Zinc(II)-catalyzed disproportionation in rubber: The mechanism of sulfur vulcanization revisited

Article abstract of DOI:10.1002/chem.19970031117

Model studies have shown that cross-link precursors, that is, intermediates in the sulfur vulcanization of rubber, are transformed into cross-links by a nonsymmetric but regioselective disproportionation mechanism. Thus, two equivalents of the cross-link precursor of the type R-S-S-X are transformed into X-S-X and the actual cross-link R-S-S-S-R. Exchange of sulfur atoms is a prerequisite. A mechanism involving an S(N)i' reaction with an allylic moiety, suggested in the literature, has not been observed. The disproportionation reaction is catalyzed by rubber-soluble zinc-dithio-carbamate complexes, an important class of vulcanization accelerators. By virtue of ligand-functional-group exchange reactions these complexes serve to transport and exchange sulfur atoms.