Effects of Thiol Substitution on the Kinetics and Efficiency of Thiol-Michael Reactions and Polymerizations

The kinetic effects of the substitution and functionality of the thiol in thiol-Michael reactions were investigated using model monofunctional thiols and multifunctional thiols used in various cross-linking polymerizations. The differences in kinetic rates and final conversions were observed via Fourier transform infrared spectroscopy. The shelf life of these polymers and their mechanical properties were analyzed using a rheometer to measure viscosity changes over time. It was concluded that for monofunctional systems, the reaction rate is dependent on both electronic and steric interactions. For systems with a propagation rate-limiting step (propionate), the secondary thiol was faster than the primary thiol due to increased reactivity of the thiolate anion, by as much as much as a 60% increase in the rate. However, more sterically hindered internal alkenes resulted in primary and secondary rates about equal to each other. For systems with a chain transfer-limiting step (alkyl thiol), the rate was dependent on the p K a of the thiol and ease of deprotonation; in these cases, the primary thiol was the fastest. Though primary and secondary thiols had relatively mild differences in rates, reactions of tertiary thiols were slower than either of the others. For polymerizing systems using multifunctional thiols, the results varied depending on the substitution and functionality. When reacting with a difunctional alkene, the secondary thiol was 74–95% faster than the primary thiol, depending on the type of thiol assessed, and as the functionality of the alkene increased, the rates became more comparable. In the tetrafunctional alkene systems, the primary thiol was 57% faster than the secondary thiol. The shelf life of the systems produced varied results. Typically, in systems with the difunctional thiol, the primary thiol formulation was significantly less stable and gelled more rapidly than the resin with the corresponding secondary thiol. However, in the tetrafunctional thiol systems, the resin containing the secondary thiol gelled more rapidly than that containing the primary thiol. All systems typically gelled within 30 days regardless of substitution, although no additional formulation adjustments were made to stabilize any of these systems beyond changing the thiol structure.