The Reaction of Sulfur and Sulfur Compounds with Olefinic Substances. Part XII. Effects of Vulcanizing Additives on the Reaction of Sulfur with 2,6-Dimethylocta-2,6-Diene, and Their Bearing on the Mechanism of Sulfur Vulcanization of Natural Rubber

Abstract

Abstract To summarize, we conclude that the course of the interaction of sulfur with an olefin alone is determined principally by the electrophilic reactivity of labile polysulfides and by the reactions of cationic-type intermediates, and that the essential function of zinc salts is to stimulate the reactivity of complementary nucleophilic species. Accelerators of the thiazole-type specifically promote the latter reactivity, as almost certainly do other common vulcanization accelerators, e.g., certain amines, metal dithiocarbamates and xanthates, and thiuram sulfides. It may be noted, for example, that reaction of 2,6-dimethylocta-2,6-diene with sulfur, zinc dibutyldithiocarbamate, and zinc oxide at room temperature, i.e., under conditions where direct sulfur-olefin reaction does not occur, yields a dialkenyl polysulfide and negligible cyclic sulfide (a result also found with tetramethylthiuram disulfide and zinc oxide plus zinc propionate at 140° C), indicative of sulfuration by the nucleophilic mechanism becoming almost fully dominant under especially favorable reaction conditions.