Photocatalysis of viologens for photoinitiated polymerization using carboxylic acid as electron donors

Abstract

Hydroxyl radicals (OH), together with carbon-centered radicals (R), derived from photoinduced electron transfer (PET) interactions between viologen and carboxylic acid ion pairs ([bipy++⋯RCOOH]), were developed to photoinitiate the polymerization of various acrylate monomers in the presence of oxygen. The mechanism of the radicals’ formation was established using nanosecond laser flash photolysis and electron spin resonance methods. Carboxylate anion (electron donnor, D) gave one electron to viologen (electron acceptor, A) via either intermolecule (D/A) or intramolecular (D-A) PET, to yield carboxylate radicals (RCOO) that later underwent decarboxylation, yielding R radicals which can initiate the photopolymerization of many acrylate monomers. Simultaneously, reduced viologen cation radical (bipy+) was oxidized by dissolved O2 and returned back to bipy++ dications. OH radicals emerged from the reaction of H+ with the superoxide radicals (O2−) that formed during the oxidation of bipy+.. In this way, viologens were observed to act as photocatalysts in the radical generation process based on their strong electron acceptor, as well as efficient light absorption nature. Photopolymerization performance with D-A type viologens, for instance, bipy++−(CH2)nCOOH, extra carboxylic acid was not a requirement.