Thiol−Ene Photopolymerization Mechanism and Rate Limiting Step Changes for Various Vinyl Functional Group Chemistries

Abstract

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 (kp/kCT). Ratios of kinetic parameters (kp/kCT) were found to vary significantly with the ene functional group chemistry and to have a dramatic impact on polymerization kinetics. For high ratios of kp/kCT, polymerization rates are first order in thiol functional group concentration and nearly independent of ene functional group concentration. For kp/kCT values near unity, polymerization rates are approximately 1/2 order in both thiol and ene functional group concentrations. When kCT is much greater than kp, 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 (Rp ∝ [SH]). For thiol−norbornene and thiol−vinyl ether systems, polymerizations are nearly 1/2 order in both thiol and ene functional group concentrations (Rp ∝ [SH]1/2[CC]1/2). In thiol−vinyl silazane systems, polymerization rates are approximately first order in ene functional group concentration (Rp ∝ [CC]) and independent of thiol functional group concentration. A theory is proposed which states that the effect of functional group chemistry on kp/kCT is controlled primarily by ene functional group electron density (kp) and carbon radical stability (kCT).