Photopolymerization of vinyl donor/vinyl acceptor systems and thiol-ene systems
Despite the increased productivity afforded by photoinitiated systems, certain large areas of the coatings industry have not enjoyed the benefits of photocuring due to symptomatic problems associated with traditional photopolymerization. Four major problems with the traditional photoinitiator systems are light reactive photoinitiator molecules left in the cured polymer matrix, leachable small molecule photo by-products, the high cost of photoinitiators, and the inhibiting effect of oxygen on curing. This prevents photocuring from being used in numerous applications: outdoor coatings, food packaging, and rapid curing of ultrathin or thick films. The investigation reported herein attempts to use electron donor/acceptor complexes (EDA complexes) and related molecular associations to circumvent these symptomatic problems. Two similar areas were investigated: the effects of Lewis acids on photopolymerization of EDA monomer systems and thiol-ene photopolymerizations. Lewis acids were used to enhance formation of EDA complexes, resulting in higher concentrations of EDA complex and increased rates of both thermal and photopolymerization. Photobleaching and thick cures of 1 cm were observed; however, the Lewis acid enhanced EDA systems also displayed large decreases in reactivity upon absorption of water, and Lewis acid modified monomer mixtures had substantial dark thermal cures that reduced shelf life to just hours. Thiol-ene photopolymerizations were investigated to establish the donor/acceptor character of these systems. UV-Vis spectroscopy did not identify observable EDA complexes except for thiol-styrene. However, thiol-ene systems were found to have many characteristics that classify them as weak EDA complexed systems. Various aspects of thiol-ene polymerizations were also investigated. Viscosity, monomer functionality, and ionization potential of the monomers were found to greatly influence the rate of polymerization. For both types of systems, the donor/acceptor character of the monomers afforded high rates of polymerization and allowed for photopolymerization without added photoinitiator. However, in the case of thiol-ene systems, direct participation of EDA complexes in initiating or propagation of free-radical chain processes was not proven.