Visible light induced free radical promoted cationic polymerization using thioxanthone derivatives

The free radical promoted cationic polymerization cyclohexene oxide (CHO), was achieved by visible light irradiation (υ_inc = 430-490 nm) of methylene chloride solutions containing thioxanthone-fluorene carboxylic acid (TX-FLCOOH) or thioxanthone-carbazole (TX-C) and cationic salts, such as diphenyliodonium hexafluorophosphate (Ph₂I⁺PFâ6-) or silver hexafluorophosphate (Ag⁺PFâ6-) in the presence of hydrogen donors. A feasible initiation mechanism involves the photogeneration of ketyl radicals by hydrogen abstraction in the first step. Subsequent oxidation of ketyl radicals by the oxidizing salts yields Bronsted acids capable of initiating the polymerization of CHO. In agreement with the proposed mechanism, the polymerization was completely inhibited by 2,2,6,6-tetramethylpiperidinyl-1- oxy and di-2,6-di-tert-butylpyridine as radical and acid scavengers, respectively. Additionally polymerization efficiency was directly related to the reduction potential of the cationic salts, that is, Ag⁺PF ₆- (Ea_red^1/2 = +0.8 V) was found to be more efficient than Ph₂I⁺PF₆- (E_red ^1/2 = -0.2 V). In addition to CHO, vinyl monomers such as isobutyl vinyl ether and N-vinyl carbazole, and a bisepoxide such as 3,4-epoxycyclohexyl- 3′,4′-epoxycyclohexene carboxylate, were polymerized in the presence of TX-FLCOOH or TX-C and iodonium salt with high efficiency.