The Mechanism of Zinc(II)-Dithiocarbamate-Accelerated Vulcanization Uncovered; Theoretical and Experimental Evidence

Abstract

The mechanism of cross-link formation in sulfur vulcanization mediated by bis(dimethyldithiocarbamato)zinc(II), ZDMC, has been uncovered, utilizing a combination of Density-Functional calculations and model experiments. These studies have revealed that, in a three-stage process, ZDMC exhibits a unique combination of catalytic activity: (1) It mediates the reaction between sulfur and rubber. This is achieved by incorporating sulfur atoms in the zinc-dithiocarbamate ring and inducing their insertion into an allylic C−H bond via an ene-like reaction. This ene reaction yields a rubber-bound polythiothiol and is only slightly endothermic, even though an activation energy of ∼90 kJ mol-1 is required. (2) The resulting polythiothiols engage in equilibrated metathesis reactions to yield polysulfides, the initial sulfur cross-links. (3) In a hitherto unsuspected mechanistic step ZDMC has been found to shift the metathesis equilibrium to the side of cross-links by mediating desulfhydration of the polythiothiols, producing sulfides and H2S. Thus, the combined results of theoretical and experimental work have allowed to put forward a novel mechanism for ZDMC-mediated sulfur cross-link formation that successively comprises (a) homogeneous catalysis of thiol formation from sulfur and rubber, (b) an equilibrium between polythiothiol intermediates and cross-links and (c) ZDMC-induced desulfhydration.