Photoinitiator-free radical photopolymerization using polybrominated and polychlorinated aromatic methacrylates: Investigations on the mechanisms of initiation

Abstract

In this study, pentabrominated and pentachlorinated phenyl methacrylate (PBPMA, PCPMA) were studied with respect to their suitability to initiate a photoinduced radical polymerization of an acrylic formulation without conventional photoinitiator. Real-time FTIR spectroscopy (RTIR) was used to study the kinetics of the polymerization reaction. Both halogenated monomers were found to be effective as initiators. Moreover, different experimental techniques such as laser flash photolysis (LFP), product analysis by GC/MS and FTIR spectroscopy as well as quantum chemical modeling were used to elucidate possible reaction pathways for the initiation of a photopolymerization reaction. A release of bromine radicals was observed by LFP after irradiation of PBPMA, which may initiate the polymerization similarly to the tetrabromophenyl methacrylate radicals. In contrast, no formation of free chlorine radicals from PCPMA was detected. Rather, GC/MS measurements after stationary photolysis indicated the formation of bicyclic isomers from PCPMA upon irradiation. Extensive quantum chemical modeling substantiated the formation of two different isomers. Accordingly, we propose that relocalization of the chlorine atom from the ortho position and subsequent ring closure via the methacrylate group may occur upon irradiation, which finally results in the formation of two bicyclic isomers with either a five- or a six-membered lactone ring. Spectra simulation and additional experiments by FTIR spectroscopy seem to indicate that the initiation of the photopolymerization with PCPMA might preferably proceed via the bicyclic molecule with the six-membered lactone ring and subsequent radical formation.