Chain transfer agents in vinyl polymerizations photoinduced by bimolecular photoinitiators

The behavior of 2-mercaptoethanol and phenols as chain transfer agents in the polymerization of methyl methacrylate and acrylamide at 25 °C photoinitiated by bimolecular initiators based on xanthene dyes and aromatic ketones in the presence of amine was studied. Addition of millimolar concentrations of 2-mercaptoethanol or phenol to the methyl methacrylate polymerization reduced notably the polymer molecular weights, with minor changes of the polymerization rate. The photochemical behavior of the initiators in the polymerization media was analyzed. These studies allowed explaining the behavior of alkyl thiols and phenols as chain transfer agents in terms of the competition of the coinitiator and chain transfer agents to deactivate the triplet state of the photoinitiator, and the characteristics of the intermediates formed in the deactivation processes. Chain transfer constants were determined, and the values are similar to that obtained in the polymerization photoinduced by the α-cleavage of aliphatic azo compounds.