Zinc(II)‐Catalyzed Disproportionation in Rubber: The mechanism of Sulfur Vulcanization Revisited

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 crosslink 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 SNi′ reaction with an allylic moiety, suggested in the literature, has not been observed. The disproportionation reaction is catalyzed by rubber‐soluble zinc‐dithiocarbamate complexes, an important class of vulcanization accelerators. By virtue of ligand‐functional‐group exchange reactions these complexes serve to transport and exchange sulfur atoms.