Modeling Diffusion-Limited Kinetics of Thiol–Ene Photopolymerizations

A reaction kinetic model incorporating free volume theory was developed to elucidate and predict diffusion-controlled kinetics and attainment of limited functional group conversions in radical-mediated thiol–ene photopolymerizations. The glass transition temperature (Tg) of the reaction mixture was experimentally correlated with conversion by using a combination of photo-DSC and temperature-sweep DSC measurements. The subsequently determined Tg values of the thiol–ene monomer mixture (0% conversion) and fully polymerized photopolymer (100% conversion) were incorporated into the kinetic model via free volume theory, enabling a highly accurate prediction of both the kinetics and the ultimate conversion, within 5% of experimental values. Leveraging this accurate description of the kinetics, systematic simulations were conducted to evaluate the impact of the cross-linking density, reaction temperature and plasticizer loading on thiol–ene photopolymerization kinetics. Notably, the model predicts thiol–ene reaction mixtures at limited conversions with Tg at or below the reaction temperature, a phenomenon rarely observed in chain-growth polymerizations, where Tg typically exceeds the reaction temperature under diffusion-controlled conditions. These findings provide critical insights into thiol–ene photopolymerizations and highlight potential strategies for thiol–ene photopolymer development in diverse applications where diffusion-limited kinetics constrains the material performance.