Redox initiation of bulk thiol–ene polymerizations

The unique formation–structure–property attributes and reaction behavior of the thiol–ene “click” reaction have been explored extensively for photochemically and thermally initiated reactions but have been much less explored for redox initiation. Therefore, the objective of this work is to characterize fully the impact of the initiation system, monomer structure, degree of functionalization, and inhibitor level on the redox-mediated thiol–ene polymerization rate and behavior. Moreover, this study confirms the ability of redox initiation to achieve full conversion of desired thiol–ene “click” products for small molecules in solution. For the multifunctional thiol–ene systems, polymerization rate was shown to be comparable to photo- and thermally initiated systems, but with the additional advantages of unlimited depth of cure and mild reaction conditions. Additionally, the network properties of the redox-initiated thiol–ene systems were on par with a photocured material formulated with identical monomers and radical initiating potential. Lastly, control over the polymerization rate and preceding induction period was garnered from the concentration of inhibitor included in the reaction mixture. The mechanism of action of quinone inhibition in redox-mediated thiol–ene polymerizations is shown to depend on both the presence of an aniline reducing agent and the concentration of inhibitor, with quinone concentrations in great excess of oxidizing agent concentrations actually leading to heightened polymerization rates when aniline is present.