains and/or by achieving significant local increases of their concentration [358].

# 5. Radiation Chemistry of Natural Polymers

There are a large number of natural polymers and to review the radiation chemistry of all of them is beyond the scope of this review. This review will focus on five classes of natural polymers, polysaccharides, lignin, natural rubber (cis-1,4 polyisoprene), proteins, and RNA/DNA.

## 5.1. Radiation Chemistry of Polysaccharides

Polysaccharides are the most abundant natural polymers. They are long chains of carbohydrate (sugar) molecules, specifically polymeric carbohydrates are composed of monosaccharide units bound together by glycosidic linkages, as seen in Figure 28. Polysaccharides are widely distributed and are found in plant cell walls, seeds, and roots; algae; animals; bacteria; and fungi [359]. The most abundant monosaccharide in nature is the six-carbon sugar glucose and its derivatives as seen in cellulose and chitin. Polysaccharides can be amorphous as in hemicelluloses; crystalline such as cellulose and chitin and semicrystalline including amylopectin. Amorphous polysaccharides are most often highly branched polymers with linear polymers being more crystalline. The amount of crystallinity has been shown to affect the radiation chemistry of polysaccharides [360]. The type of glycosidic linkage can have a major effect on the physical and chemical properties of polysaccharides. Cellulose and amylose are both polymers of glucose, (1→4) linked D-glucose units. The difference is that cellulose is β (1→4) linked D-glucose units and amylose is α (1→4) linked D-glucose units. While both cellulose and amylose are insoluble in cold water amylose is soluble in hot water and cellulose is not [359]. This is attributed to the higher amount of hydrogen bonding, both inter and intra, in cellulose, Figure 29. Polysaccharides can also be made up of more than one saccharide. Alginate is a linear copolymer of β-(1-4) linked d-mannuronic acid and β-(1-4)-linked l-guluronic acid units [359]. There are many review articles and books on polysaccharides for additional information see [361–363].

## 5.2. Sold State Irradiation

When polysaccharides are irradiated in the solid state the energy is absorbed directly by the polymer and radicals are formed. This type of interaction is called a direct effect. When a polymer is irradiated two reactions can take place chain scission and crosslinking, with one predominating. In the case of polysaccharides chain scission predominates. In polysaccharides the absorbed energy causes
the breakage of the glycosidic bond leading to a reduction of the molecular mass of the polymer [270]. After chain scission there are still free radicals present which can lead to ring opening of sugar units. These radicals can also lead to additional fragmentation of the polymer chain. Many studies have been conducted on the irradiation of natural polysaccharides and their derivatives in the sold state including cellulose and its derivatives [360,364–381], chitin and chitosan [382–388], starch [389,390], alginate [391,392], pectin [393,394], and dextran [395,396] to name a few. It should be noted that polysaccharides are hydroscopic and absorbed moisture can affect the radiation chemistry [397].

<figure><img src="image_38.png" /><figcaption>**Figure 28.** Chemical structure of the polysaccharide cellulose, note the glycosidic linkages between the glucose repeating units.</figcaption></figure>

<figure><img src="image_39.png" /><figcaption>**Figure 29.** Crystalline structure of cellulose, note the inter and intra hydrogen bonding (dashed lines denote hydrogen bonds).</figcaption></figure>

### 5.3. *Aqueous Irradiation*

In the case of a dilute aqueous solution of polysaccharides the energy will most likely be absorbed by the water producing the primary radiolysis products of hydrated electrons ($e_{aq}$), hydrogen atoms ($\cdot H$) and hydroxyl radicals ($\cdot OH$). These primary radiolysis products can produce secondary products including hydrogen peroxide, perhydroxyl radical, and super oxide [398]. The most reactive of the primary radiolysis products towards polysaccharides is the hydroxyl radical. The hydroxyl radical randomly extracts hydrogen atoms from carbon hydrogen bonds. In polysaccharides the extraction is not selective, with all carbon-bound hydrogens equally likely to be extracted. The rate constant for the reaction of hydrogen atom is about an order of magnitude lower than for $\cdot OH$ [398]. As with $\cdot OH$, $\cdot H$ extracts hydrogen randomly from carbon hydrogen bonds. The reaction rate of hydrated electrons towards is at least two orders of magnitude lower than for $\cdot OH$. This is due to the fact that while $e_{aq}$ react quickly with carbonyl groups they are slow to react with ethers and alcohol groups [399]. The extraction of hydrogens by $\cdot H$ and $\cdot OH$ produce radicals that lead to ring opening and chain scission [399]. There are many articles on the irradiation of polysaccharides in solution [400–407].
the breakage of the glycosidic bond leading to a reduction of the molecular mass of the polymer [270]. After chain scission there are still free radicals present which can lead to ring opening of sugar units. These radicals can also lead to additional fragmentation of the polymer chain. Many studies have been conducted on the irradiation of natural polysaccharides and their derivatives in the sold state including cellulose and its derivatives [360,364–381], chitin and chitosan [382–388], starch [389,390], alginate [391,392], pectin [393,394], and dextran [395,396] to name a few. It should be noted that polysaccharides are hydroscopic and absorbed moisture can affect the radiation chemistry [397].

<figure><img src="image_38.png" /><figcaption>**Figure 28.** Chemical structure of the polysaccharide cellulose, note the glycosidic linkages between the glucose repeating units.</figcaption></figure>

<figure><img src="image_39.png" /><figcaption>**Figure 29.** Crystalline structure of cellulose, note the inter and intra hydrogen bonding (dashed lines denote hydrogen bonds).</figcaption></figure>

### 5.3. Aqueous Irradiation

In the case of a dilute aqueous solution of polysaccharides the energy will most likely be absorbed by the water producing the primary radiolysis products of hydrated electrons ($e_{aq}$), hydrogen atoms ($\cdot H$) and hydroxyl radicals ($\cdot OH$). These primary radiolysis products can produce secondary products including hydrogen peroxide, perhydroxyl radical, and super oxide [398]. The most reactive of the primary radiolysis products towards polysaccharides is the hydroxyl radical. The hydroxyl radical randomly extracts hydrogen atoms from carbon hydrogen bonds. In polysaccharides the extraction is not selective, with all carbon-bound hydrogens equally likely to be extracted. The rate constant for the reaction of hydrogen atom is about an order of magnitude lower than for $\cdot OH$ [398]. As with $\cdot OH$, $\cdot H$ extracts hydrogen randomly from carbon hydrogen bonds. The reaction rate of hydrated electrons towards is at least two orders of magnitude lower than for $\cdot OH$. This is due to the fact that while $e_{aq}$ react quickly with carbonyl groups they are slow to react with ethers and alcohol groups [399]. The extraction of hydrogens by $\cdot H$ and $\cdot OH$ produce radicals that lead to ring opening and chain scission [399]. There are many articles on the irradiation of polysaccharides in solution [400–407].
5.4. *The Effect of Oxygen*

The above discussion assumed the irradiations were conducted in an oxygen-free environment.
In the presence of oxygen, carbon-centered radicals can react with oxygen to produce peroxyl radicals.
As with carbon-centered radicals, peroxyl radicals can lead to ring opening, chain scission and stable
products most often containing carbonyl groups [270]. Unlike many polymers, which tend to degrade
much more in the presence of oxygen, polysaccharides degrade both in the presence of oxygen
and without. In fact, some studies show that oxygen may reduce that amount of degradation in
polysaccharides [359].

5.5. *Radiation Crosslinking of Polysaccharides*

While polysaccharides most often degrade upon irradiation, crosslinking can be accomplished
under the right conditions. In concentrated solutions, cellulose derivatives have been shown to
crosslink. This crosslinking is dependent on degree and type of substitution and at higher dose
hydrogels can be formed [375-377,408,409]. Other polysaccharides have also been studied including
substituted chitin and chitosan, and gum arabic [382,410,411].

Polysaccharides can also be crosslinked in solution and in the solid state in the presence of an acetylene gas. Carboxymethyl cellulose, dextran, pullulan, gum arabic and others have been studied [270,412–415]. Crosslinking is dependent on the degree of chain branching and degree and type of substitution [270]. At higher doses hydrogels can be formed.

5.6. *Radical Lifetime in Polysaccharides*

Polysaccharide radicals in solution decay very quickly when irradiation is suspended. Thus, there is no post irradiation effect. When crystalline and semicrystalline polysaccharides are irradiated in the solid-state radicals can be trapped in the crystalline structure after irradiation has stopped. These trapped radicals can slowly migrate to the amorphous region of the polymer where they undergo reactions including ring opening and chain scission [416-421]. Trapped radical can remain in the crystalline region for months. When processing crystalline polysaccharides, or other polymers, in the solid-state, post irradiation effects must be understood.

5.7. *Radiation Chemistry of Lignin:*

Lignin is a group of amorphous polyphenolic polymers that are produced in vascular plants
and some algae. The structure of lignin is unknown, but it is believed that there are a number of
different lignin structures. While the true structure(s) of lignin is not known, a model structure is
illustrated in Figure 30 [422]. When lignin or model compounds are irradiated, phenoxyl and peroxyl
radicals are produced and the glass transition of lignin is increased [423–427]. While there appears to
be some reactions in lignin upon irradiation work by LaVerne et al. [360] has shown that the hydrogen
production G-value for lignin is an order of magnitude lower than cellulose.
### 5.4. The Effect of Oxygen

The above discussion assumed the irradiations were conducted in an oxygen-free environment. In the presence of oxygen, carbon-centered radicals can react with oxygen to produce peroxyl radicals. As with carbon-centered radicals, peroxyl radicals can lead to ring opening, chain scission and stable products most often containing carbonyl groups [270]. Unlike many polymers, which tend to degrade much more in the presence of oxygen, polysaccharides degrade both in the presence of oxygen and without. In fact, some studies show that oxygen may reduce that amount of degradation in polysaccharides [359].

### 5.5. Radiation Crosslinking of Polysaccharides

While polysaccharides most often degrade upon irradiation, crosslinking can be accomplished under the right conditions. In concentrated solutions, cellulose derivatives have been shown to crosslink. This crosslinking is dependent on degree and type of substitution and at higher dose hydrogels can be formed [375-377,408,409]. Other polysaccharides have also been studied including substituted chitin and chitosan, and gum arabic [382,410,411].

Polysaccharides can also be crosslinked in solution and in the solid state in the presence of an acetylene gas. Carboxymethyl cellulose, dextran, pullulan, gum arabic and others have been studied [270,412-415]. Crosslinking is dependent on the degree of chain branching and degree and type of substitution [270]. At higher doses hydrogels can be formed.

### 5.6. Radical Lifetime in Polysaccharides

Polysaccharide radicals in solution decay very quickly when irradiation is suspended. Thus, there is no post irradiation effect. When crystalline and semicrystalline polysaccharides are irradiated in the solid-state radicals can be trapped in the crystalline structure after irradiation has stopped. These trapped radicals can slowly migrate to the amorphous region of the polymer where they undergo reactions including ring opening and chain scission [416-421]. Trapped radical can remain in the crystalline region for months. When processing crystalline polysaccharides, or other polymers, in the solid-state, post irradiation effects must be understood.

### 5.7. Radiation Chemistry of Lignin:

Lignin is a group of amorphous polyphenolic polymers that are produced in vascular plants and some algae. The structure of lignin is unknown, but it is believed that there are a number of different lignin structures. While the true structure(s) of lignin is not known, a model structure is illustrated in Figure 30 [422]. When lignin or model compounds are irradiated, phenoxyl and peroxyl radicals are produced and the glass transition of lignin is increased [423-427]. While there appears to be some reactions in lignin upon irradiation work by LaVerne et al. [360] has shown that the hydrogen production G-value for lignin is an order of magnitude lower than cellulose.
<figure><img src="image_40.png" /><figcaption>**Figure 30.** Model structure of lignin [428].</figcaption></figure>

## 5.8. Radiation Chemistry of Natural Rubber

Natural rubber also known as latex, India rubber and Amazonian rubber is a polymer of cis-1,4-polyisoprene with a molecular mass range of 100,000 to 1,000,000 Daltons. The worldwide production of natural rubber in 2018 was about 13.9 million metric tons. For most commercial use the rubber needs to be vulcanized (crosslinked). The main reaction when natural rubber is irradiated is crosslinking. In 1933 a patent was issued for the radiation crosslinking of rubber [429]. One of the main advantages of radiation crosslinking is that it is conducted at room temperatures which prevents thermal degradation. Many studies have been published on the crosslinking of rubber and rubber blends [430–441]. Blending rubber with other polymers changes the physical and chemical properties of the final polymer. While rubber can be crosslinked without the use of accelerator the dose required is high thus an accelerator is used for economic purposes.

## 5.9. Radiation Chemistry of Peptides and Proteins

Proteins are the most abundant organic compounds in animals. They are macromolecules of amino acids linked together by a peptide bond between the C1 carbon of one amino acid and the N2 nitrogen of the other. Molecules of less than 50 amino acids are called peptides while greater than 50 are proteins. In the solid or frozen-in-solution irradiation of proteins, only direct effects occur. This leads to random bond breakage resulting in fragmentation, backbone and disulfide bond breakage, and loss of helicity [442–448]. In aqueous solutions the most abundant reactions are due to primary radiation products of water. The hydroxyl radical is believed to be the most reactive primary product towards proteins. The radical will extract hydrogens from any accessible part of the protein with the backbone peptide and thiol hydrogens the most susceptible. Side change amino acids can also be attached with aromatic side chains more reactive than aliphatic [442,449–453]. More recently methionine oxidation by HO, H, eaq and H₂O₂ has been studied [454,455]. The radiation effects of the primary radiation products of water are much greater than those of direct effects [442].

## 5.10. Radiation Chemistry of RNA and DNA

It has been very well known that ionizing radiation induces damage to biological systems. The mechanisms of radiation-induced damage are very complicated and vary tremendously from biological system to another. For example, the mechanisms involved from the radiation effects
<figure><img src="image_40.png" /><figcaption>**Figure 30.** Model structure of lignin [428].</figcaption></figure>

## 5.8. Radiation Chemistry of Natural Rubber

Natural rubber also known as latex, India rubber and Amazonian rubber is a polymer of cis-1,4-polyisoprene with a molecular mass range of 100,000 to 1,000,000 Daltons. The worldwide production of natural rubber in 2018 was about 13.9 million metric tons. For most commercial use the rubber needs to be vulcanized (crosslinked). The main reaction when natural rubber is irradiated is crosslinking. In 1933 a patent was issued for the radiation crosslinking of rubber [429]. One of the main advantages of radiation crosslinking is that it is conducted at room temperatures which prevents thermal degradation. Many studies have been published on the crosslinking of rubber and rubber blends [430–441]. Blending rubber with other polymers changes the physical and chemical properties of the final polymer. While rubber can be crosslinked without the use of accelerator the dose required is high thus an accelerator is used for economic purposes.

## 5.9. Radiation Chemistry of Peptides and Proteins

Proteins are the most abundant organic compounds in animals. They are macromolecules of amino acids linked together by a peptide bond between the C1 carbon of one amino acid and the N2 nitrogen of the other. Molecules of less than 50 amino acids are called peptides while greater than 50 are proteins. In the solid or frozen-in-solution irradiation of proteins, only direct effects occur. This leads to random bond breakage resulting in fragmentation, backbone and disulfide bond breakage, and loss of helicity [442–448]. In aqueous solutions the most abundant reactions are due to primary radiation products of water. The hydroxyl radical is believed to be the most reactive primary product towards proteins. The radical will extract hydrogens from any accessible part of the protein with the backbone peptide and thiol hydrogens the most susceptible. Side change amino acids can also be attached with aromatic side chains more reactive than aliphatic [442,449–453]. More recently methionine oxidation by HO, H, eaq and H₂O₂ has been studied [454,455]. The radiation effects of the primary radiation products of water are much greater than those of direct effects [442].

## 5.10. Radiation Chemistry of RNA and DNA

It has been very well known that ionizing radiation induces damage to biological systems. The mechanisms of radiation-induced damage are very complicated and vary tremendously from biological system to another. For example, the mechanisms involved from the radiation effects
on DNA and RNA, proteins, fatty acids, and carbohydrates are different. The mechanisms of the radiation-induced damages strongly depend on the type of irradiation (high LET such as protons and alpha particles versus low LET such as gamma rays, X-rays, and electrons), dose and dose-rate, the presence of the molecular oxygen, the presence of antioxidants, and the presence of water.

Since most biological organisms contain a large amount of water, the research on radiation effects on biological systems has been concentrated in the presence of H₂O. Under these conditions, most of the radiation effects are indirect, caused by the radiolysis of H₂O and the radiolytically produced •OH, eaq⁻, and H-atoms. These reactive species attack the DNA and RNA molecules at a diffusion controlled limit with reaction rate constants 10⁸–10⁹ L mol⁻¹s⁻¹. The radiolytically produced free radicals (DNA• and RNA•), DNA and RNA cations (DNA⁺ and RNA⁺), and DNA and RNA anions (DNA•⁻ and RNA•⁻) undergo various complicated mechanisms leading to deadly damages such as single and double strand breaks and base and phosphate release. These damages are enhanced tremendously by the presence of O₂. DNA• and RNA• radicals react with O₂ to produce the corresponding peroxyl radicals; DNAO₂• and RNAO₂•. These peroxyl radicals are very active and undergo various reactions, such as O₂•⁻-elimination, and formation or decay of tetraoxides (DNAOO-OODNA and RNAOO-OORNA) leading to more damages. Similarly, the results of attacking the protein molecules by •OH, eaq⁻, and H-atoms, from radiolysis of H₂O, lead to the amine-release and damages. As for the carbohydrates, the •OH radicals attack the glycosidic bonds leading to direct sessions on the backbone of the molecules. Finally, the fatty acids and liposomes are very prone to ionizing radiation. The •OH radicals abstract H atoms from the backbone of the fatty acids or liposomes producing C-centered free radicals. These free radicals react with O₂ to produce the corresponding peroxyl radicals (RO₂•). The RO₂• abstract H atoms from another fatty acid (or liposome) producing another free radical and initiating a chain reaction.

In the absence of H₂O, the ionizing radiation interacts directly and rapidly with the biological molecules producing intermediates such as excited molecules, radicals, cations, and anions. These intermediates have a short life in the microsecond timescale or even less. Again, depending on the presence and absence of oxygen, the presence of traces amounts of water, the dose, and the dose-rate, these intermediates undergo various reactions. Similar to the indirect interaction (in the presence of H₂O), radiolysis of DNA and RNA lead to single and double strand breaks, and base and phosphate release. Due to very strong hydrogen bonding, even very dry samples of DNA and RNA contain traces of water, which can contribute to indirect effects on the radiolysis of DNA and RNA. Although very little work has been published on the direct effects on proteins, fatty acids, and liposomes, most of the published results show that the direct ionizing radiation has the same effects as the indirect interactions. In both cases, the radiolytic products are amine-release, and oxidation in the proteins, and fatty acids and liposomes, respectively.

## 6. Summary

This survey emphasizes the evolution of research on radiation-initiated polymerization. Early investigations on radiation-initiated polymerization were mainly driven by the need for basic knowledge in this new field at the interface between polymer science and radiation chemistry, and by the perspective to develop processes competing with conventional methods for polymer production. Current activities are now oriented toward the study of radiation-induced processes of more complex chemical blends and/or in more complex media, using the recent developments for radiation sources and analytical methods. Besides the now well-established use of EB-curing in graphic arts, adhesives, and coatings, a variety of new applications are envisioned for advanced structural materials and for the production of key technological components for the healthcare and energy sectors. Promising prospects are under evaluation.

The use of radiation as an initiation source in controlled free radical graft copolymerization seems to open the door to designing new tunable surfaces in a controlled manner. The graft copolymers thus prepared may also lead to more complex structures via further chain growth or block
on DNA and RNA, proteins, fatty acids, and carbohydrates are different. The mechanisms of the radiation-induced damages strongly depend on the type of irradiation (high LET such as protons and alpha particles versus low LET such as gamma rays, X-rays, and electrons), dose and dose-rate, the presence of the molecular oxygen, the presence of antioxidants, and the presence of water.

Since most biological organisms contain a large amount of water, the research on radiation effects on biological systems has been concentrated in the presence of H₂O. Under these conditions, most of the radiation effects are indirect, caused by the radiolysis of H₂O and the radiolytically produced •OH, eaq⁻, and H-atoms. These reactive species attack the DNA and RNA molecules at a diffusion controlled limit with reaction rate constants 10⁸–10⁹ L mol⁻¹s⁻¹. The radiolytically produced free radicals (DNA• and RNA•), DNA and RNA cations (DNA⁺ and RNA⁺), and DNA and RNA anions (DNA•⁻ and RNA•⁻) undergo various complicated mechanisms leading to deadly damages such as single and double strand breaks and base and phosphate release. These damages are enhanced tremendously by the presence of O₂. DNA• and RNA• radicals react with O₂ to produce the corresponding peroxyl radicals; DNAO₂• and RNAO₂•. These peroxyl radicals are very active and undergo various reactions, such as O₂•⁻-elimination, and formation or decay of tetraoxides (DNAOO-OODNA and RNAOO-OORNA) leading to more damages. Similarly, the results of attacking the protein molecules by •OH, eaq⁻, and H-atoms, from radiolysis of H₂O, lead to the amine-release and damages. As for the carbohydrates, the •OH radicals attack the glycosidic bonds leading to direct sessions on the backbone of the molecules. Finally, the fatty acids and liposomes are very prone to ionizing radiation. The •OH radicals abstract H atoms from the backbone of the fatty acids or liposomes producing C-centered free radicals. These free radicals react with O₂ to produce the corresponding peroxyl radicals (RO₂•). The RO₂• abstract H atoms from another fatty acid (or liposome) producing another free radical and initiating a chain reaction.

In the absence of H₂O, the ionizing radiation interacts directly and rapidly with the biological molecules producing intermediates such as excited molecules, radicals, cations, and anions. These intermediates have a short life in the microsecond timescale or even less. Again, depending on the presence and absence of oxygen, the presence of traces amounts of water, the dose, and the dose-rate, these intermediates undergo various reactions. Similar to the indirect interaction (in the presence of H₂O), radiolysis of DNA and RNA lead to single and double strand breaks, and base and phosphate release. Due to very strong hydrogen bonding, even very dry samples of DNA and RNA contain traces of water, which can contribute to indirect effects on the radiolysis of DNA and RNA. Although very little work has been published on the direct effects on proteins, fatty acids, and liposomes, most of the published results show that the direct ionizing radiation has the same effects as the indirect interactions. In both cases, the radiolytic products are amine-release, and oxidation in the proteins, and fatty acids and liposomes, respectively.

## 6. Summary

This survey emphasizes the evolution of research on radiation-initiated polymerization. Early investigations on radiation-initiated polymerization were mainly driven by the need for basic knowledge in this new field at the interface between polymer science and radiation chemistry, and by the perspective to develop processes competing with conventional methods for polymer production. Current activities are now oriented toward the study of radiation-induced processes of more complex chemical blends and/or in more complex media, using the recent developments for radiation sources and analytical methods. Besides the now well-established use of EB-curing in graphic arts, adhesives, and coatings, a variety of new applications are envisioned for advanced structural materials and for the production of key technological components for the healthcare and energy sectors. Promising prospects are under evaluation.

The use of radiation as an initiation source in controlled free radical graft copolymerization seems to open the door to designing new tunable surfaces in a controlled manner. The graft copolymers thus prepared may also lead to more complex structures via further chain growth or block
extension by subsequent addition of monomer due to postpolymerization activity of their chain ends. Radiation-induced RAFT-mediated graft polymerization seems to be a very powerful technique for the achievement of tailored polymeric surfaces with well-defined properties.

Nanogels have been studied as potential carriers for the delivery of genes, proteins, and drugs; use in tissue engineering; as biosensors; and other materials. Since most of these applications are biomedical applications being able to synthesis nanogel without the need for toxic chemicals is important. The use of ionizing radiation for the crosslinking of the nanogels reduces or eliminates the need for toxic chemicals.

With more and more interest in the use of natural materials and green chemistry understanding how to control the effects of ionizing radiation on natural polymers is important.

**Funding:** This work has been funded in part by the National Science Centre, Poland (grant numbers UMO-2017/25/B/ST4/01110 and UMO-2019/33/B/ST5/02125) and by the International Atomic Energy Agency (grant number CRP F22070).

**Acknowledgments:** Clelia Dispenza acknowledges the FFR_D09-Fondo di Finanziamento per la Ricerca di Ateneo 2018/2021. M.-Cl. Clochard would like to thank Didier Lairez for SANS experiments done at LLB Saclay (France) and also Uliana Pinaeva for her contribution in some bibliographic research in the Section 3.

**Conflicts of Interest:** The authors declare no conflict of interest.

## References and Note

1. Charlesby, A. *Atomic Radiation and Polymers*; Pergamon Press: Oxford, UK, 1960.

2. Chapiro, A. *Radiation Chemistry of Polymeric Systems*; Interscience: New York, NY, USA, 1962.

3. Dole, M. *The Radiation Chemistry of Macromolecules*; Academic Press: New York, NY, USA, 1972.

4. Schnabel, W. *Polymer Degradation. Principles and Practical Applications*; Hanser: Muenchen, Germany, 1981.

5. Rosiak, J.M. Radiation Effects of Polymers. In *ACS Symposium Series*; ACS Symposium, Series; Clough, R.L., Shalaby, S.W., Eds.; American Chemical Society: Washington, DC, USA, 1991; Volume 475.

6. Singh, A.; Silverman, J. (Eds.) *Radiation Processing of Polymers*; Hanser: Muenchen, Germany, 1992.

7. Ivanov, V.S. *Radiation Chemistry of Polymers*; VSP: Utrecht, The Netherlands, 1992.

8. Coqueret, X. Obtaining High-Performance Polymeric materials by Radiation. In *Radiation Chemistry: From Basics to Applications in Material and Life Sciences*; Spotheim-Maurizot, M., Mostafavi, M., Douki, T., Belloni, J., Eds.; EDP Sciences: Les Ulis, France, 2008; pp. 131–150.

9. Drobny, J.G. *Ionizing Radiation and Polymers: Principles, Technology, and Applications*; William Andrew: Norwich, NY, USA, 2013.

10. Coqueret, X.; Sabharwal, S.; Khairul Zaman, H.M.D.; Czechowska-Biskup, R.; Wach, R.A.; Rosiak, J.M.; Ulanski, P.; Gulrez, S.K.H.; Al-Assaf, S. Introdcution to the radiation chemistry of polymers. In *The Radiation Chemistry of Polysaccharides*; Al-Assaf, S., Coqueret, X., Khairul Zaman, H.M.D., Sen, M., Ulanski, P., Eds.; International Atomic Energy Agency: Vienna, Austria, 2016.

11. Odian, G. *Principles of Polymerization*, 4th ed.; John Wiley & Sons: Hoboken, NJ, USA, 2004.

12. Ferry, M.N.; Ngono-Ravache, Y.; Aymes-Chodur, C.; Clochard, M.C.; Coqueret, X.; Cortella, L.; Pellizzi, E.; Rouif, S.; Esnouf, S. Ionizing radiation effects in polymers. In *Reference Module in Materials Science and Materials Engineering*; Hashmi, S., Ed.; Elsevier: Oxford, UK, 2016; pp. 131–149.

13. Wojnarovits, L. Radiation chemistry. In *Handbook of Nuclear Chemistry*; Vértes, A.N.S., Klencsár, Z., Lovas, R.G., Rösch, F., Eds.; Springer: Boston, MA, USA, 2011; pp. 1263–1331.

14. Chapiro, A. Radiation-Induced Polymerization. *Radiat. Phys. Chem* **1979**, *14*, 101–116. [CrossRef]

15. Hopwood, F.L.; Phillips, J.T. Polymerization of liquids by irradiation with neutrons and other rays. *Nature* **1939**, *143*, 640. [CrossRef]

16. Chapiro, A. Polymerisation par les rayons-gamma.2. *C.R. Acad. Sci. Chim.* **1949**, *229*, 827–829.

17. Zivic, N.; Kuroishi, P.K.; Dumur, F.; Gigmes, D.; Dove, A.P.; Sardon, H. Recent advances and challenges in the design of organic photoacid and photobase generators for polymerizations. *Angew.Chem. Int. Ed.* **2019**, *58*, 10410–10422. [CrossRef]

18. Zandi Shafagh, R.; Vastesson, A.; Guo, W.; van der Wijngaart, W.; Haraldsson, T. E-beam nanostructuring and direct click biofunctionalization of thiol-ene resist. *ACS Nano* **2018**, *12*, 9940–9946. [CrossRef]
extension by subsequent addition of monomer due to postpolymerization activity of their chain ends. Radiation-induced RAFT-mediated graft polymerization seems to be a very powerful technique for the achievement of tailored polymeric surfaces with well-defined properties.

Nanogels have been studied as potential carriers for the delivery of genes, proteins, and drugs; use in tissue engineering; as biosensors; and other materials. Since most of these applications are biomedical applications being able to synthesis nanogel without the need for toxic chemicals is important. The use of ionizing radiation for the crosslinking of the nanogels reduces or eliminates the need for toxic chemicals.

With more and more interest in the use of natural materials and green chemistry understanding how to control the effects of ionizing radiation on natural polymers is important.

**Funding:** This work has been funded in part by the National Science Centre, Poland (grant numbers UMO-2017/25/B/ST4/01110 and UMO-2019/33/B/ST5/02125) and by the International Atomic Energy Agency (grant number CRP F22070).

**Acknowledgments:** Clelia Dispenza acknowledges the FFR_D09-Fondo di Finanziamento per la Ricerca di Ateneo 2018/2021. M.-Cl. Clochard would like to thank Didier Lairez for SANS experiments done at LLB Saclay (France) and also Uliana Pinaeva for her contribution in some bibliographic research in the Section 3.

**Conflicts of Interest:** The authors declare no conflict of interest.

## References and Note

1. Charlesby, A. *Atomic Radiation and Polymers*; Pergamon Press: Oxford, UK, 1960.

2. Chapiro, A. *Radiation Chemistry of Polymeric Systems*; Interscience: New York, NY, USA, 1962.

3. Dole, M. *The Radiation Chemistry of Macromolecules*; Academic Press: New York, NY, USA, 1972.

4. Schnabel, W. *Polymer Degradation. Principles and Practical Applications*; Hanser: Muenchen, Germany, 1981.

5. Rosiak, J.M. Radiation Effects of Polymers. In *ACS Symposium Series*; ACS Symposium, Series; Clough, R.L., Shalaby, S.W., Eds.; American Chemical Society: Washington, DC, USA, 1991; Volume 475.

6. Singh, A.; Silverman, J. (Eds.) *Radiation Processing of Polymers*; Hanser: Muenchen, Germany, 1992.

7. Ivanov, V.S. *Radiation Chemistry of Polymers*; VSP: Utrecht, The Netherlands, 1992.

8. Coqueret, X. Obtaining High-Performance Polymeric materials by Radiation. In *Radiation Chemistry: From Basics to Applications in Material and Life Sciences*; Spotheim-Maurizot, M., Mostafavi, M., Douki, T., Belloni, J., Eds.; EDP Sciences: Les Ulis, France, 2008; pp. 131–150.

9. Drobny, J.G. *Ionizing Radiation and Polymers: Principles, Technology, and Applications*; William Andrew: Norwich, NY, USA, 2013.

10. Coqueret, X.; Sabharwal, S.; Khairul Zaman, H.M.D.; Czechowska-Biskup, R.; Wach, R.A.; Rosiak, J.M.; Ulanski, P.; Gulrez, S.K.H.; Al-Assaf, S. Introdcution to the radiation chemistry of polymers. In *The Radiation Chemistry of Polysaccharides*; Al-Assaf, S., Coqueret, X., Khairul Zaman, H.M.D., Sen, M., Ulanski, P., Eds.; International Atomic Energy Agency: Vienna, Austria, 2016.

11. Odian, G. *Principles of Polymerization*, 4th ed.; John Wiley & Sons: Hoboken, NJ, USA, 2004.

12. Ferry, M.N.; Ngono-Ravache, Y.; Aymes-Chodur, C.; Clochard, M.C.; Coqueret, X.; Cortella, L.; Pellizzi, E.; Rouif, S.; Esnouf, S. Ionizing radiation effects in polymers. In *Reference Module in Materials Science and Materials Engineering*; Hashmi, S., Ed.; Elsevier: Oxford, UK, 2016; pp. 131–149.

13. Wojnarovits, L. Radiation chemistry. In *Handbook of Nuclear Chemistry*; Vértes, A.N.S., Klencsár, Z., Lovas, R.G., Rösch, F., Eds.; Springer: Boston, MA, USA, 2011; pp. 1263–1331.

14. Chapiro, A. Radiation-Induced Polymerization. *Radiat. Phys. Chem* **1979**, *14*, 101–116. [CrossRef]

15. Hopwood, F.L.; Phillips, J.T. Polymerization of liquids by irradiation with neutrons and other rays. *Nature* **1939**, *143*, 640. [CrossRef]

16. Chapiro, A. Polymerisation par les rayons-gamma.2. *C.R. Acad. Sci. Chim.* **1949**, *229*, 827–829.

17. Zivic, N.; Kuroishi, P.K.; Dumur, F.; Gigmes, D.; Dove, A.P.; Sardon, H. Recent advances and challenges in the design of organic photoacid and photobase generators for polymerizations. *Angew.Chem. Int. Ed.* **2019**, *58*, 10410–10422. [CrossRef]

18. Zandi Shafagh, R.; Vastesson, A.; Guo, W.; van der Wijngaart, W.; Haraldsson, T. E-beam nanostructuring and direct click biofunctionalization of thiol-ene resist. *ACS Nano* **2018**, *12*, 9940–9946. [CrossRef]
19. Sarapas, J.M.; Tew, G.N. Thiol-ene step-growth as a versatile route to functional polymers. *Angew. Chem. Int. Ed. Engl.* **2016**, *55*, 15860–15863. [CrossRef]

20. Kinoshita, S. Basics and applications of uv/eb curing technology. *J. Photopolym. Sci. Technol.* **2006**, *19*, 93–98. [CrossRef]

21. Wojnarovits, L.; TakACS, E.; Dobo, J.; Foldiak, G. Pulse-radiolysis studies on the polymerization of 1,6-hexanediol diacrylate in cyclohexane solvent. *Radiat. Phys. Chem.* **1992**, *39*, 59–63. [CrossRef]

22. Silverman, J.; Weiss, D.E. The role of pulse frequency and acrylic acid in the radiation-induced bulk polymerization of 2-ethylhexyl acrylate. *Radiat. Phys. Chem.* **2003**, *67*, 347–352. [CrossRef]

23. Defoort, B.; Larnac, G.; Coqueret, X. Electron-beam initiated polymerization of acrylate compositions 4: Effects of pulsed irradiation parameters on curing kinetics. *Radiat. Phys. Chem.* **2001**, *62*, 47–53. [CrossRef]

24. Feng, H.X.; Al-Sheikhly, M.; Silverman, J.; Weiss, D.E.; Neta, P. Polymerization of neat 2-ethylhexyl acrylate induced by a pulsed electron beam. *J. Polym. Sci. Part A Polym. Chem.* **2003**, *41*, 196–203. [CrossRef]

25. Berger, K. Energy curing: Advancing the graphic arts through innovation and performance. *RadTech Rep.* **2001**, *15*, 44–46.

26. Sakaguchi, S.; Sakurai, T.; Ma, J.; Sugimoto, M.; Yamaki, T.; Chiba, A.; Saito, Y.; Seki, S. Conjugated nanowire sensors via high-energy single-particle-induced linear polymerization of 9,9'-spirobi[9h-fluorene] derivatives. *J. Phys. Chem. B* **2018**, *122*, 8614–8623. [CrossRef] [PubMed]

27. Sakaguchi, S.; Kamiya, K.; Sakurai, T.; Seki, S. Interactions of single particle with organic matters: A facile bottom-up approach to low dimensional nanostructures. *Quantum Beam Sci.* **2020**, *4*, 7. [CrossRef]

28. Beziers, D.; Perilleux, P.; Grenie, Y. Composite structures obtained by ionization curing. *Radiat. Phys. Chem.* **1996**, *48*, 171–177. [CrossRef]

29. Vinje, J.; Beckwith, K.S.; Sikorski, P. Electron beam lithography fabrication of su-8 polymer structures for cell studies. *J. Microelectromech. Syst.* **2020**, *29*, 160–169. [CrossRef]

30. Furtak-Wrona, K.; Kozik-Ostrowka, P.; Jadwiszczak, K.; Maigret, J.E.; Aguie-Beghin, V.; Coqueret, X. Polyurethane acrylate networks including cellulose nanocrystals: A comparison between uv and eb- curing. *Radiat. Phys. Chem.* **2018**, *142*, 94–99. [CrossRef]

31. Herman, J.A.; Roberge, P. X-ray induced polymerization of vinyl iodide in solution. *J. Polym. Sci.* **1962**, *62*, S116–S118. [CrossRef]

32. Herman, J.A.; Roberge, P.C. Radiolysis of vinyl iodide.3. Formation of polymer in carbon tetrachloride solutions. *Trans. Faraday Soc.* **1969**, *65*, 1315–1324. [CrossRef]

33. Roberge, P.C.; Herman, J.A. Radiolysis of vinyl iodide.2. Solutions of vinyl iodide in carbon tetrachloride. *Trans. Faraday Soc.* **1969**, *65*, 1303–1314. [CrossRef]

34. Anderson, W.S. Radiation-induced cationic polymerization of butadiene. *J. Phys. Chem.* **1959**, *63*, 765–766. [CrossRef]

35. Verdurmen, E.M.; German, A.L.; Sudol, E.D.; Gilbert, R.G. Particle growth in butadiene emulsion polymerization. 2. Gamma-radiolysis. *Macromol. Chem. Phys.* **1994**, *195*, 635–640. [CrossRef]

36. Zhang, J.; Zhang, Z.P.; Ying, S.K. Study of gamma-ray radiation-induced polymerization of butadiene in ethanol. *Int. J. Radiat. Appl. Instrum. Part C Radiat. Phys. Chem.* **1991**, *37*, 263–266.

37. Chatani, Y.; Nakatani, S.; Tadokoro, H. Structural evidence of radiation-induced thiourea canal polymerization of 2,3-disubstituted-1,3-butadienes. *Acta Cryst.* **A** **1972**, *28*, S123. [CrossRef]

38. Miyata, M.; Takemoto, K. Radiation-induced polymerization of 2,3-disubstituted butadienes in deoxycholic-acid inclusion compounds. *J. Polym. Sci. Part C Polym. Symp.* **1975**, *13*, 221–223. [CrossRef]

39. Cataldo, F.; Ragni, P.; Ursini, O.; Angelini, G. Synthesis of highly crystalline poly(dimethylbutadiene) (PDMB) by radiation-induced inclusion polymerization: A comparison with pdmbs synthesized by bulk and emulsion polymerization. *Radiat. Phys. Chem.* **2008**, *77*, 941–948. [CrossRef]

40. Cataldo, F.; Ursini, O.; Ragni, P.; Rosati, A. Radiation-induced polymerization of 2,3-dimethyl-1,3-butadiene clathrate in deoxycholic acid. *J. Radioanal. Nucl. Chem.* **2009**, *280*, 99–106. [CrossRef]

41. Yoshii, F.; Abe, T.; Kobayashi, Y. Radiation-induced in-source polymerization of acrylonitrile in urea canal complex—Radiation-induced polymerization of vinyl monomer in urea canal complex.4. *Kobunshi Ronbunshu* **1975**, *32*, 477–483.

42. Zou, J.-T.; Wang, Y.-S.; Pang, W.-M.; Shi, L.; Lu, F. Radiation-induced inclusion polymerization of acrylonitrile in urea canals: Toward synthesis of completely isotactic polyacrylonitrile with controlled molecular weight. *Macromolecules* **2013**, *46*, 1765–1771. [CrossRef]
19. Sarapas, J.M.; Tew, G.N. Thiol-ene step-growth as a versatile route to functional polymers. *Angew. Chem. Int. Ed. Engl.* **2016**, *55*, 15860–15863. [CrossRef]

20. Kinoshita, S. Basics and applications of uv/eb curing technology. *J. Photopolym. Sci. Technol.* **2006**, *19*, 93–98. [CrossRef]

21. Wojnarovits, L.; TakACS, E.; Dobo, J.; Foldiak, G. Pulse-radiolysis studies on the polymerization of 1,6-hexanediol diacrylate in cyclohexane solvent. *Radiat. Phys. Chem.* **1992**, *39*, 59–63. [CrossRef]

22. Silverman, J.; Weiss, D.E. The role of pulse frequency and acrylic acid in the radiation-induced bulk polymerization of 2-ethylhexyl acrylate. *Radiat. Phys. Chem.* **2003**, *67*, 347–352. [CrossRef]

23. Defoort, B.; Larnac, G.; Coqueret, X. Electron-beam initiated polymerization of acrylate compositions 4: Effects of pulsed irradiation parameters on curing kinetics. *Radiat. Phys. Chem.* **2001**, *62*, 47–53. [CrossRef]

24. Feng, H.X.; Al-Sheikhly, M.; Silverman, J.; Weiss, D.E.; Neta, P. Polymerization of neat 2-ethylhexyl acrylate induced by a pulsed electron beam. *J. Polym. Sci. Part A Polym. Chem.* **2003**, *41*, 196–203. [CrossRef]

25. Berger, K. Energy curing: Advancing the graphic arts through innovation and performance. *RadTech Rep.* **2001**, *15*, 44–46.

26. Sakaguchi, S.; Sakurai, T.; Ma, J.; Sugimoto, M.; Yamaki, T.; Chiba, A.; Saito, Y.; Seki, S. Conjugated nanowire sensors via high-energy single-particle-induced linear polymerization of 9,9'-spirobi[9h-fluorene] derivatives. *J. Phys. Chem. B* **2018**, *122*, 8614–8623. [CrossRef] [PubMed]

27. Sakaguchi, S.; Kamiya, K.; Sakurai, T.; Seki, S. Interactions of single particle with organic matters: A facile bottom-up approach to low dimensional nanostructures. *Quantum Beam Sci.* **2020**, *4*, 7. [CrossRef]

28. Beziers, D.; Perilleux, P.; Grenie, Y. Composite structures obtained by ionization curing. *Radiat. Phys. Chem.* **1996**, *48*, 171–177. [CrossRef]

29. Vinje, J.; Beckwith, K.S.; Sikorski, P. Electron beam lithography fabrication of su-8 polymer structures for cell studies. *J. Microelectromech. Syst.* **2020**, *29*, 160–169. [CrossRef]

30. Furtak-Wrona, K.; Kozik-Ostrowka, P.; Jadwiszczak, K.; Maigret, J.E.; Aguie-Beghin, V.; Coqueret, X. Polyurethane acrylate networks including cellulose nanocrystals: A comparison between uv and eb- curing. *Radiat. Phys. Chem.* **2018**, *142*, 94–99. [CrossRef]

31. Herman, J.A.; Roberge, P. X-ray induced polymerization of vinyl iodide in solution. *J. Polym. Sci.* **1962**, *62*, S116–S118. [CrossRef]

32. Herman, J.A.; Roberge, P.C. Radiolysis of vinyl iodide.3. Formation of polymer in carbon tetrachloride solutions. *Trans. Faraday Soc.* **1969**, *65*, 1315–1324. [CrossRef]

33. Roberge, P.C.; Herman, J.A. Radiolysis of vinyl iodide.2. Solutions of vinyl iodide in carbon tetrachloride. *Trans. Faraday Soc.* **1969**, *65*, 1303–1314. [CrossRef]

34. Anderson, W.S. Radiation-induced cationic polymerization of butadiene. *J. Phys. Chem.* **1959**, *63*, 765–766. [CrossRef]

35. Verdurmen, E.M.; German, A.L.; Sudol, E.D.; Gilbert, R.G. Particle growth in butadiene emulsion polymerization. 2. Gamma-radiolysis. *Macromol. Chem. Phys.* **1994**, *195*, 635–640. [CrossRef]

36. Zhang, J.; Zhang, Z.P.; Ying, S.K. Study of gamma-ray radiation-induced polymerization of butadiene in ethanol. *Int. J. Radiat. Appl. Instrum. Part C Radiat. Phys. Chem.* **1991**, *37*, 263–266.

37. Chatani, Y.; Nakatani, S.; Tadokoro, H. Structural evidence of radiation-induced thiourea canal polymerization of 2,3-disubstituted-1,3-butadienes. *Acta Cryst.* **A** **1972**, *28*, S123. [CrossRef]

38. Miyata, M.; Takemoto, K. Radiation-induced polymerization of 2,3-disubstituted butadienes in deoxycholic-acid inclusion compounds. *J. Polym. Sci. Part C Polym. Symp.* **1975**, *13*, 221–223. [CrossRef]

39. Cataldo, F.; Ragni, P.; Ursini, O.; Angelini, G. Synthesis of highly crystalline poly(dimethylbutadiene) (PDMB) by radiation-induced inclusion polymerization: A comparison with pdmbs synthesized by bulk and emulsion polymerization. *Radiat. Phys. Chem.* **2008**, *77*, 941–948. [CrossRef]

40. Cataldo, F.; Ursini, O.; Ragni, P.; Rosati, A. Radiation-induced polymerization of 2,3-dimethyl-1,3-butadiene clathrate in deoxycholic acid. *J. Radioanal. Nucl. Chem.* **2009**, *280*, 99–106. [CrossRef]

41. Yoshii, F.; Abe, T.; Kobayashi, Y. Radiation-induced in-source polymerization of acrylonitrile in urea canal complex—Radiation-induced polymerization of vinyl monomer in urea canal complex.4. *Kobunshi Ronbunshu* **1975**, *32*, 477–483.

42. Zou, J.-T.; Wang, Y.-S.; Pang, W.-M.; Shi, L.; Lu, F. Radiation-induced inclusion polymerization of acrylonitrile in urea canals: Toward synthesis of completely isotactic polyacrylonitrile with controlled molecular weight. *Macromolecules* **2013**, *46*, 1765–1771. [CrossRef]
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44. Charlesby, A. Solid-state polymerization induced by radiation. *Rep. Prog. Phys.* **1965**, *28*, 463–518. [CrossRef]

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46. Mah, S.; Yamamoto, Y.; Hayashi, K. Effects of diphenyliodonium hexafluorophosphate in the radiation-induced cationic polymerization of styrene—Formation and decay processes of styrene dimer radical cation. *J. Polym. Sci. Part A Polym. Chem.* **1982**, *20*, 2151–2158. [CrossRef]

47. Yamamoto, Y.; Teramoto, M.; Ma, X.H.; Hayashi, K. Radiation-induced cationic polymerization by aryl onium salts. *Abstr. Pap. Am. Chem. Soc.* **1989**, *198*, 43.

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49. Colombo, P.; Chapman, R.N.; Steinberg, M.; Fontana, J.; Kukacka, L.E. High-pressure ethylene polymerization reactions with cobalt-60 gamma radiation. *Trans. Am. Nuc. Soc.* **1964**, *7*, 313–3214.

50. Wiley, R.H.; Parrish, C.F.; Lipscomb, N.T.; Guillet, J.E. Kinetics of gamma-radiation-induced polymerization of ethylene in alkyl chlorides. *J. Polym. Sci. Part A Polym. Chem.* **1964**, *2*, 2503–2511. [CrossRef]

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52. Takehisa, M.; Machi, S.; Watanabe, H.; Ueno, T.; Takahashi, S.; Tsuchiya, R.; Otaguro, K.; Motoda, I.; Takasaka, Y.; Miyanaga, K.; et al. Radiation-induced polymerization of ethylene in a pilot-plant.1. Bulk process. *J. Appl. Polym. Sci.* **1979**, *24*, 853–864. [CrossRef]

53. Takehisa, M.; Watanabe, H.; Kurihara, H.; Takasaka, Y.; Maruyama, Y.; Miyanaga, K.; Suwa, T.; Nakajima, H.; Yamaguchi, K.; Tohei, M.; et al. Radiation-induced polymerization of ethylene in a pilot-plant. 2. Development of wet-wall process. *J. Appl. Polym. Sci.* **1979**, *24*, 865–882. [CrossRef]

54. Takehisa, M.; Watanabe, H.; Kurihara, H.; Yamaguchi, K.; Nakajima, H.; Yagi, T.; Watanabe, T.; Sugo, T.; Suwa, T.; Maruyama, S.; et al. Radiation-induced polymerization of ethylene in pilot-plant. 3. Heavy-phase recycling process. *J. Appl. Polym. Sci.* **1979**, *24*, 1831–1844. [CrossRef]

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58. Wiley, R.H.; Lipscomb, M.T. Gamma-radiation-induced polymerization of styrene. *J. Polym. Sci.* **1960**, *45*, 271–274. [CrossRef]

59. Ueno, K.; Hayashi, K.; Okamura, S. Radiation induced polymerization of styrene in a dry system. *J. Polym. Sci. Part B Polym. Lett.* **1965**, *3*, 363–368. [CrossRef]

60. Ueno, K.; Williams, F.; Hayashi, K.; Okamura, S. Radiation-induced polymerization by free ions.1. Polymerization of styrene under anhydrous conditions. *Trans. Faraday Soc.* **1967**, *63*, 1478–1488. [CrossRef]

61. Stannett, V.; Meyer, J.A.; Szwarc, M.; Bahstetter, F.C. Radiation polymerization of isobutylene and styrene. *Appl. Radiat. Isot.* **1964**, *15*, 747–757. [CrossRef]

62. Gotoh, T.; Yamamoto, M.; Nishijima, Y. Radiation-induced cationic dimerization and polymerization of styrene in methylene-chloride solution.1. *J. Polym. Sci. Part A Polym. Chem.* **1981**, *19*, 1047–1060. [CrossRef]

63. Chapiro, A. Cationic polymerizations initiated by high-energy radiation. *Makromol. Chem.* **1974**, *175*, 1181–1197. [CrossRef]

64. Chen, B.C.; Stamm, R.F. Polymerization induced by ionizing radiation at low temperatures. I. Evidence for the simultaneous existence of ionic and free-radical mechanisms in the polymerization of styrene and 2,4-dimethylstyrene. *J. Polym. Sci.* **1962**, *58*, 369–388. [CrossRef]

65. Guarise, G.B.; Palma, G.; Siviero, E.; Talamini, G. Polymerization induced by gamma radiation of styrene under pressure. *Polymer* **1970**, *11*, 613–625. [CrossRef]
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48. Henley, E.J.; Ng, C.C. Gamma-radiation-induced solution polymerization of ethylene. *J. Polym. Sci.* **1959**, *36*, 511–517. [CrossRef]

49. Colombo, P.; Chapman, R.N.; Steinberg, M.; Fontana, J.; Kukacka, L.E. High-pressure ethylene polymerization reactions with cobalt-60 gamma radiation. *Trans. Am. Nuc. Soc.* **1964**, *7*, 313–3214.

50. Wiley, R.H.; Parrish, C.F.; Lipscomb, N.T.; Guillet, J.E. Kinetics of gamma-radiation-induced polymerization of ethylene in alkyl chlorides. *J. Polym. Sci. Part A Polym. Chem.* **1964**, *2*, 2503–2511. [CrossRef]

51. Hagiwara, M.; Mitsui, H.; Machi, S.; Kagiya, T. Liquid carbon dioxide as a solvent for radiation polymerization of ethylene. *J. Polym. Sci. Part A Polym. Chem.* **1968**, *6*, 603–608. [CrossRef]

52. Takehisa, M.; Machi, S.; Watanabe, H.; Ueno, T.; Takahashi, S.; Tsuchiya, R.; Otaguro, K.; Motoda, I.; Takasaka, Y.; Miyanaga, K.; et al. Radiation-induced polymerization of ethylene in a pilot-plant.1. Bulk process. *J. Appl. Polym. Sci.* **1979**, *24*, 853–864. [CrossRef]

53. Takehisa, M.; Watanabe, H.; Kurihara, H.; Takasaka, Y.; Maruyama, Y.; Miyanaga, K.; Suwa, T.; Nakajima, H.; Yamaguchi, K.; Tohei, M.; et al. Radiation-induced polymerization of ethylene in a pilot-plant. 2. Development of wet-wall process. *J. Appl. Polym. Sci.* **1979**, *24*, 865–882. [CrossRef]

54. Takehisa, M.; Watanabe, H.; Kurihara, H.; Yamaguchi, K.; Nakajima, H.; Yagi, T.; Watanabe, T.; Sugo, T.; Suwa, T.; Maruyama, S.; et al. Radiation-induced polymerization of ethylene in pilot-plant. 3. Heavy-phase recycling process. *J. Appl. Polym. Sci.* **1979**, *24*, 1831–1844. [CrossRef]

55. Watanabe, H.; Machi, S.; Kurihara, H.; Wada, T.; Yamaguchi, K.; Watanabe, T.; Takehisa, M. Radiation-induced polymerization of ethylene in pilot-plant. 4. Kinetic-analysis. *J. Appl. Polym. Sci.* **1980**, *25*, 277–285. [CrossRef]

56. Yamaguchi, K.; Watanabe, H.; Sugo, T.; Watanabe, T.; Takehisa, M.; Machi, S. Radiation-induced polymerization of ethylene in pilot-plant. 5. Molecular-weight distribution of polyethylene. *J. Appl. Polym. Sci.* **1980**, *25*, 1633–1638. [CrossRef]

57. Machi, S.; Tamura, T.; Hagiwara, M.; Gotoda, M.; Kagiya, T. Short-chain branching in gamma-radiation-induced polymerization of ethylene. *J. Polym. Sci. Part A Polym. Chem.* **1966**, *4*, 283–291. [CrossRef]

58. Wiley, R.H.; Lipscomb, M.T. Gamma-radiation-induced polymerization of styrene. *J. Polym. Sci.* **1960**, *45*, 271–274. [CrossRef]

59. Ueno, K.; Hayashi, K.; Okamura, S. Radiation induced polymerization of styrene in a dry system. *J. Polym. Sci. Part B Polym. Lett.* **1965**, *3*, 363–368. [CrossRef]

60. Ueno, K.; Williams, F.; Hayashi, K.; Okamura, S. Radiation-induced polymerization by free ions.1. Polymerization of styrene under anhydrous conditions. *Trans. Faraday Soc.* **1967**, *63*, 1478–1488. [CrossRef]

61. Stannett, V.; Meyer, J.A.; Szwarc, M.; Bahstetter, F.C. Radiation polymerization of isobutylene and styrene. *Appl. Radiat. Isot.* **1964**, *15*, 747–757. [CrossRef]

62. Gotoh, T.; Yamamoto, M.; Nishijima, Y. Radiation-induced cationic dimerization and polymerization of styrene in methylene-chloride solution.1. *J. Polym. Sci. Part A Polym. Chem.* **1981**, *19*, 1047–1060. [CrossRef]

63. Chapiro, A. Cationic polymerizations initiated by high-energy radiation. *Makromol. Chem.* **1974**, *175*, 1181–1197. [CrossRef]

64. Chen, B.C.; Stamm, R.F. Polymerization induced by ionizing radiation at low temperatures. I. Evidence for the simultaneous existence of ionic and free-radical mechanisms in the polymerization of styrene and 2,4-dimethylstyrene. *J. Polym. Sci.* **1962**, *58*, 369–388. [CrossRef]

65. Guarise, G.B.; Palma, G.; Siviero, E.; Talamini, G. Polymerization induced by gamma radiation of styrene under pressure. *Polymer* **1970**, *11*, 613–625. [CrossRef]
66. Tabata, Y.; Sobue, H.; Oda, E. Radiation induced ionic polymerization of butadiene. *J. Phys. Chem.* **1961**, *65*, 1645-1647. [CrossRef]

67. Ishigure, K.; Oneill, T.; Stahel, E.P.; Stannett, V. Radiation-induced polymerization and copolymerization of butadiene in emulsion. *J. Macromol. Sci. Chem.* **1974**, *8*, 353-372. [CrossRef]

68. Radzinskii, S.A.; Sheinker, A.P.; Abkin, A.D. Radiation-induced polymerization of butadiene. *Vysok. Soedin. Seriya A* **1977**, *19*, 2066-2072.

69. Verdurmen, E.M.; Verstegen, J.M.; German, A.L. Particle growth in butadiene emulsion polymerization—4. The promoting effect of mercaptans. *Macromol. Chem. Phys.* **1994**, *195*, 647-659. [CrossRef]

70. Challa, R.R.; Drew, J.H.; Stahel, E.P.; Stannett, V. Radiation-induced emulsion polymerization of vinyl-acetate in a pilot-plant reactor.2. Kinetics. *J. Appl. Polym. Sci.* **1986**, *31*, 27-38. [CrossRef]

71. Verdurmen, E.M.; Geurts, J.M.; German, A.L. Particle growth in butadiene emulsion polymerization, 1. The use of Fremy salt as aqueous radical scavenger. *Macromol. Chem. Phys.* **1994**, *195*, 621-633. [CrossRef]

72. Verdurmen, E.M.; Geurts, J.M.; German, A.L. Particle growth in butadiene emulsion polymerization, 3. Radical adsorption and desorption rate coefficients. *Macromol. Chem. Phys.* **1994**, *195*, 641-645. [CrossRef]

73. Sundardi, F. Emulsion polymerization of vinyl-acetate initiated by intermittent gamma-radiation. *J. Appl. Polym. Sci.* **1979**, *24*, 1031-1038. [CrossRef]

74. Xu, X.L.; Zhang, Z.C.; Zhang, M.W. Microemulsion polymerization of butyl acrylate initiated by gamma rays. *J. Appl. Polym. Sci.* **1996**, *62*, 1179-1183. [CrossRef]

75. Yu, H.; Peng, J.; Zhai, M.; Li, J.; Wei, G.; Qiao, J. Radiation-induced copolymerization of styrene/n-butyl acrylate in the presence of ultra-fine powdered styrene-butadiene rubber. *Radiat. Phys. Chem.* **2007**, *76*, 1736-1740. [CrossRef]

76. Okada, T.; Ishigaki, I.; Suwa, T.; Machi, S. Synthesis of cationic flocculant by radiation-induced co-polymerization of methyl-chloride salt of n,n-dimethylaminoethyl methacrylate with acrylamide in aqueous-solution. *J. Appl. Polym. Sci.* **1979**, *24*, 1713-1721. [CrossRef]

77. Tabata, Y.; Ito, W.; Oshima, K. Radiation-induced polymerization of tetrafluoroethylene. *J. Macromol. Sci. Part A Chem.* **1970**, *4*, 789-799. [CrossRef]

78. Suwa, T.; Watanabe, T.; Okamoto, J.; Machi, S. Emulsifier-free emulsion polymerization of tetrafluoroethylene by radiation.5. Effect of reaction conditions on the stability of polytetrafluoroethylene latex. *J. Polym. Sci. Part A Polym. Chem.* **1979**, *17*, 503-516. [CrossRef]

79. Brown, D.W.; Lowry, R.E. Radiation-induced co-polymerization of tetrafluoroethylene and styrene at high-pressure. *J. Polym. Sci. Part A Polym. Chem.* **1979**, *17*, 759-768. [CrossRef]

80. Lowry, R.E.; Brown, D.W.; Wall, L.A. Radiation-induced polymerization of hexafluoropropylene at high temperature and pressure. *J. Polym. Sci. Part A Polym. Chem.* **1966**, *4*, 2229-2240. [CrossRef]

81. Matsuda, O.; Watanabe, T.; Tabata, Y.; Machi, S. Radiation-induced co-polymerization of methyl trifluoroacrylate with ethylene. *J. Polym. Sci. Part A Polym. Chem.* **1979**, *17*, 1789-1793. [CrossRef]

82. Matsuda, O.; Watanabe, T.; Tabata, Y.; Machi, S. Radiation-induced co-polymerization of methyl trifluoroacrylate with propylene. *J. Polym. Sci. Part A Polym. Chem.* **1979**, *17*, 1795-1800. [CrossRef]

83. Allcock, H.R. X-ray-induced polymerization of diphenylvinylphosphine oxide. *J. Polym. Sci. Part A Polym. Chem.* **1964**, *2*, 4087-4095. [CrossRef]

84. Wiley, R.H.; Gensheimer, D.E. Gamma-radiation-induced polymerization of ethenesulfonamide. *J. Polym. Sci.* **1960**, *42*, 119-123. [CrossRef]

85. Kumar, V.; Bhardwaj, Y.K.; Sabharwal, S.; Mohan, H. The role of radiolytically generated species in radiation-induced polymerization of vinylbenzyltrimethylammonium chloride (VBT) in aqueous solution: Steady-state and pulse radiolysis study. *J. Radiat. Res.* **2003**, *44*, 161-169. [CrossRef] [PubMed]

86. Davidson, R.S.; Ellis, R.J.; Wilkinson, S.A. The electron-beam curing of dialkyltin diacrylates. *Polymer* **1992**, *33*, 1836-1842. [CrossRef]

87. Batten, R.J.; Davidson, R.S.; Ellis, R.J.; Wilkinson, S.A. New surface-coating materials—Electron-beam curing of some silicon-containing acrylates. *Polymer* **1992**, *33*, 3037-3043. [CrossRef]

88. Tyagi, M.; Seshadri, G.; Sabharwal, S.; Niyogi, U.K.; Khandal, R.K. Studies on development of polymeric materials using gamma irradiation for contact and intra-ocular lenses. *Int. J. Polym. Sci.* **2009**. [CrossRef]

89. Lipscomb, N.T.; Purcell, T.O. Intensity dependence of gamma-radiation-induced polymerization of solid N-(1,1-dimethyl-3-oxobutyl)-acrylamide (diacetone acrylamide). *J. Polym. Sci. Part B Polym. Lett.* **1971**, *9*, 831-837. [CrossRef]
66. Tabata, Y.; Sobue, H.; Oda, E. Radiation induced ionic polymerization of butadiene. *J. Phys. Chem.* **1961**, *65*, 1645-1647. [CrossRef]

67. Ishigure, K.; Oneill, T.; Stahel, E.P.; Stannett, V. Radiation-induced polymerization and copolymerization of butadiene in emulsion. *J. Macromol. Sci. Chem.* **1974**, *8*, 353-372. [CrossRef]

68. Radzinskii, S.A.; Sheinker, A.P.; Abkin, A.D. Radiation-induced polymerization of butadiene. *Vysok. Soedin. Seriya A* **1977**, *19*, 2066-2072.

69. Verdurmen, E.M.; Verstegen, J.M.; German, A.L. Particle growth in butadiene emulsion polymerization—4. The promoting effect of mercaptans. *Macromol. Chem. Phys.* **1994**, *195*, 647-659. [CrossRef]

70. Challa, R.R.; Drew, J.H.; Stahel, E.P.; Stannett, V. Radiation-induced emulsion polymerization of vinyl-acetate in a pilot-plant reactor.2. Kinetics. *J. Appl. Polym. Sci.* **1986**, *31*, 27-38. [CrossRef]

71. Verdurmen, E.M.; Geurts, J.M.; German, A.L. Particle growth in butadiene emulsion polymerization, 1. The use of Fremy salt as aqueous radical scavenger. *Macromol. Chem. Phys.* **1994**, *195*, 621-633. [CrossRef]

72. Verdurmen, E.M.; Geurts, J.M.; German, A.L. Particle growth in butadiene emulsion polymerization, 3. Radical adsorption and desorption rate coefficients. *Macromol. Chem. Phys.* **1994**, *195*, 641-645. [CrossRef]

73. Sundardi, F. Emulsion polymerization of vinyl-acetate initiated by intermittent gamma-radiation. *J. Appl. Polym. Sci.* **1979**, *24*, 1031-1038. [CrossRef]

74. Xu, X.L.; Zhang, Z.C.; Zhang, M.W. Microemulsion polymerization of butyl acrylate initiated by gamma rays. *J. Appl. Polym. Sci.* **1996**, *62*, 1179-1183. [CrossRef]

75. Yu, H.; Peng, J.; Zhai, M.; Li, J.; Wei, G.; Qiao, J. Radiation-induced copolymerization of styrene/n-butyl acrylate in the presence of ultra-fine powdered styrene-butadiene rubber. *Radiat. Phys. Chem.* **2007**, *76*, 1736-1740. [CrossRef]

76. Okada, T.; Ishigaki, I.; Suwa, T.; Machi, S. Synthesis of cationic flocculant by radiation-induced co-polymerization of methyl-chloride salt of n,n-dimethylaminoethyl methacrylate with acrylamide in aqueous-solution. *J. Appl. Polym. Sci.* **1979**, *24*, 1713-1721. [CrossRef]

77. Tabata, Y.; Ito, W.; Oshima, K. Radiation-induced polymerization of tetrafluoroethylene. *J. Macromol. Sci. Part A Chem.* **1970**, *4*, 789-799. [CrossRef]

78. Suwa, T.; Watanabe, T.; Okamoto, J.; Machi, S. Emulsifier-free emulsion polymerization of tetrafluoroethylene by radiation.5. Effect of reaction conditions on the stability of polytetrafluoroethylene latex. *J. Polym. Sci. Part A Polym. Chem.* **1979**, *17*, 503-516. [CrossRef]

79. Brown, D.W.; Lowry, R.E. Radiation-induced co-polymerization of tetrafluoroethylene and styrene at high-pressure. *J. Polym. Sci. Part A Polym. Chem.* **1979**, *17*, 759-768. [CrossRef]

80. Lowry, R.E.; Brown, D.W.; Wall, L.A. Radiation-induced polymerization of hexafluoropropylene at high temperature and pressure. *J. Polym. Sci. Part A Polym. Chem.* **1966**, *4*, 2229-2240. [CrossRef]

81. Matsuda, O.; Watanabe, T.; Tabata, Y.; Machi, S. Radiation-induced co-polymerization of methyl trifluoroacrylate with ethylene. *J. Polym. Sci. Part A Polym. Chem.* **1979**, *17*, 1789-1793. [CrossRef]

82. Matsuda, O.; Watanabe, T.; Tabata, Y.; Machi, S. Radiation-induced co-polymerization of methyl trifluoroacrylate with propylene. *J. Polym. Sci. Part A Polym. Chem.* **1979**, *17*, 1795-1800. [CrossRef]

83. Allcock, H.R. X-ray-induced polymerization of diphenylvinylphosphine oxide. *J. Polym. Sci. Part A Polym. Chem.* **1964**, *2*, 4087-4095. [CrossRef]

84. Wiley, R.H.; Gensheimer, D.E. Gamma-radiation-induced polymerization of ethenesulfonamide. *J. Polym. Sci.* **1960**, *42*, 119-123. [CrossRef]

85. Kumar, V.; Bhardwaj, Y.K.; Sabharwal, S.; Mohan, H. The role of radiolytically generated species in radiation-induced polymerization of vinylbenzyltrimethylammonium chloride (VBT) in aqueous solution: Steady-state and pulse radiolysis study. *J. Radiat. Res.* **2003**, *44*, 161-169. [CrossRef] [PubMed]

86. Davidson, R.S.; Ellis, R.J.; Wilkinson, S.A. The electron-beam curing of dialkyltin diacrylates. *Polymer* **1992**, *33*, 1836-1842. [CrossRef]

87. Batten, R.J.; Davidson, R.S.; Ellis, R.J.; Wilkinson, S.A. New surface-coating materials—Electron-beam curing of some silicon-containing acrylates. *Polymer* **1992**, *33*, 3037-3043. [CrossRef]

88. Tyagi, M.; Seshadri, G.; Sabharwal, S.; Niyogi, U.K.; Khandal, R.K. Studies on development of polymeric materials using gamma irradiation for contact and intra-ocular lenses. *Int. J. Polym. Sci.* **2009**. [CrossRef]

89. Lipscomb, N.T.; Purcell, T.O. Intensity dependence of gamma-radiation-induced polymerization of solid N-(1,1-dimethyl-3-oxobutyl)-acrylamide (diacetone acrylamide). *J. Polym. Sci. Part B Polym. Lett.* **1971**, *9*, 831-837. [CrossRef]
90. Hardy, G.; Varga, J.; Nagy, G.; Cser, F.; Ero, J. Gamma-radiation-initiated polymerization of N-vinylsuccinimide in liquid and solid states. *J. Polym. Sci. Part C Polym. Symp.* **1967**, *16*, 2583–2596. [CrossRef]

91. Hardy, G.; Nagy, G.; Varga, J.; Nemesheg, G. Investigations in field of radiation-induced solid-state polymerization. *J. Eur. Polym. J.* **1973**, *9*, 399–410. [CrossRef]

92. Hayakawa, K.; Yamakita, H.; Kawase, K. Gamma-radiation-induced and ultraviolet-radiation-induced solid-state polymerization of maleimide in a few binary-systems. *J. Polym. Sci. Part A Polym. Chem.* **1972**, *10*, 1363–1375. [CrossRef]

93. Allayarov, S.R.; Barkalov, I.M.; Kiryukhin, D.P. Radiation-induced polymerization of trifluorochloroethylene in perfluoroalkane glass. *High Energy Chem.* **1987**, *21*, 112–115.

94. Allayarov, S.R.; Kiryukhin, D.P.; Barkalov, I.M. Radiation-induced polymerization of vinyl-chloride in glassy matrix of perfluoroalkanes. *Vysok. Soedin. Seriya A* **1983**, *25*, 1655–1659.

95. Usanmaz, A.; Ozdemir, T.; Polat, O. Solid state polymerization of N-vinylcaprolactam via gamma irradiation and characterization. *J. Macromol. Sci. Chem.* **2009**, *46*, 597–606. [CrossRef]

96. Wegner, G. Topochemical polymerization of monomers with conjugated triple bonds. *Angew. Chem. Int. Ed.* **1971**, *10*, 355–363.

97. Tabata, Y.; Shibano, H.; Hara, K.; Sobue, H. Radiation-induced cationic polymerization of ethylene. *J. Polym. Sci. Part A Polym. Chem.* **1963**, *1*, 1049–1053. [CrossRef]

98. Ueno, K.; Yamaoka, H.; Hayashi, K.; Okamura, S. Studies on radiation-induced ionic polymerization. 2. Effect of solvent on polymerization of isobutene at low temperature. *Appl. Radiat. Isot.* **1966**, *17*, 595–602. [CrossRef]

99. Marek, M.T.L.; Pecka, J. Process For Radiation Polymerization and Copolymerization of Monomers with Olefinic Double Bonds Using Halide Catalysts. U.S. Patent US3997417A, 14 December 1976.

100. Cataldo, F.; Ragni, P.; Ursini, O. Radiation-induced polymerization of beta(-)pinene: A further insight. *J. Radioanal. Nucl. Chem.* **2007**, *272*, 29–36. [CrossRef]

101. Goineau, A.M.; Kohler, J.; Stannett, V. Radiation-induced polymerization of a series of vinyl ethers. *J. Macromol. Sci. Chem.* **1977**, *11*, 99–114. [CrossRef]

102. Parrish, C.F.; Harmer, D.E. Solid-state polymerization of 1,2,3,4-diepoxybutane initiated by cobalt-60 gamma-radiation. *J. Polym. Sci. Part A Polym. Chem.* **1967**, *5*, 1015–1020. [CrossRef]

103. Nishii, M.; Hayashi, K.; Okamura, S. Radiation-induced polymerization of pentoxane in solid state. *J. Polym. Sci. Part B Polym. Lett.* **1969**, *7*, 891–895. [CrossRef]

104. Ishigaki, I.; Ito, A.; Iwai, T.; Hayashi, K. Radiation-induced postpolymerization of trioxane in solid-state. 5. Studies on copolymer composition. *J. Polym. Sci. Part A Polym. Chem.* **1972**, *10*, 1883–1893. [CrossRef]

105. Naylor, D.M.; Stannett, V.T.; Deffieux, A.; Sigwalt, P. The radiation-induced polymerization of dimethylcyclosiloxanes in the liquid-state. 3. Copolymerization of D₃ with D₄ and D₄ with D₅ reactivities and interpretation. *Polymer* **1994**, *35*, 1764–1768. [CrossRef]

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109. Yamaoka, H.; Williams, F.; Hayashi, K. Radiation-induced polymerization of nitroethylene. *Trans. Faraday Soc.* **1967**, *63*, 376–381. [CrossRef]

110. Ogasawara, M.; Arai, S.; Imamura, M. Radiation-induced polymerization of nitroethylene as studied by low-temperature pulse-radiolysis. *Abstr. Pap. Am. Chem. Soc.* **1979**, *176*. [CrossRef]

111. DinhNgoc, B.; Schnabel, W. Primary reactions during free anionic-polymerization of beta-nitrostyrene. *Z. Nat. A* **1978**, *33*, 253–256. [CrossRef]

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91. Hardy, G.; Nagy, G.; Varga, J.; Nemesheg, G. Investigations in field of radiation-induced solid-state polymerization. *J. Eur. Polym. J.* **1973**, *9*, 399–410. [CrossRef]

92. Hayakawa, K.; Yamakita, H.; Kawase, K. Gamma-radiation-induced and ultraviolet-radiation-induced solid-state polymerization of maleimide in a few binary-systems. *J. Polym. Sci. Part A Polym. Chem.* **1972**, *10*, 1363–1375. [CrossRef]

93. Allayarov, S.R.; Barkalov, I.M.; Kiryukhin, D.P. Radiation-induced polymerization of trifluorochloroethylene in perfluoroalkane glass. *High Energy Chem.* **1987**, *21*, 112–115.

94. Allayarov, S.R.; Kiryukhin, D.P.; Barkalov, I.M. Radiation-induced polymerization of vinyl-chloride in glassy matrix of perfluoroalkanes. *Vysok. Soedin. Seriya A* **1983**, *25*, 1655–1659.

95. Usanmaz, A.; Ozdemir, T.; Polat, O. Solid state polymerization of N-vinylcaprolactam via gamma irradiation and characterization. *J. Macromol. Sci. Chem.* **2009**, *46*, 597–606. [CrossRef]

96. Wegner, G. Topochemical polymerization of monomers with conjugated triple bonds. *Angew. Chem. Int. Ed.* **1971**, *10*, 355–363.

97. Tabata, Y.; Shibano, H.; Hara, K.; Sobue, H. Radiation-induced cationic polymerization of ethylene. *J. Polym. Sci. Part A Polym. Chem.* **1963**, *1*, 1049–1053. [CrossRef]

98. Ueno, K.; Yamaoka, H.; Hayashi, K.; Okamura, S. Studies on radiation-induced ionic polymerization. 2. Effect of solvent on polymerization of isobutene at low temperature. *Appl. Radiat. Isot.* **1966**, *17*, 595–602. [CrossRef]

99. Marek, M.T.L.; Pecka, J. Process For Radiation Polymerization and Copolymerization of Monomers with Olefinic Double Bonds Using Halide Catalysts. U.S. Patent US3997417A, 14 December 1976.

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116. Barsbay, M.; Güven, O. A short review of radiation-induced raft-mediated graft copolymerization: A powerful combination for modifying the surface properties of polymers in a controlled manner. *Radiat. Phys. Chem.* **2009**, *78*, 1054-1059. [CrossRef]

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120. Cortella, L.; Albino, C.; Tran, Q.-K.; Froment, K. 50 years of French experience in using gamma rays as a tool for cultural heritage remedial conservation. *Radiat. Phys. Chem.* **2020**, *171*, 8726. [CrossRef]

121. Takács, E.; Wojnárovits, L. Pulse radiolysis studies on the polymerization of acrylates and methacrylates. *Radiat. Phys. Chem.* **1996**, *47*, 441-444. [CrossRef]

122. Takács, E.; Wojnárovits, L. Kinetic study on the radiation-induced polymerization of HDDA and HDDMA. *Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. At.* **1997**, *131*, 295-299. [CrossRef]

123. Knolle, W.; Mehnert, R. On the mechanism of the electron-initiated curing of acrylates. *Radiat. Phys. Chem.* **1995**, *46*, 963-974. [CrossRef]

124. Kozicki, M.; Kujawa, P.; Rosiak, J.M. Pulse radiolysis study of diacrylate macromonomer in aqueous solution. *Radiat. Phys. Chem.* **2002**, *65*, 133-139. [CrossRef]

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126. Fouassier, J.P.; Lalevée, J. *Photoinitiators for Polymer Synthesis: Scope, Reactivity, and Efficiency*; John Wiley & Sons: Weinheim, Germany, 2012.

127. Emmi, S.S.; Corda, U.; Fuochi, P.; Lavalle, M.; Alessi, S.; Spadaro, G. Pulse radiolysis and theoretical investigation on the initial mechanism of the e-beam polymerization of epoxy resins. The results obtained on (phenoxymethyl)oxirane. *Radiat. Phys. Chem.* **2007**, *76*, 1251-1256. [CrossRef]

128. Davidson, R.S.; Wilkinson, S.A. Electron-beam-induced polymerization of epoxides. *J. Photochem. Photobiol. A* **1991**, *58*, 123-134. [CrossRef]

129. Schalek, R.L.; Defoort, B.; Drzal, L.T. A TEM investigation of the network structure of electron beam cured epoxy polymers. *Microsc. Today* **2018**, *10*, 24-25. [CrossRef]

130. Defoort, B.; Defoort, D.; Coqueret, X. Electron-beam initiated polymerization of acrylate compositions, 2. Simulation of thermal effects in thin films. *Macromol. Theory Simul.* **2000**, *9*, 725-734. [CrossRef]

131. Decker, C.; Moussa, K. Photopolymerization of multifunctional monomers.4. Acrylates with carbamate or oxazolidone structure. *Eur. Polym. J.* **1991**, *27*, 403-411. [CrossRef]

132. Vandeberg, J.; Krongauz, V.V. Methods for curing optical fiber coatings and inks by low power electron beam radiation. U.S. Patent WO 98/41484, 24 September 1998.

133. Taylor, D.H.; Cahil, V. *Advances in Energy Cure Inkjet*; Uv+Eb Technol: Boston, MA, USA, 2017; pp. 12-14.

134. Du, Z.; Janke, C.J.; Li, J.; Wood, D.L., III. High-speed electron beam curing of thick electrode for high energy density li-ion batteries. *Green Energy Environ.* **2019**, *4*, 375-381. [CrossRef]

135. Defoort, B.; Lopitaux, G.; Dupillier, J.M.; Larnac, G.; Coqueret, X. Electron-beam initiated polymerization of acrylate compositions, 6-Influence of processing parameters on the curing kinetics of an epoxy acrylate blend. *Macromol. Chem. Phys.* **2001**, *202*, 3149-3156. [CrossRef]

136. Chuda, K.; Smolinski, W.; Defoort, B.; Rudz, W.; Gawdzik, B.; Rayss, J.; Coqueret, X. Effects of vitrification on the isothermal polymerization of acrylate blends under radiation. *Polymery* **2004**, *49*, 505-513. [CrossRef]

137. Al-Sheikhly, M.; McLaughlin, W. On the mechanisms of radiation-induced curing of epoxy-fiber composites. *Radiat. Phys. Chem.* **1996**, *48*, 201-206. [CrossRef]

138. Krzeminski, M.; Molinari, M.; Troyon, M.; Coqueret, X. Characterization by atomic force microscopy of the nanoheterogeneities produced by the radiation-induced cross-linking polymerization of aromatic diacrylates. *Macromolecules* **2010**, *43*, 8121-8127. [CrossRef]
115. Wang, H.; Li, Q.; Dai, J.; Du, F.; Zheng, H.; Bai, R. Real-time and in situ investigation of "living"/controlled photopolymerization in the presence of a trithiocarbonate. *Macromolecules* **2013**, *46*, 2576-2582. [CrossRef]

116. Barsbay, M.; Güven, O. A short review of radiation-induced raft-mediated graft copolymerization: A powerful combination for modifying the surface properties of polymers in a controlled manner. *Radiat. Phys. Chem.* **2009**, *78*, 1054-1059. [CrossRef]

117. Nasef, M.M.; Gursel, S.A.; Karabelli, D.; Güven, O. Radiation-grafted materials for energy conversion and energy storage applications. *Prog. Polym. Sci.* **2016**, *63*, 1-41. [CrossRef]

118. Bongiovanni, R.; Sangermano, M. UV-curing science and technology. In *Encyclopedia of Polymer Science and Technology*; Wiley Online Library: Hoboken, NJ, USA, 2014; pp. 1-20. [CrossRef]

119. Wicks, Z.W.; Jones, F.N.; Pappas, S.P.; Wicks, D.A. *Organic Coatings: Science and Technology*, 4th ed.; John Wiley & Sons: New York, NY, USA, 2017; p. 512.

120. Cortella, L.; Albino, C.; Tran, Q.-K.; Froment, K. 50 years of French experience in using gamma rays as a tool for cultural heritage remedial conservation. *Radiat. Phys. Chem.* **2020**, *171*, 8726. [CrossRef]

121. Takács, E.; Wojnárovits, L. Pulse radiolysis studies on the polymerization of acrylates and methacrylates. *Radiat. Phys. Chem.* **1996**, *47*, 441-444. [CrossRef]

122. Takács, E.; Wojnárovits, L. Kinetic study on the radiation-induced polymerization of HDDA and HDDMA. *Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. At.* **1997**, *131*, 295-299. [CrossRef]

123. Knolle, W.; Mehnert, R. On the mechanism of the electron-initiated curing of acrylates. *Radiat. Phys. Chem.* **1995**, *46*, 963-974. [CrossRef]

124. Kozicki, M.; Kujawa, P.; Rosiak, J.M. Pulse radiolysis study of diacrylate macromonomer in aqueous solution. *Radiat. Phys. Chem.* **2002**, *65*, 133-139. [CrossRef]

125. TakACS, E.; Dajka, K.; Wojnarovits, L.; Emmi, S.S. Protonation kinetics of acrylate radical anions. *Phys. Chem. Chem. Phys.* **2000**, *2*, 1431-1433. [CrossRef]

126. Fouassier, J.P.; Lalevée, J. *Photoinitiators for Polymer Synthesis: Scope, Reactivity, and Efficiency*; John Wiley & Sons: Weinheim, Germany, 2012.

127. Emmi, S.S.; Corda, U.; Fuochi, P.; Lavalle, M.; Alessi, S.; Spadaro, G. Pulse radiolysis and theoretical investigation on the initial mechanism of the e-beam polymerization of epoxy resins. The results obtained on (phenoxymethyl)oxirane. *Radiat. Phys. Chem.* **2007**, *76*, 1251-1256. [CrossRef]

128. Davidson, R.S.; Wilkinson, S.A. Electron-beam-induced polymerization of epoxides. *J. Photochem. Photobiol. A* **1991**, *58*, 123-134. [CrossRef]

129. Schalek, R.L.; Defoort, B.; Drzal, L.T. A TEM investigation of the network structure of electron beam cured epoxy polymers. *Microsc. Today* **2018**, *10*, 24-25. [CrossRef]

130. Defoort, B.; Defoort, D.; Coqueret, X. Electron-beam initiated polymerization of acrylate compositions, 2. Simulation of thermal effects in thin films. *Macromol. Theory Simul.* **2000**, *9*, 725-734. [CrossRef]

131. Decker, C.; Moussa, K. Photopolymerization of multifunctional monomers.4. Acrylates with carbamate or oxazolidone structure. *Eur. Polym. J.* **1991**, *27*, 403-411. [CrossRef]

132. Vandeberg, J.; Krongauz, V.V. Methods for curing optical fiber coatings and inks by low power electron beam radiation. U.S. Patent WO 98/41484, 24 September 1998.

133. Taylor, D.H.; Cahil, V. *Advances in Energy Cure Inkjet*; Uv+Eb Technol: Boston, MA, USA, 2017; pp. 12-14.

134. Du, Z.; Janke, C.J.; Li, J.; Wood, D.L., III. High-speed electron beam curing of thick electrode for high energy density li-ion batteries. *Green Energy Environ.* **2019**, *4*, 375-381. [CrossRef]

135. Defoort, B.; Lopitaux, G.; Dupillier, J.M.; Larnac, G.; Coqueret, X. Electron-beam initiated polymerization of acrylate compositions, 6-Influence of processing parameters on the curing kinetics of an epoxy acrylate blend. *Macromol. Chem. Phys.* **2001**, *202*, 3149-3156. [CrossRef]

136. Chuda, K.; Smolinski, W.; Defoort, B.; Rudz, W.; Gawdzik, B.; Rayss, J.; Coqueret, X. Effects of vitrification on the isothermal polymerization of acrylate blends under radiation. *Polymery* **2004**, *49*, 505-513. [CrossRef]

137. Al-Sheikhly, M.; McLaughlin, W. On the mechanisms of radiation-induced curing of epoxy-fiber composites. *Radiat. Phys. Chem.* **1996**, *48*, 201-206. [CrossRef]

138. Krzeminski, M.; Molinari, M.; Troyon, M.; Coqueret, X. Characterization by atomic force microscopy of the nanoheterogeneities produced by the radiation-induced cross-linking polymerization of aromatic diacrylates. *Macromolecules* **2010**, *43*, 8121-8127. [CrossRef]
139. McHugh, J.; Fideu, P.; Herrmann, A.; Stark, W. Determination and review of specific heat capacity measurements during isothermal cure of an epoxy using TM-DSC and standard DSC techniques. *Polym. Test.* **2010**, *29*, 59–765. [CrossRef]

140. Coqueret, X.; Krzeminski, M.; Ponsaud, P.; Defoort, B. Recent advances in electron-beam curing of carbon fiber-reinforced composites. *Radiat. Phys. Chem.* **2009**, *78*, 557–561. [CrossRef]

141. Defoort, B.; Coqueret, X.; Larnac, G.; Dupillier, J.M. Electron-beam curing of acrylate resins for composites: Modeling reaction kinetics. In *Proceedings of the 45th International SAMPE Symposium and Exhibition, books 1 and 2*. Long Beach, CA, USA, 21–25 May 2000; SAMPE International Business Office: Diamond Bar, CA, USA, 2000; Volume 45, pp. 2223–2234.

142. Defoort, B.; Boursereau, F.; Dupillier, J.M.; Larnac, G.; Lopitaux, G.; Coqueret, X. Investigations to improve the properties of eb cured composites: A status report. In *Affordable Materials Technology-Platform to Global Value and Performance; Books 1 and 2; In Proceedings of the 47th International SAMPE Symposium and Exhibition; SAMPE International Business Office: Diamond Bar, CA, USA, 2002; Volume 47*, pp. 607–617.

143. Martin, A.; Pietras-Ozga, D.; Ponsaud, P.; Kowandy, C.; Barczak, M.; Defoort, B.; Coqueret, X. Radiation-curing of acrylate composites including carbon fibres: A customized surface modification for improving. *Mechanical performances. Radiat. Phys. Chem.* **2014**, *105*, 63–68. [CrossRef]

144. Manning, P.; Chapiro, A. Radiation chemistry of polymeric systems. In *High Polymers; John Wiley & Sons: New York, NY, USA, London, UK*, 1962; Volume XV, pp. 712. E7 18s. net. *Endeavour* **1963**, *22*, 100. [CrossRef]

145. Nasef, M.M.; Güven, O. Radiation-grafted copolymers for separation and purification purposes: Status, challenges and future directions. *Prog. Polym. Sci.* **2012**, *37*, 1597–1656. [CrossRef]

146. Hassan, M.I.U.; Taimur, S.; Yasin, T. Upcycling of polypropylene waste by surface modification using radiation-induced grafting. *Appl. Surf. Sci.* **2017**, *422*, 720–730. [CrossRef]

147. Gao, Q.; Hua, J.; Li, R.; Xing, Z.; Pang, L.; Zhang, M.; Xu, L.; Wu, G. Radiation-induced graft polymerization for the preparation of highly efficient UHMWPE fibrous adsorbent for Cr(VI) removal. *Rad. Phys. Chem.* **2017**, *130*, 92–102. [CrossRef]

148. Kavakli, C.; Barsbay, M.; Tilki, S.; Güven, O.; Kavakli, P.A. Activation of polyethylene/propylene non-woven fabric by radiation-induced grafting for the removal of Cr(VI) from aqueous solutions. *Water Air Soil Pollut.* **2016**, *227*, 473. [CrossRef]

149. Kavakli, P.A.; Kavakli, C.; Seko, N.; Tamada, M.; Güven, O. Radiation induced emulsion graft polymerization of 4-vinylpyridine onto PE/PP non woven fabric for As(V) adsorption. *Radi. Phys. Chem.* **2016**, *127*, 13–20. [CrossRef]

150. Gamma-radiation grafting on chitosan polymer substrate: a) Zhuang, S.; Yin, Y.; Wang, J. Removal of cobalt ions from aqueous solution using chitosan grafted with maleic acid by gamma radiation. *Nucl. Eng. Techn.* **2018**, *50*, 211–215. [CrossRef] b) Crini, G. Recent Developments in Polysaccharide-Based Materials Used as Adsorbents in Wastewater Treatment. *Progr. Polym. Sci.* **2005**, *30*, 38–70. [CrossRef] c) Sokker, H.H.; Abdel Ghaffar, A.M.; Gad, Y.H.; Aly, A.S. Synthesis and characterization of hydrogels based on grafted chitosan for the controlled drug release. *Carbohydr. Polym.* **2009**, *75*, 222–229. d) Casimiro, M.H.; Ferreira, L.M.; Leal, J.P.; Pereira, C.C.L.; Monteiro, B. Ionizing Radiation for Preparation and Functionalization of Membranes and Their Biomedical and Environmental Applications. *Membranes* **2019**, *9*, 163. [CrossRef] e) Cai, H.; Zhang, Z.P.; Sun, P.C.; He, B.L.; Zhu, X.X. Synthesis and characterization of thermo- and pH- sensitive hydrogels based on Chitosan-grafted N-isopropylacrylamide via γ-radiation. *Radiat. Phys. Chem.* **2005**, *74*, 26–30. [CrossRef]

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152. Chen, G.; Wang, Y.; Weng, H.; Wu, Z.; He, K.; Zhang, P.; Guo, Z.; Lin, M. Selective separation of Pd(II) on pyridine-functionalized graphene oxide prepared by radiation-induced simultaneous grafting polymerization and reduction. *Acs App. Mater. Interf.* **2019**, *11*, 24560–24570. [CrossRef]

153. Ueki, Y.; Seko, N. Synthesis of fibrous metal adsorbent with a piperazinyl-dithiocarbamate group by radiation-induced grafting and its performance. *ACS Omega* **2020**, *5*, 2947–2959. [CrossRef]

154. Shin, I.H.; Hong, S.; Lima, S.J.; Son, Y.-S.S.; Kim, T.-H. Surface modification of PVDF membrane by radiation-induced graft polymerization for novel membrane bioreactor. *J. Indust. Eng. Chem.* **2017**, *46*, 103–110. [CrossRef]
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140. Coqueret, X.; Krzeminski, M.; Ponsaud, P.; Defoort, B. Recent advances in electron-beam curing of carbon fiber-reinforced composites. *Radiat. Phys. Chem.* **2009**, *78*, 557–561. [CrossRef]

141. Defoort, B.; Coqueret, X.; Larnac, G.; Dupillier, J.M. Electron-beam curing of acrylate resins for composites: Modeling reaction kinetics. In *Proceedings of the 45th International SAMPE Symposium and Exhibition, books 1 and 2*. Long Beach, CA, USA, 21–25 May 2000; SAMPE International Business Office: Diamond Bar, CA, USA, 2000; Volume 45, pp. 2223–2234.

142. Defoort, B.; Boursereau, F.; Dupillier, J.M.; Larnac, G.; Lopitaux, G.; Coqueret, X. Investigations to improve the properties of eb cured composites: A status report. In *Affordable Materials Technology-Platform to Global Value and Performance; Books 1 and 2; In Proceedings of the 47th International SAMPE Symposium and Exhibition; SAMPE International Business Office: Diamond Bar, CA, USA, 2002; Volume 47*, pp. 607–617.

143. Martin, A.; Pietras-Ozga, D.; Ponsaud, P.; Kowandy, C.; Barczak, M.; Defoort, B.; Coqueret, X. Radiation-curing of acrylate composites including carbon fibres: A customized surface modification for improving. *Mechanical performances. Radiat. Phys. Chem.* **2014**, *105*, 63–68. [CrossRef]

144. Manning, P.; Chapiro, A. Radiation chemistry of polymeric systems. In *High Polymers; John Wiley & Sons: New York, NY, USA, London, UK*, 1962; Volume XV, pp. 712. E7 18s. net. *Endeavour* **1963**, *22*, 100. [CrossRef]

145. Nasef, M.M.; Güven, O. Radiation-grafted copolymers for separation and purification purposes: Status, challenges and future directions. *Prog. Polym. Sci.* **2012**, *37*, 1597–1656. [CrossRef]

146. Hassan, M.I.U.; Taimur, S.; Yasin, T. Upcycling of polypropylene waste by surface modification using radiation-induced grafting. *Appl. Surf. Sci.* **2017**, *422*, 720–730. [CrossRef]

147. Gao, Q.; Hua, J.; Li, R.; Xing, Z.; Pang, L.; Zhang, M.; Xu, L.; Wu, G. Radiation-induced graft polymerization for the preparation of highly efficient UHMWPE fibrous adsorbent for Cr(VI) removal. *Rad. Phys. Chem.* **2017**, *130*, 92–102. [CrossRef]

148. Kavakli, C.; Barsbay, M.; Tilki, S.; Güven, O.; Kavakli, P.A. Activation of polyethylene/propylene non-woven fabric by radiation-induced grafting for the removal of Cr(VI) from aqueous solutions. *Water Air Soil Pollut.* **2016**, *227*, 473. [CrossRef]

149. Kavakli, P.A.; Kavakli, C.; Seko, N.; Tamada, M.; Güven, O. Radiation induced emulsion graft polymerization of 4-vinylpyridine onto PE/PP non woven fabric for As(V) adsorption. *Radi. Phys. Chem.* **2016**, *127*, 13–20. [CrossRef]

150. Gamma-radiation grafting on chitosan polymer substrate: a) Zhuang, S.; Yin, Y.; Wang, J. Removal of cobalt ions from aqueous solution using chitosan grafted with maleic acid by gamma radiation. *Nucl. Eng. Techn.* **2018**, *50*, 211–215. [CrossRef] b) Crini, G. Recent Developments in Polysaccharide-Based Materials Used as Adsorbents in Wastewater Treatment. *Progr. Polym. Sci.* **2005**, *30*, 38–70. [CrossRef] c) Sokker, H.H.; Abdel Ghaffar, A.M.; Gad, Y.H.; Aly, A.S. Synthesis and characterization of hydrogels based on grafted chitosan for the controlled drug release. *Carbohydr. Polym.* **2009**, *75*, 222–229. d) Casimiro, M.H.; Ferreira, L.M.; Leal, J.P.; Pereira, C.C.L.; Monteiro, B. Ionizing Radiation for Preparation and Functionalization of Membranes and Their Biomedical and Environmental Applications. *Membranes* **2019**, *9*, 163. [CrossRef] e) Cai, H.; Zhang, Z.P.; Sun, P.C.; He, B.L.; Zhu, X.X. Synthesis and characterization of thermo- and pH- sensitive hydrogels based on Chitosan-grafted N-isopropylacrylamide via γ-radiation. *Radiat. Phys. Chem.* **2005**, *74*, 26–30. [CrossRef]

151. Saito, K.; Fujiwara, K.; Sugo, T. Commercial Products by Radiation-Induced Graft Polymerization. In *Innovative Polymeric Adsorbents*; Springer: Singapore, 2018. [CrossRef]

152. Chen, G.; Wang, Y.; Weng, H.; Wu, Z.; He, K.; Zhang, P.; Guo, Z.; Lin, M. Selective separation of Pd(II) on pyridine-functionalized graphene oxide prepared by radiation-induced simultaneous grafting polymerization and reduction. *Acs App. Mater. Interf.* **2019**, *11*, 24560–24570. [CrossRef]

153. Ueki, Y.; Seko, N. Synthesis of fibrous metal adsorbent with a piperazinyl-dithiocarbamate group by radiation-induced grafting and its performance. *ACS Omega* **2020**, *5*, 2947–2959. [CrossRef]

154. Shin, I.H.; Hong, S.; Lima, S.J.; Son, Y.-S.S.; Kim, T.-H. Surface modification of PVDF membrane by radiation-induced graft polymerization for novel membrane bioreactor. *J. Indust. Eng. Chem.* **2017**, *46*, 103–110. [CrossRef]
155. Omichi, M.; Yamashita, S.; Okura, Y.; Ikutomo, R.; Ueki, Y.; Seko, N.; Kakuchi, R. Surface engineering of fluoropolymer films via the attachment of crown ether derivatives based on the combination of radiation-induced graft polymerization and the Kabacknik-fields reaction. *Polymers* **2019**, *11*, 1337. [CrossRef] [PubMed]

156. Fastow, E.; Cook, S.; Dean, P.; Ott, P.; Wilson, J.; Yoon, H.; Dietz, T.; Bateman, F.; Al-Sheikhly, M. Single-step synthesis of atmospheric CO2 sorbents through radiation-induced graft polymerization on commercial-grade fabrics. *Rad. Res.* **2019**, *192*, 219–230. [CrossRef]

157. Ponce-Gonzalez, J.; Ouachan, I.; Varcoe, J.R.; Whelligan, D.K. Radiation-induced grafting of a butyl-spacer styrene monomer onto ETFE: The synthesis of the most alkali stable radiation-grafted anion-exchange membranes to date. *J. Mater. Chem. A* **2018**, *6*, 823–827. [CrossRef]

158. Yoshimura, K.; Hiroki, A.; Yu, H.-C.; Zhao, Y.; Shishitani, H.; Yamaguchi, S.; Tanaka, H.; Maekawa, Y. Alkaline durable 2-methylimidazolium containing anion-conducting electrolyte membranes synthesized by radiation-induced grafting for direct hydrazine hydrate fuel cells. *J. Memb. Sci.* **2019**, *574*, 403–410. [CrossRef]

159. Sawada, S.-I.; Hasegawa, S.; Zhao, Y.; Maekawa, Y. Block-type proton exchange membranes prepared by a combination of radiation-induced grafting and atom-transfer radical polymerization. *J. Memb. Sci.* **2017**, *532*, 105–114. [CrossRef]

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