Vulcanization Accelerator and Activator Complexes. 2. Chemistry of Amine and Zinc Carboxylate Complexes of Zinc and Cadmium Benzothiazolyl Mercaptides

The behavior of zinc benzothiazolyl mercaptide (ZMBT) and its complexes with amines and zinc carboxylates in effecting nucleophilic displacement reactions related to those which occur during accelerated sulfur vulcanization has been studied. Ligands influence the activity of ZMBT in nucleophilic displacement reactions (a) by increasing the solubility of ZMBT in the reaction medium and (b) by increasing the polarity of the Zn—S bond; this increase in polarity only enhances the intrinsic activity of the catalyst if a second zinc species (e.g., a zinc carboxylate) is present. Complexes of zinc carboxylates with ZMBT are very active catalysts for disulfide interchange reactions, but not for sulfur insertion reactions. In the latter, an amine is also necessary to act as a co-catalyst with traces of hydrogen sulfide in order to convert octa-atomic sulfur into a more reactive open-chain polysulfide species. A combination of ZMBT, an amine and a zinc carboxylate provides the best catalyst for effecting sulfur insertion reactions at high temperatures; the active species may be a ternary complex. Amine complexes of ZMBT, either in the absence or presence of a zinc carboxylate, are good accelerators for the vulcanization of cis-1,4-polyisoprene with sulfur.