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.