The role of thiol intermediates in 2-mercaptobenzothiazole accelerated sulfur vulcanization of rubber model compounds

The model compound, 2,3-dimethyl-2-butene (TME), was vulcanized using 2-mercaptobenzothiazole (MBT) and sulfur. MBT was not consumed during the vulcanization reaction. The resultant crosslink products were bis(alkenyl) in nature. 2,3-Dimethyl-2-buten-1-thiol (TME-SH) was identified as being present in the vulcanization mixture by a postcolumn derivatization technique. The appearance of thiol was coincident with crosslinking. Polysulfanes (H2Sn) were formed on crosslinking. Studies of the reaction of TME-SH and sulfur indicated a rapid reaction to form crosslink products and polysulfanes. No monosulfidic crosslink species were formed in these reactions. Closer investigation revealed the presence of small quantities of what appeared to be highly reactive polysulfidic thiols. This is the first time that such species have been identified in vulcanization systems. Consequently, MBT-accelerated vulcanization of TME is proposed to occur via the reaction of MBT and S-8 to form polysulfidic MBT, which then reacts with TME to form polysulfidic thiols. These thiols then rapidly react via a metathesis reaction pathway to provide crosslink products and polysulfanes. (C) 2003 Wiley Periodicals, Inc.