Sulfur-vulcanisation of BR

Solid-state 13C NMR spectroscopy was used to study accelerated [33] and unaccelerated [34] sulfur-vulcanisation and sulfur-donor (TMTD) [35] vulcanisation of czs-polybutadiene (BR). Olefinic and methylene carbons of the czs-BR repeating unit typically resonate at 129.5 and 27.5 ppm, respectively. The dominant products occurring in the vulcanisation 

The peak at 33 ppm is assigned to the trans structure of 1,4-BR. An increasing intensity at 33 ppm peak with cure in both sulfur-cured and accelerated sulfur-cured BR postulates the occurrence of cis-to-trans chain isomerisation in these systems. The resonances at 38 and 50 ppm are assigned to cyclic monosulfide and polysulfidic crosslink structures. The expected monosulfidic junctions are not detected in this study possibly due to the low concentration of these species [33]. 

Similar vulcanisation products with reduced structural modifications are obtained in the sulfur-donor (TMTD) vulcanised BR [35]. By comparing the GHPD and DEPT results of sulfur-vulcanised BR with the results of sulfur-donor vulcanisation, detailed chemical shift assignments are possible where the peaks from inter- and intra-molecular sulfurisations and those from accelerator fragments can be distinguished. 

Side reactions including cis-to-trans isomerisation and sulfidic cyclisation are observed along with the formation of crosslinks in the BR cured with sulfur alone. In the sulfur-donor vulcanisation of BR, cis-to-trans isomerisation is the predominant feature of the vulcanisation reaction sequence and seems to obey first-order kinetics with respect to the concentration of accelerator. 

The effect of the accelerator to sulfur ratio on the vulcanisation chemistry was also investigated by comparing the vulcanisation products from conventional, SEV and EV formulation systems. With increase in accelerator to sulfur ratio (from conventional to EV) there is a lowering in the sulfur rank. Also, the cis-to-trans isomerisation increases with the amount of accelerator. 

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