In a study of the mechanism of the sulfur vulcanization of unsaturated rubber, 2,3-dimethyl-2-butene (C6H12) was used as a simple, low-molecular model alkene. Only equivalent allylic positions are present in this alkene. Treating C6H12 with a mixture of ZnO, S8 and the accelerator tetramethylthiuramdisulflde at 140°C for 20 minutes yields a mixture of addition products (C6H11—Sn—C6H11) and also intermediate products (C6H11—Sn—S(S)CN(CH3)2). The formation of C6H11—Sn—C6H11 from these intermediate products only proceeds in the presence of the zinc dimethyldithiocarbamate complex and free alkene.

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To study the mechanism of the sulfur vulcanization of rubber, 2,3-dimethyl-2-butene (C6H12) was used as a simple, low-molecular-weight model alkene. Only equivalent allylic positions are present in this alkene. Treating C6H12 with a mixture of ZnO, S8, and the accelerator tetramethylthiuramdisulfide at 140°C yields a mixture of addition products (C6H11—Sn—C6H11). RP-HPLC in combination with MS and 1H-NMR shows that the products differ only in the length of the sulfur bridge. Small quantities of isomerized products have been found, in which a 1,3-shift of the double bond has occurred.

Vulcanization of ENBH (C9H14), a model for ENB containing EPDM rubber, with a system consisting of zinc oxide, stearic acid, sulfur, TMTD, and MBT at 140°C for 1 h yields slightly more than 30 different crosslinked products C9H13-Sn-C9H13 where n=2,3,4, and 5. In all of the products, the original ENBH structure is maintained (no shift of the double bond) and attachment of the sulfur bridge occurs only at the two allylic positions 3 and 9. Monosulfides (n=1) are probably also formed but no structures could be determined. Only small amounts of the MBT-Sn-C9H13 coupling products and noncrosslinked cyclic sulfides are formed.