Photoacid Generation by Stepwise Two-Photon Absorption: Photoinitiated Cationic Polymerization of Cyclohexene Oxide by Using Benzodioxinone in the Presence of Iodonium Salt

Photoinitiated cationic polymerization1-4 is of great practical interest due to their applicability for the curing of coatings and printing inks and for resist technology. The major classes of photoinitiators which can successfully initiate cationic polym-erization include diaryliodonium,5 triarylsulfonium,6 and alkoxy-pyridinium7,8 salts. Besides their direct activation (direct light absorption), several indirect sensitization methods are reported to extend photopolymerization sensitivity to longer wavelengths. This way practical application of the photoinitiated cationic polymerization can become acceptable since medium- and high-pressure mercury lamps that are frequently used as light sources provide emissions at 313 and 366 nm. Successfully employed indirect ways include oxidation of photochemically generated free radicals,9 electron-transfer reactions in the exciplexes,10-12 and charge-transfer complexes.13,14 Among these methods, free-