Role of thiol oxidation by air in the mechanism of the self-initiated thermal thiol-ene polymerization

A spontaneous thiol-ene polymerization has been reported in the literature for a broad range of monomers, generally under conditions avoiding solvents, heating and the intentional formation of radicals. In the search for a mechanism accounting for the self-initiated thiol-ene polymerization, this study demonstrates the significant role played by thiyl radicals produced by thiol oxidation through dissolved atmospheric oxygen in the thiol-ene mixture. To determine the factors underlying the reactivity, self-initiated polymerization kinetics, thiol solvation and the nature of initiating radicals are studied by means of various techniques including real-time FTIR, 1 H NMR spectroscopy and spin-trapping experiments. Our data support that the driving force for thiol air oxidation is the polarization of the S-H bond. Thiol substituents with electron-withdrawing properties and/or hydrogen-bonded thiol-solvent (or thiol-alkene) complexes favor self-initiation by increasing S-H bond polarization, thereby easing the electron transfer to molecular oxygen. We highlight the role of thiyl initiating radicals generated by air oxidation of thiols in the mechanism accounting for the self-initiation of thiol-ene polymerization. The driving force of this reaction is polarization of the S-H bond.