Photoinduced electron transfer polymerization. 4. 4-carboxybenzophenone-sulfur-containing carboxylic acids photoredox pairs as a photoinitiating system for free-radical polymerization

A series of sulfur-containing carboxylic acids (SCCA) was investigated as electron donors in photoinduced free-radical polymerizations, in conjunction with 4-carboxybenzophenone (CB) as sensitizer. These carboxylic acids included (phenylthio)acetic acid, S-benzylthioglycolic acid, 4-(methylthio)phenylacetic acid, 2-(methylthio)ethanoic acid, 4-(methylthio)benzoic acid, 2,2'-thiodiethanoic acid, and 3,3'-thiodipropionic acid. The results were compared to the simplest thioether, dimethyl sulfide. The mechanism of the radicals' formation was established using nanosecond laser flash photolysis and photochemical steady-state measurements of carbon dioxide formation. Photopolymerizations were carried out in aqueous solutions of acrylamide. Several conclusions follow from the experimental data: (1) The rates of polymerization of the CB/SCCA/acrylamide systems are affected by the yields of secondary processes that follow photoinduced electron transfer, e.g. decarboxylation or deprotonation. This is in addition to the reactivity of the free radicals themselves. (2) The highest initiation yield is observed for the system where there is efficient diffusion apart of the radical-ion pairs, followed by efficient decarboxylation. The resulting radicals are localized on carbons adjacent to the sulfur atom. (3) Free radicals with aromatic moieties are better initiators than are the analogous aliphatic radicals. (4) Radicals that retain the carboxylate moiety are inefficient initiators. (5) Photopolymerization of these systems appears to proceed by the conventional mechanism where termination is bimolecular.