ZINC(II)-CATALYZED DISPROPORTIONATION IN RUBBER - THE MECHANISM OF SULFUR VULCANIZATION REVISITED

Model studies have shown that cross-link precursors, that is, intermed iates in the sulfur vulcanization of rubber, are transformed into cros s-links by a nonsymmetric but regioselective disproportionation mechan ism. 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, Ex change 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 rub ber-soluble zinc-dithiocarbamate complexes, an important class of vulc anization accelerators. By virtue of ligand-functional-group exchange reactions these complexes serve to transport and exchange sulfur atoms .