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 e lectron donors in photoinduced free-radical polymerizations, in conjunction with 4-carboxybenzophenone (CB) as sensitizer. These carboxylic acids incl uded (phenylthio)acetic acid, S-benzylthioglycolic acid, 4-(methylthio)phen ylacetic acid, 2-(methylthio)ethanoic acid, 4-(methylthio)benzoic acid, 2,2 '-thiodiethanoic acid, and 3,3'-thiodipropionic acid. The results were comp ared to the simplest thioether, dimethyl sulfide. The mechanism of the radi cals' formation was established using nanosecond laser flash photolysis and photochemical steady-state measurements of carbon dioxide formation. Photo polymerizations were carried out in aqueous solutions of acrylamide. Severa l conclusions follow from the experimental data: (1) The rates of polymeriz ation of the CB/SCCA/acrylamide systems are affected by the yields of secon dary processes that follow photoinduced electron transfer, e.g. decarboxyla tion or deprotonation. This is in addition to the reactivity of the free ra dicals themselves. (2) The highest initiation yield is observed for the sys tem where there is efficient diffusion apart of the radical-ion pairs, foll owed by efficient decarboxylation. The resulting radicals are localized on carbons adjacent to the sulfur atom. (3) Free radicals with aromatic moieti es are better initiators than are the analogous aliphatic radicals. (4) Rad icals that retain the carboxylate moiety are inefficient initiators. (5) Ph otopolymerization of these systems appears to proceed by the conventional m echanism where termination is bimolecular.