Sulfur Vulcanization of Simple Model Olefins, Part II: Characterization and Reactivity of Intermediate Vulcanization Products of 2,3-Dimethyl-2-Butene

Abstract

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.

The role of dimethyldithiocarbamic acid in accelerated sulfur vulcanization

“…TMTU does not form in vulcanizates where Hdmtc is trapped by ZnO. 14 As noted above, Geyser and McGill 3 considered TMTU formation as an integral part of the crosslinking reaction, although in a similar study of model compound vulcanization by TMTD/sulfur, Versloot et al 2 did not report TMTU. Furthermore, Versloot et al…”

“…Geyser and McGill, 3 who reported TMTU, completely submerged the reaction tubes in the oil bath at 150°C, while Versloot et al, 2 who employed partially submerged reaction tubes, did not report TMTU in the products that precipitated on the cold sections of the reaction tubes. Dma ⅐ dmtc was detected, which implies that some Hdmtc escaped being trapped by ZnO present in the system, yet as volatiles could precipitate on the upper sections of the reaction tube, TMTU was not produced, only dma ⅐ dmtc.…”

“…TMTU has been reported by some 1,3 but not all 2 authors as being present in the extractable products at the end of the vulcanization process.…”

“…No such breakdown has been observed at vulcanization temperatures in the absence of rubber in any studies in these laboratories. 11,12 Versloot et al, 2 also, did not report TMTU formation on heating TMTD. The addition of dimethylamine to TMTD leads to the production 13 of dimethylammonium dimethyldithiocarbamate (dma.dmtc) and TMTU, and it has been suggested 5 that differences in TMTU formation may be ascribed to the presence of an impurity, such as an amine, that may or may not be trapped in the system, depending on the experimental setup employed.…”

“…The accelerated sulfur vulcanization of polyisoprene (IR) 1 and of model compounds 2,3 by tetramethylthiuram disulfide (TMTD), in the absence of ZnO, has been the subject of a number of studies. It is generally agreed 1,[4][5][6] that accelerator polysulfide intermediates react with the olefin in a substitution reaction, leading to the formation of accelerator terminated polysulfidic pendent groups and the liberation of dimethyldithiocarbamic acid (Hdmtc).…”