Molecular Weight Development during Thiol−Ene Photopolymerizations

A kinetic model is presented for the pregelation period of thiol−ene photopolymerizations utilizing multifunctional thiol and ene monomers. The model involves the moment equations for thiyl radicals, carbon radicals, and polymers, and it predicts the chain length averages as a function of the reaction time or functional group conversion. Cyclization reactions are taken into account with the assumption that the probability of cyclization is proportional to the number of local pendant vinyl groups connected to the thiyl radical center. The cyclization reaction is arbitrarily truncated at a critical size, N, in that only cycles with sizes less than N are allowed during gelation. The relative rate constant of cyclization with respect to propagation was calculated using the kinetic model and the experimental gelation data obtained for a thiol−ene system consisting of divinyl and trithiol monomers. It was shown that if the functional group conversion is taken as the independent variable, the reaction kinetics of the ring-free thiol−ene system is identical to that of usual step-growth reactions. In the time scale of the reactions, however, the rate expressions for intermolecular reactions in thiol−enes are first-order reactions overall, and the reaction rates as well as the gelation times can easily be manipulated by the kinetic parameters. The cyclization rate in thiol−ene reactions is slower compared to that in step-growth reactions. This result arises from intramolecular chain transfer reactions reducing the probability of favorable intramolecular collisions between the functional groups.