ThreeBond Technical News
Issued January 1, 2023

101

Development of Organic-inorganic Hybrid Materials Using
Photo-crosslinking Reactions
Introduction
Polymeric materials such as inks, coatings and adhesives are now used for a wide range ofapplications. Molecular design that gives these materials the desired physical properties can beachieved by using polymerization of the reactive monomers that serve as raw materials (crosslinking
reactions) to create three-dimensional crosslinking structures. Furthermore, there are many cases
in which complex compositions are created by adding fillers or functional compounds to suit thecharacteristics required for practical application. Two main methods are used to create crosslinked
materials: thermal curing and photo curing. However, many types of work require rapid processing
and simple processes, and photo-crosslinking reactions are an effective solution in such cases.
Organic-inorganic hybrid materials are also gaining prominence as functional materials thatposses the characteristics of both organic polymers and inorganic nanomaterials. By using photocrosslinking reactions, the hybridization of polymers and inorganic components mainly composedof silica can be achieved extremely easily. This article provides an overview of photo-crosslinking
reactions and describes photo-crosslinked organic-inorganic hybrid materials developed by theauthors.

Table of Contents
Introduction............................................................. 1
1. Types and Characteristics of
Photo-crosslinking Reactions............................ 2
2. Development of Hybrid Materials Using
Photo-radical Polymerization/
Photo-sol-gel Reactions.................................... 3

3. Development of Hybrid Materials Using
Thiol-ene Reactions........................................... 6
Conclusion.............................................................. 81

1.

Types and Characteristics of
Photo-crosslinking Reactions

Photo curing requires less energy than thermal curing,making it more environmentally friendly, and is used
to manufacture various industrial products. Photocrosslinking reactions that lead to photo curing consistof photoinitiators, which produce active species throughexposure to light, and reactive monomers/oligomers. These
reactions are an indispensable part of modern industry. These
reactions can be subdivided into radical polymerization,
cationic polymerization, anionic polymerization and thiolene reactions. The respective advantages and disadvantagesof each type are summarized in Table 1. The next section
provides an overview of each type of reaction.
In terms of practical application, radical polymerization
is currently the most-used type of photo curing. It is theresult of polymerization reactions between photo-radical
initiators, which create radicals with photo-irradiation,
and acrylic monomers. Example reaction mechanisms
are shown in Fig. 1.1) The reaction speed is high, andrapid curing occurs through photo-irradiation. However,polymerization is easily inhibited by oxygen in the air, andsurface curing may fail in some cases. There are many

types of commercially-available acrylic monomers andoligomers, and various formulations can be implemented
to achieve the desired physical properties. Consequently,
these materials are used in numerous fields for applications
ranging from adhesives to hard coatings. Hard coatings
with high crosslinking density often experience issues as aresult of curling and cracking due to curing shrinkage. In
order to resolve these issues, a flexible coating is required.
Cationic polymerization comprises reactions between
photo-acid generators, which create acid under photoirradiation, and reactive resins such as epoxides.
Example reaction mechanisms are shown in Fig. 2.
Termination reactions occur less easily than with radical
polymerization. As a result, polymerization continues
even after photoirradiation has stopped. In the case of
ring-opening polymerization of epoxy resin, there is low
Initial reaction

Photoacid generator

Propagation reaction

Initial reaction

Propagation reaction
Termination reaction

Termination reaction

Fig. 2 Photo-cationic Polymerization Reaction
Mechanism

Fig. 1 Photo-radical Polymerization Reaction
Mechanism

Table 1 Chracteristics of Photo-assisted Polymerization and Reaction
Polymerization
speed

Inhibition by
oxygenVolumetricshrinkage

Adhesion

Other

Radical polymerization

Very highOccursHigh

Poor

Wide variety of monomers, lowmaterial cost, inhibited by oxygen

Cationic polymerization

Average

None

Low

Good

Corrosion of metal substrates,
significantly affected by humidity

Anionic polymerization

High

None

Low

Good

Low sensitivity

Very high

None

Low

Good

Odor occurs, dark reaction control
is required

Thiol-ene reaction

2

volumetric shrinkage due to polymerization and adhesion
to the substrate is good. However, polymerization is
stopped by the influence of water during the reaction. Thismeans that cationic polymerization has the disadvantagesof being easily affected by atmospheric humidity andhaving unstable reactions. In addition, the use of strong
acids generated by the photo-acid generators means that
corrosion of metals is also a concern.
Anionic polymerization comprises photo-base generators,
which create base catalysts under photo-irradiation, andbase-reactive resins. Reaction mechanisms are shown in
Fig. 3. Photo-base generators are less sensitive than photoradical initiators and photo-acid generators, and researchand development of high-efficiency photo-base generators
had been comparatively slow. However, the existence ofoutstanding photo-base generators has been reported inrecent years, and the field is gaining prominence once
more.2) As shown in Table 1, anionic polymerization
may be able to overcome some disadvantages of radical
polymerization and cationic polymerization. As a result,further development can be expected.

Initial reaction

Photo base generator

Propagation reaction

Fig. 3 Photo-anionic Polymerization Reaction
Mechanism

Thiol-ene reactions are addition reactions between thiolgroups and carbon-carbon double bonds (ene) underphoto-irradiation. They are regarded as click chemistry
reactions because they have high chemoselectivity, highfunctional group tolerance and low tendency to produceside reactions. The assumed reaction mechanism is asshown in Fig. 4. Even if carbon radicals generated by theaddition of thiyl radicals to double bonds (the propagation
reaction in Fig. 4) are denatured to peroxy radicals due tothe effects of oxygen, thiyl radicals are generated throughhydrogen abstraction from thiols and the addition reaction
proceeds again without stopping. In other words, unlike
photo-radical polymerization, thiol-ene reactions are notinhibited by oxygen, and this capacity to resolve a major
problem seen with photoradical curing reactions is theirmost notable feature. Furthermore, because thiol-ene
reactions are addition reactions, the curing shrinkage
is low and the resulting thioether compounds are highly
flexible. These characteristics reduce the risk of curling.
Crosslinked materials created through reactions between
polyfunctional thiol compounds and olefin compounds are
flexible and can also provide toughness.
In terms of light sources, there has been a recent shift
away from conventional high-pressure mercury lamps
and metal halogen lamps towards the use of LEDs. At
present, 365 nm is the most widely-used wavelength for
LEDs, and monochromatic light is emitted at wavelengths
such as 375 nm, 385 nm, 395 nm and 405 nm. With highpressure mercury lamps and metal halogen lamps, multiple
line spectra that emit in the ultraviolet region are used forphotoinitiator decomposition. By comparison, LEDs use
monochromatic light, resulting in slower photoinitiatordecomposition and lower sensitivity in the photocrosslinking reaction. It is therefore necessary to either
select monochromatic LEDs that can be easily absorbed byphotoinitiators or to develop photoinitiators that are wellsuited to LED light.

Initial reaction
Propagation
reaction

Termination
reactionFig. 4 Photo-thiol-ene Reaction Mechanism

2.

Development of Hybrid Materials
Using Photo-radical Polymerization/
Photo-sol-gel Reactions

Light-cured organic hard coatings generally comprise
structures formed by three-dimensional crosslinkingof multifunctional acrylates that are given surface
hardness. multifunctional acrylates with a high numberof functional groups and finer crosslinked structures tend
to have higher hardness. However, this also increases thecuring contraction rate, which leads to drawbacks such asincreased warping and the occurrence of cracks. In contrast,
inorganic hard coatings are based on sol-gel reactions of
inorganic compounds such as silica and are characterized
by forming hard coatings with excellent chemical

3

resistance and heat resistance. Issues with these substances
include liquid storage stability and process complexity. In
order to overcome these drawbacks, organic-inorganic
hybrid hard coatings that combine the advantages of lightcured organic hard coatings and inorganic hard coatings
are garnering attention. Carbon-functional alkoxysilanes
used as silane coupling agents in these compounds are
extremely useful as raw materials for organic-inorganic
hybrids. By inciting polymerization reactions and solgel reactions simultaneously with carbon-functional
alkoxysilanes, organic-inorganic hybrid products can begiven high crosslinking density. Through sol-gel reactions
using photopolymerization and hydrolysis/condensation ofcarbon-functional alkoxysilanes containing groups suchas vinyl groups, acryl groups, and epoxy groups, photoirradiation processes can be used in the rapid preparationof organic-inorganic hybrids. Here, the preparation ofacryl/silica organic-inorganic hybrids through photobinary-crosslinking reactions is described.
The chemical structure of a benzoin sulfonate compoundis shown in Fig. 5. It is a photoacid/radical generator(PARG) that simultaneously generates sulfonic acid(cationic species) and radical species through photolysis. It
is well suited to the creation of organic-inorganic hybrids
through photo-radical polymerization and alkoxysilane
hydrolysis/condensation.3) Acryl/silica organic-inorganic
hybrids with high silica content were prepared byadding benzoin tosylate (BT) to the multifunctional
acrylate monomers pentaerythritol triacrylate (PETA)
and trimethylolpropane triacrylate (TMPTA) andtetraethoxysilane (TEOS), then performing photo curing.4)
For example, when 1 to 3 mol% of BT was added to PETA/TEOS (50/50), the crosslinking reaction was complete inabout two minutes. For organic-inorganic hybrids in which
the composition ratio of TEOS was changed to 0, 20, 40
and 50 mol%, the gel contents were 99%, 91%, 72% and
60% respectively. The fact that gel content decreased as
the amount of TEOS was increased could be attributed toa weight reduction caused by hydrolysis of ethoxysilane
groups to silanol groups (ethanol elimination) and

condensation to siloxane bonds. A radical photoinitiator(benzoin ethyl ether) and photoacid generator(diaryliodonium tetrakis(pentafluorophenylborate)) wereadded separately to PETA/TEOS (50/50), photo-crosslinking
was performed, and the gel content was measured (Fig.
6). Radical polymerization was found to be fast andhydrolysis/condensation reactions of TEOS were found to
be comparatively slow. Meanwhile, Fig. 7 shows the resultsof real-time infrared spectroscopy of changes in C=C bondsof acryl groups and Si-O-Si bonds in photo-crosslinking
using BT. It was confirmed that radical polymerizationof acryl groups and hydrolysis/condensation reactions ofalkoxysilane groups proceed almost simultaneously. It
is conceivable that improving the dispersibility of eachacrylic/silica formation can lead to improved performance.

Fig. 6 Photo-curing of PETA/TEOS (50/50) with
Radical Photoinitiators and Photo-acid
Generators
Si-O-Si: 1,030 cm-1 absorption change

C=C: 1,630 cm-1 absorption change

Fig. 5 Chemical Structure of Benzoin Sulfonate

4

Fig. 7 Changes in Infrared Absorption (C=C:
1,630 cm-1 and Si-O-Si: 1,030 cm-1)
of Photo-curing PETA/TEOS/BT (50/50/1)

Accordingly, it was observed that, if a small amount (5 to
10 mol%) of 3-acryloxypropyltrimethoxysilane (APTMS)
is added to a PETA/TEOS composition, the effects of
the acryl groups and methoxysilane groups present in
the APTMS improve dispersion of PETA and TEOS,
resulting in improvements to both surface hardness andadhesion to substrates.5) It was also found that a uniform
photo-crosslinked coating can be obtained regardlessof the TEOS content. Furthermore, when considering
the preparation of hybrid films using dipentaerythritol
penta-/hexa-acrylate (DPHA) as a multifunctional acrylic
monomer, DPHA/TEOS/APTMS photo-cured coatings
using BT as a photo-radical/acid generator can be expected
to have extremely high transparency, excellent adhesion
and surface hardness, providing significant practical value
as transparent negative resists.
Organic-inorganic hybrids in which organic and inorganic
formations are dispersed on a nanometer scale aretransparent, making them useful as optical materials.
Bisphenoxyethanolfluorene diglycidyl ether (BPEFG)
containing a bisphenylfluorene skeleton with two phenylgroups bonded to the 9-position carbon, an epoxy monomerwith a high refractive index and low birefringence, is
expected to support the development of such applications
as an optical material with a high refractive index of
1.62 or higher (Fig. 8). Based on well-known results
regarding the preparation of organic-inorganic hybrids
through hydrolysis/condensation of silane coupling
agents containing epoxy groups with photocationicpolymerization6), the synthesis of epoxyfluorene organic-

inorganic hybrids was examined. However, when thealkoxysilane hydrolysis/condensation reactions are
performed under UV irradiation, one problem is thatcracks tend to occur due to shrinkage as a result of thegeneration of alcohol and water. Consequently, a method
was adopted whereby random-type polysilsesquioxane
obtained through hydrolysis/condensation of silane
coupling agents containing epoxy groups was mixed
with BPEFG, and photo-cationic polymerization was
performed.7) Glycidoxypropyltriethoxysilane (GPTES)
and epoxycyclohexylethyltrimethoxysilane (ECHETMS)
were used as silane coupling agent containing epoxy
groups for condensation with phenyltrimethoxysilane(PTMS) to prepare two co-condensation products(copolysilsesquioxane GPTES/PTMS and ECHETMS/
PTMS). 1 to 2 wt% of photoacid generator was added tothe mixture of BPEFG and copolysilsesquioxanes. The
solution was spin-coated onto a substrate and irradiatedwith UV light (using a high-pressure mercury vapor lamp)
at 1,300 mJ/cm 2 and post-exposure baking was performed
at 150°C to form a thin, cured film. Cationic polymerization
with BPEFG was performed on epoxy groups in these
copolysilsesquioxanes to form stable organic-inorganic
hybrids. Although BPEFG has a refractive index of 1.625,
the creation of a hybrid with a silica component resulted ina slight decrease in refractive index. However, the hybrid
showed excellent adhesion and heat resistance properties
while retaining a high refractive index of about 1.6.
Furthermore, UV-visible light spectra recorded the hybrid
thin film (thickness 5 μm) showed light transmittance of
95% or more in the visible spectrum, indicating that thethin film had extremely high transparency. Table 2 shows
the results of ellipsometry measurements of the refractive
index of hybrid thin films generated on silicon wafers. The
refractive index can be controlled by adjusting the ratio of
BPEFG and copolysilsesquioxane, and these materials are
extremely attractive as optical materials.

Fig. 8 Structure of Bisphenoxyethanolfluorene
Diglycidylether (BPEFG)
Table 2 Refractive Index of Epoxyfluorene-based Organic-inorganic Hybrids
Hybrid formation

Refractive index

BPEFG

GPTES/PTESECHETMS/PTMS

1

1

-

1.591

1

-

1

1.596

2

1

-

1.612

2

-

1

1.615

5

3.

Development of Hybrid Materials
Using Thiol-ene Reactions

The creation of organic-inorganic hybrids through thesimultaneous use of photo-thiol-ene reactions between
multifunctional thiol compounds and vinylsilane
coupling agents and sol-gel reactions using alkoxysilylgroups was examined.8-10) Vinyltrimethoxysilane wasused as a vinylsilane coupling agent and was mixed withtrimethylolpropane tris(3-mercaptopropionate) (TMMP)
as a multifunctional thiol compound. The solution
was irradiated with light, and the thiol-ene reactions
proceeded efficiently. However, the difference in reaction
rate compared to the sol-gel reactions (alkoxysilyl grouphydrolysis/condensation) meant that it was difficult toobtain a uniform, stable crosslinked film. Consequently,
vinyltriacetoxysilane (VTAS) was used as a vinylsilane
coupling agent with greater hydrolyzability. A small
amount of photo-acid generator was added, and theFig. 9 Photo-crosslinking Through Thiol-ene
Reaction/Simultaneous Sol-gel Reaction

generated acid caused the VTAS to smoothly hydrolyze
with moisture in the air. Efficient sol-gel reactions werethen observed (Fig. 9). Thiol-ene reactions using TMMP, a
trifunctional thiol, require three times the molar amount of
VTAS, and this hydrolysis produces 9 times molar amountof acetic acid. It is suggested that these features promote
sol-gel reactions. Organic-inorganic hybrids createdthrough two-dimensional photo-crosslinking reactions
comprising these photo-thiol-ene reactions and sol-gel
reactions are made up of covalent bonds. Acetic acid wasremoved by post heating, and transparent, odorless thinfilms were produced.
As shown in Table 3, these organic-inorganic hybrids weremade into hard coatings with a pencil hardness of about 2H
on a PET film and 4H to 5H on a glass substrate. These thinfilms had a refractive index of about 1.55. This relatively
high refractive index is due to sulfur element. As thiol-ene
organic-inorganic hybrid films with TMMP do not containany aromatic rings, resulting in a high Abbe number of
59.9, and it was also found that the creation of a coatingwith low chromatic dispersion is possible. Furthermore,mandrel bending test results showed excellent bending
properties. At the same time, it was also confirmed thatthe material exhibits high durability even against repeatedbending.
As described above, random-type polysilsesquioxane iseffective as an inorganic component for use in organicinorganic hybrids. Consequently, the preparation of
hybrids using thiol-containing polysilsesquioxane was
examined. Thiol-containing polysilsesquioxane wasobtained by performing hydrolysis and condensationfrommercaptopropyltrimethoxysilane(MPTMS).
By controlling the reaction conditions, random-type
polysilsesquioxane with a molecular weight of about 950

Table 3 Characteristics of Organic-inorganic Hybrids from Thiol-ene Reaction/So-gel Reaction

6

Polyfunctional
thiol

TEOS
(wt%)

Pencil hardness
on PET

Pencil hardness
on glass

Refractive index

Mandrel bend test
(mm)

TMMP

0

2H

5H

1.540

2

TMMP

17

3H

6H

1.539

2

DPMP

0

2H

5H

1.553

2

TEMPIC

0

2H

4H

1.558

2

to 1,000 was synthesized without gelation. Mixtures of thisthiol-containing polysilsesquioxane and polyfunctional
olefins such as triallyl isocyanurate (TAIC) were irradiatedwith light, and photo-crosslinked organic-inorganic hybrids
with high transparency, high refractive indices, high heat
resistance, and low shrinkage were produced throughphoto-thiol-ene reactions. (Fig. 10).11, 12) The progression of
thiol-ene reactions was confirmed by the decrease of S-H
and allyl C=C peaks in the Raman spectroscopy. Basedon the UV-visible absorption spectrum shown in Fig. 11,
the photo-crosslinked organic-inorganic hybrid films(thickness 30 μm) were found to have high transparencyof 95% or more in the visible light region. The solventfree viscous liquids of these compositions were dripped
onto glass plates and cured with UV-light, a process
which enabled lens shapes to be formed with ease. The
refractive index and Abbe number of these hybrid films
were 1.56 and 46 respectively. These relatively high values
indicate that these materials can be expected to contributeto the development of materials for optical applications.
In addition, curing shrinkage was evaluated throughcomparison with a DPHA photo-radical polymer coated on
a PET film. While warping occurred due to large curing
shrinkage with the photo-cured DPHA film, the hybrid
films made from thiol-containing silsesquioxane and
TAIC demonstrated low curing shrinkage and exhibited

almost no warping. It was also confirmed that the coating
film had excellent flexibility and high durability against
bending. These characteristics due to crosslinking throughthiol-ene reactions and are of significant interest. Witha pyrolysis temperature of over 300°C, the excellent
heat resistance is another important characteristic. The
organic-inorganic hybrid materials developed in this casehave high flexibility and are useful as a flexible coating
materials. Imparting more flexibility can be expected toresult in demonstration of self-healing properties. It was
subsequently confirmed that adding more polyfunctional
thiol results in high elasticity and self-repair of dents.13)
As is shown in the estimated structure of the threedimensional crosslinked hybrid shown in Fig. 12, it isbelieved that hard silsesquioxane is bound by soft sulfide.
A metal needle was used to scratch the surface of organicinorganic hybrid thin films. Observation with a laser
microscope showed that these scratches were repaired
naturally in about 10 minutes. An example is shown in
Fig. 13. With these hybrid thin films, polysilsesquioxane
surface segregation has also been confirmed, which has
also been found to lead to the development of self-healing
films with low-friction surfaces.

Fig. 12 Estimated Structure of Ternary
Crosslinked Hybrid Created from Thiolcontaining Polysilsesquioxane

Fig. 10 Organic-inorganic Hybrids Prepared via
Thiol-containing Polysilsesquioxane/TAIC
Thiol-ene Reactions

Fig. 11 UV-visible Spectra of Thiol-containing
Polysilsesquioxane/TAIC Hybrid Film

Fig. 13 Self-healing Behavior of Hybrid Thin
Films Created from Thiol-containing
Polysilsesquioxane7

Conclusion
Photo-crosslinking is a useful method in the design and manufacture of various materials, including coatings and adhesives.
This article outlined four types of photo-crosslinking reaction and described the preparation and properties of organicinorganic hybrids made using each reaction. By binding inorganic nanomaterials to crosslinked polymers, organic-inorganic
hybrids are attractive as materials that exhibit complementary properties and new functions. The organic-inorganic hybrids
generated through photo-crosslinking reactions described in this article facilitate the production of functional materials.
The process of simultaneously photo-curing acryl groups, epoxy groups and alkoxysilane groups contained in a silane
coupling agent to produce organic-inorganic hybrids is also beneficial in terms of practical considerations such as energy
saving and environmental concerns. Organic-inorganic hybrids obtained by photo-cationic polymerization of bisphenyl
fluorenedin glycidyl ether, a monomer with a high-refractive index, and epoxy-containing polysilsesquioxane are expected
to provide hard coatings with high refractive indices. In addition, organic-inorganic hybrids that can be generated bysimultaneous thiol-ene/sol-gel binary reactions demonstrated a high refractive index as a result of the effects of sulfur
while also exhibiting the contradictory properties of hard coating and flexibility. Thiol-ene reaction crosslinking with thiolcontaining silsesquioxane was shown to produce organic-inorganic hybrids with high refractive indices and toughness.
Thin films with a controlled refractive index are used for applications such as anti-reflection films and ITO refractive index
matching layers. They make a significant contribution to improving the visibility of devices, making them an indispensable
component in information terminals. These flexible organic-inorganic hybrid materials with high refractive indices can be
expected to contribute to the development of new optical materials.
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Kimihiro Matsukawa
Specially Appointed Professor, Materials Innovation Lab, Kyoto Institute of Technology
President, Matsukawa R&D Solutions● ThreeBond is a trademark or registered trademark of ThreeBond Holdings Co., Ltd.

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