Relative reactivity and selectivity of vinyl sulfones and acrylates towards the thiol-Michael addition reaction and polymerization

The reactivity, selectivity and kinetics of vinyl sulfones and acrylates in base and nucleophile-catalyzed thiol-Michael addition reactions were examined in detail in this study. The vinyl sulfones react selectively and more rapidly with thiols in the presence of acrylates, which was clearly indicated from reactions of hexanethiol (HT), ethyl vinyl sulfone (EVS) and hexyl acrylate (HA) at a molar ratio of 2 : 1 : 1. EVS reaches 100% conversion with minimal consumption (<10%) of HA, which demonstrates the high selectivity of vinyl sulfones over acrylates. The reaction rate of EVS with HT was approximately 7 times higher than that of HA. A detailed study of the kinetics of the nucleophile-catalyzed thiol-Michael addition reaction was carried out, and it was shown that the delay observed in the initial stages of the nucleophile-catalyzed thiol-Michael addition reaction is due to the relatively slow attack of the nucleophiles on the vinyl. The presence of protic species other than thiols in the reaction mixture has also been shown to significantly impede the reaction rate, and in extreme cases, has been shown to inhibit the Michael addition reaction. These results provided a better understanding of the conditions under which the thiol-Michael addition reaction can or cannot be considered as a click reaction. Finally, the high reaction selectivity of vinyl sulfones over acrylates via thiol-Michael addition reaction in ternary systems is used to control gelation behavior in crosslinked polymer networks formed by thiol-Michael addition reactions.