An Appropriate Model Compound for the Accelerated Sulfur Vulcanization of Polyisoprene: I. The Mechanism of Bisbenzothiazole-2,2′-Disulfide Accelerated Vulcanization of Squalene in the Absence of ZnO

Model compound vulcanization is a useful technique for studying the mechanism of accelerated sulfur vulcanization. A technique based on gel permeation chromatography is presented which allows for the use of squalene as model compound. Squalene is shown to be a more realistic model compound for polyisoprene vulcanization in that the release of 2-mercaptobenzothiazole accompanies the formation of crosslinks. This situation accurately mirrors the situation in polyisoprene, unlike simpler model compounds such as 2-methyl-2-pentene. It is likely that a realistic model compound requires the presence of at least 2 isoprene units to allow for reactions at adjacent methylenic carbons. A mechanism for bisbenzothiazole-2,2′-disulfide vulcanization is proposed where the reaction of bisbenzothiazole-2,2′-polysulfides with squalene occurs in a concerted fashion to produce pendent groups and no 2-mercaptobenzothiazole. These pendent groups then react slowly with methylenic hydrogens on squalene to produce initial crosslinks and 2-mercaptobenzothiazole. The latter then reacts, in the presence of sulfur, with squalene to produce polysulfidic thiols which in turn react rapidly with benzothiazole groups to form further crosslinks and more 2-mercaptobenzothiazole. The whole process is autocatalytic.