3. Application of photopolymerization
technology
As a widely-used technology, photopolymerization technology has been
used to prepare a variety of products, including coatings, inks,
adhesives, printing materials and other applications since Inmont
corporation obtained the first UV-curable ink in 1946. In addition to
these traditional applications mentioned above, today
photopolymerization has been given new applications in thiol-ene click
reaction, 3D printing, photoresist, hydrogels and other fields, which
will be described in detail below.
3.1. Thiol-ene click
reaction
The researches on the reaction between thiols and carbon-carbon double
bond has a history of one hundred years.[64] In
1905, Posner et al. first carried out the researches on the chemical
reaction between thiols and enes,[65-66] then,
Kharasch et al. proposed the reaction mechanism of thiols and enes
monomers in 1938,[67] which is still widely
accepted today. The photopolymerization mechanism of thiols and enes is
demonstrated in Figure 14,[67-68] first step, the
sulfur radicals are generated by the hydrogen abstraction reaction with
the free radicals which originate from the cleavage of PIs under
irradiation at light sources. The second step, sulfur radicals attack
the carbon-carbon double bond and produces alkyl radicals,
simultaneously, alkyl radicals can also react with thiols to form sulfur
radicals, thus continuously cycling. The third step, radicals combine
with each other to terminate the polymerization reaction.
Figure 14 Photopolymerization mechanism between thiols and enes.
The field of click chemistry has been rapid development since Sharpless
first proposed the new concept in 2001.[64, 69]Reaction of click chemistry has many advantages including high yield,
short reaction time, solvent tolerance, a wide variety of functional
groups, regional and chemical selectivity, insensitivity to oxygen, easy
purification and atomic economy up to 100 %.[64,
70-72] The photopolymerization of thiols and enes carry many of the
attributes of click chemistry, and has been applied in many fields such
as hydrogels, dental prosthetics, optical materials, bonding, coatings
and so on.
In recent years, preparation of organosilicon polymers by UV
photopolymerization obtained more and more attention. However, the
polymerization reactions between organosilicon monomers modified with
acrylate are sensitive to oxygen, resulting in sticky surface and even
failure of curing process. On the other hand, the shrinkage rate of the
material after curing is very large, which may lead to the deformation
of material surface.[73] Cationic curing is not
sensitive to oxygen,[74] however, its photocuring
rate is lower than radical curing. Based on these limitations, Wang et
al.[68] synthesized the novel sulfhydryl
functionalized fluorosilicone polymer (PTFPMS-SH) and a series of
fluorosilicone polymers (PTFPMS-Vi) with different vinyl contents, then
the curing coatings are obtained by thiol-ene click reaction, as
displayed in the Figure 15. The results show that the conversion of C=C
bond can reach more than 95 % within 5 s, and the obtained
fluorosilicone coatings show a transparency greater than 90 %. The
contact angle of the coatings ranges from 100° to 110°, which showing
excellent hydrophobicity. In addition, the fluorosilicone coatings show
potential application as a protective coating for electronic devices due
to excellent corrosion resistance.