Thiol-ene photopolymerization mechanism and rate limiting step changes for various vinyl functional group chemistries

The mechanism and kinetics of thiol-ene photopolymerizations utilizing a tetrafunctional thiol monomer copolymerized with acrylate, norbornene, vinyl ether, and vinyl silazane functionalized ene monomers are successfully modeled and experimentally characterized. Modeling predictions demonstrate that the reaction orders in thiol-ene systems are controlled by the ratio of thiyl radical propagation to chain transfer kinetic parameters (k(p)/k(CT)). Ratios of kinetic parameters (k(p)/k(CT)) were found to vary significantly with the ene functional group chemistry and to have a dramatic impact on polymerization kinetics. For high ratios of k(p)/k(CT), polymerization rates are first order in thiol functional group concentration and nearly independent of ene functional group concentration. For k(p)/k(CT) values near unity, polymerization rates are approximately 1/2 order in both thiol and ene functional group concentrations. When k(CT) is much greater than k(p), polymerization rates are first order in ene functional group concentration and nearly independent of the thiol functional group concentration. In thiol-allyl ether and thiol-acrylate systems, the step growth polymerization rates are first order in thiol functional group concentration (R-p proportional to [SH]). For thiol-norbornene and thiol-vinyl ether systems, polymerizations are nearly 1/2 order in both thiol and ene functional group concentrations (R-p proportional to [SH](1/2)[C=C](1/2)). In thiol-vinyl silazane systems, polymerization rates are approximately first order in ene functional group concentration (R-p proportional to [C=C]) and independent of thiol functional group concentration. A theory is proposed which states that the effect of functional group chemistry on k(p)/k(CT) is controlled primarily by ene functional group electron density (k(p)) and carbon radical stability (k(CT)).