Effects of 1°, 2°, and 3° Thiols on Thiol–Ene Reactions: Polymerization Kinetics and Mechanical Behavior

The effect of thiol substitution in radical thiol–ene reactions has been studied by using model, monofunctional thiols as well as multifunctional thiol monomers along with the assessment of their subsequent polymerization reactions and polymer mechanical behavior. FT-IR was used to monitor the polymerization rate and quantify the overall conversion. While the total conversion was observed to range from 70% to 100%, the polymerization rate was found to decrease by as much as 10-fold as the thiol substitution was changed from primary to tertiary. Analogous multi-thiol monomers of similar structure but varying substitution were synthesized to observe the effect of substitution type on polymerization kinetics and polymer behavior. Methylation at the α-carbon was varied from primary to tertiary to observe these differences. Mechanical properties were assessed by using dynamic mechanical analysis and water sorption experiments, where the glass transition temperatures were found to be within 1–2 °C as thiol substitution varied. Furthermore, primary thiol films absorbed 1–3% more water than secondary thiol films. Resin shelf stability experiments were performed by using rheometry to measure storage time-dependent viscosity changes, and it was found that secondary thiol films remained relatively stable for up to 100 times longer than their primary counterparts. It was concluded that while there are differences under relatively slow initiation conditions, at typical initiation rates all three thiol substitutions may be made to react at similar rates for both monofunctional and polymeric systems.