Complexation of the vulcanization accelerator tetramethylthiuram disulfide and related molecules with zinc compounds including zinc oxide clusters (Zn 4O4)

Abstract

Zinc chemicals are used as activators in the vulcanization of organic polymers with sulfur to produce elastic rubbers. In this work, the reactions of Zn2+, ZnMe2, Zn(OMe)2, Zn(OOCMe)2, and the heterocubane cluster Zn4O4 with the vulcanization accelerator tetramethylthiuram disulfide (TMTD) and with the related radicals and anions Me2NCS2 ., Me2NCS 3 ., Me2NCS2 -, and Me 2NCS3 - have been studied by quantum chemical methods at the MP2/6-31 + G(2df,p)//B3LYP/6-31+G* level of theory. More than 35 zinc complexes have been structurally characterized and the energies of formation from their components calculated for the first time. The binding energy of TMTD as a bidendate ligand increases in the order ZnMe2 < Zn(OOCMe)2 < Zn(OMe)2 < Zn4O 4 < Zn2+. The last two zinc species also form very stable complexes with the radicals Me2NCS2 . and Me2NCS3 .. Dissociation of the TMTD molecule at the S-S bond on reaction with the Zn4O4 cluster is predicted to be strongly exothermic, in sharp contrast to the endothermic S-S bond dissociation of the free molecule. The same holds for tetramethylthiuram trisulfide (TMTT). Surprisingly, the resulting complexes contain Zn-S as well as S-O bonds. The Zn4O4 nanocluster serves here as a model for bulk zinc oxide used as an activator in rubber vulcanization by sulfur. The further uptake of sulfur atoms by the various complexes from S8 or TMTD with formation of species derived from the radical Me2NCS 3 . or the trithiocarbamate anion Me2NCS 3 - is endothermic for mono- and dinuclear zinc dithiocarbamate (dtc) complexes such as [Zn(dtc)2] and [Zn 2(dtc)4], but exothermic in the case of polynuclear zinc oxide species containing bridging ligands as in [Zn4O 4(μ-S2CNMe2)] and [Zn4O 4(μ-dtc)]. Therefore, zinc oxide as a polynuclear species is predicted to promote the formation of trisulfido complexes, which are generally assumed to serve as catalysts for the transfer of sulfur atoms during rubber vulcanization. This prediction is in accord with the empirical knowledge that ZnO is a better activator in TMTD-accelerated rubber vulcanization than zinc dithiocarbamate. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.