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/k CT). 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 k p/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 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 (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[C=C]1/2). In thiol-vinyl silazane systems, polymerization rates are approximately first order in ene functional group concentration (Rp ∝ [C=C] 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).