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 ( 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 ∝ [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 ∝ [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 ∝ [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 ).