THIURAM-ACCELERATED SULFUR VULCANIZATION .2. THE FORMATION OF CROSS-LINK PRECURSORS

2,3-Dibutyl-2-butene (TME) was used as a model for polyisoprene in stu dying the formation of pendent groups that act as precursors to crossl ink formation during tetramethylthiuram disulfide (TMTD)-accelerated s ulfur vulcanization. TME was heated at 130 degrees C with TMTD, TMTD/s ulfur, and TMTD/sulfur/ZnO in sealed tubes for various times, after wh ich the mixture was analyzed by HPLC. Tetramethylthiuram monosulfide ( TMTM) and tetramethylthiuram polysulfides (TMTP) formed rapidly. The r ate at which these bound to the network could not be accounted for in terms of a free-radical mechanism, and a concerted mechanism was propo sed. Dimethyldithiocarbamic acid (Hdmtc), a product of pendent group f ormation, is stable at vulcanization temperatures and does not decompo se to dimethylamine. Instead, it exchanges with thiuram pendent groups . The resultant thiol pendent groups are unreactive and their formatio n delays the onset of crosslinking. After a few minutes at 130 degrees C, there appears to be no further increase in the number of pendent g roups, but this is due to the thiol groups being undetectable in the H PLC analyzer. (C) 1996 John Wiley & Sons, Inc.