Photopolymerization Using Thiol-Epoxy 'Click' Reaction: Anionic Curing through Photolatent Superbases

Photoinduced anionic curing of epoxides by thiols offers many advantages over traditional (cationic and radical) photochemical cross-linking processes. This includes insensitivity to air and moisture, low volume shrinkage, good adhesion to substrates through β-hydroxy thioether linkages, and often no requirement for a postexposure baking step. Thus, interest in the thiol-epoxy 'click' reaction for photopolymerization purposes has been growing steadily. In this regard, photolatent catalysts have been developed with the capability to generate strong organic bases (superbases) under illumination from UV to the visible and near-infrared range. Besides bulk polymerizations, the base-catalyzed ring-opening reaction can also be harnessed for lithography purposes to fabricate micro- and nanosized patterns. Use of hydrophilic monomers can lead to the preparation of hydrogels. The cross-linked networks can be incorporated with photosensitive monomers to afford photoactive properties. Alternatively, the thioether linkages can be addressed through sulfur alkylation. This post-cross-linking modification reaction transforms the neutral thermosets into zwitterionic sulfonium/carboxylate or cationic sulfonium salts. The former endows the materials with antibiofouling properties, while the latter endows them with antibacterial surface properties. Postfabrication transesterification reactions within the material, on the other hand, bring vitrimer properties to the network and allow for object reshaping. The concepts of shape memory polymers and 3D printing have also been established. The aim of this Perspective is to review this nascent but growing area of research with the help of key literature examples.