## 3 Polymerization Methods for IIP Synthesis
### Polymerization Reactions  
Polymerization is a reaction in which simpler units, called monomers, chemically combine to form longer structures. There are two types of polymerization reactions: by condensation or by addition; the latter one is the most used in the synthesis of ion imprinted polymers. For the monomers to bond by addition polymerization (polyaddition) or chain-growth polymerization, it is necessary for the unsaturation presence in the monomer structure, where the homolytic cleavage of the pi bond produces radical species. The addition polymerization reaction has three steps with different kinetics: initiation, propagation, and termination. Initiation is the slowest step from a kinetic point of view and is characterized by the formation of free radicals from a radical, which must contain a chemical bond with low dissociation energy for a homolytic decomposition to occur (Figure 9a). This homolytic decomposition can be caused by physical agents such as heat, ultraviolet radiation, or microwaves. Then, the sp\({}^{2}\) hybridized radical species react with \begin{table}
\begin{tabular}{c c c} \hline \hline
**Polymerization Methods** & **General Features** & **Examples of Polymeric Particle Morphology** \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Cont. the least substituted carbon that participates in the \(\pi\) bond in the monomer molecule, generating a free radical that initiates the polymerization reaction (Figure 9b) [85]. The propagation step occurs after the initiation and is considered the most important step in polymerization because in this step the chain grows. This step is very fast, with low activation energy. In the propagation, the active center formed at initiation is added to another monomer, generating a longer chain with the transfer of the active center from monomer to monomer, which is immediately added to another monomer at the end, and so on, until termination occurs [85], as illustrated in Figure 10.  
Finally, the termination step occurs when there is interruption of chain growth due to the disappearance of the active center due to: (a) coupling or combination of two active centers, forming a simple bond between the propagating species; (b) disproportionation, which consists of the intermolecular transfer of a hydrogen atom from one chain to another in growth, saturating one end and creating a double bond at the end of the other chain; (c) snatching of a hydrogen atom from any point in the chain, where the active center is transferred to compensate for the loss of this hydrogen, forming branches; (d) interaction of the free radical with other formed radicals or molecules present in the reaction medium, such as a solvent, initiator, impurities or molecular oxygen. The mechanism of the termination step in free radical-initiated polyddition is shown in Figure 11.  
Due to the short lifetime of the growing chains and the several events that benefit the termination of polyddition, the structure of polymers is difficult to control, with a higher polydispersity index, i.e., greater heterogeneity in particle size [86]. Thus, to control the structure of the polymer chain, reversible-deactivation radical polymerization (RDRP) emerged as an alternative for the control of propagating radical reactivity [86]. Figure 10: Mechanism of the propagation step in free-radical polyddition. Figure 9: Mechanism of the initiation step in free-radical polyddition: (**a**) homolytic decomposition; (**b**) free radical production that initiates the polymerization reaction. There are two types of radicals in RDRP polymerization. The first one can propagate the reaction and overcome the termination event, whereas the second one is bonded to agents that control the termination of polymerization. Thus, the higher the concentration of the two agent control, the lower the probability of interaction between the propagated radicals. Thereby, it is possible to obtain polymers with higher molar mass and high rates of reaction conversion, aside from low polydispersity indices.