Thioxanthone-ethylcarbazole as a soluble visible light photoinitiator for free radical and free radical promoted cationic polymerizations

Abstract

Thioxanthone-ethylcarbazole (TX-EC), 7-ethylthiochromeno[2,3-b]carbazol-13(7H)-one, as a novel visible light absorbing one-component Type II photoinitiator with high solubility, was synthesized and characterized. Its ability to initiate free radical photopolymerization of methyl methacrylate (MMA) was demonstrated and compared with that of the parent thioxanthone-carbazole (TX-C) both in the presence and absence of a co-initiator. It is found that TX-EC is a more effective photoinitiator in all cases. The mechanism of initiating radical formation for free radical polymerization in the absence of an added co-initiator involves photoexcitation of TX-EC and hydrogen abstraction of triplet states of TX-EC from the ethyl group attached to the carbazole moiety. In the presence of triethyl amine, the corresponding hydrogen abstraction reaction occurs between a triplet photoinitiator and the amine compound. The possibility of visible light photoinitiation of free radical promoted cationic polymerization of several monomers, namely cyclohexene oxide (CHO), n-butyl vinylether (BVE), N-vinyl carbazole (NVC) and 3,4-epoxycyclohexyl-3′,4′-epoxycyclohexene carboxylate (EEC) using TX-EC and Ph₂I⁺PF₆⁻ combination was also demonstrated.