The CBS-Accelerated Sulfuration of Natural Rubber and Cis-1,4-Polybutadiene

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Abstract
The results of characterization of the natural rubber vulcanizates are consistent with the results of characterization of the sulfidic products from 2-methylpent-2-ene. In both the model olefin system and the rubber system the initially formed crosslinks are polysulfidic but these are subsequently reduced to di- and monosulfidic crosslinks as the cure time is increased. Similar amounts of zinc sulfide are formed during the sulfuration of 2-methylpent-2-ene and during the vulcanization of natural rubber. The efficiency of sulfur utilization for crosslinking in natural rubber is approximately half that in comparable sulfurations of 2-methylpent-2-ene, i.e. approximately twice as many sulfur atoms are needed to obtain a chemical crosslink in natural rubber as are needed to obtain a crosslink in 2-methylpent-2-ene. This is presumed to be a consequence of the intra-molecular sulfuration that occurs in natural rubber. There is no evidence to indicate the presence of vicinal crosslinks in the natural rubber vulcanizates. Hence in agreement with the views of other workers it is concluded that the crosslinks present in accelerated sulfur vulcanizates of natural rubber are tetrafunctional and dialkenyl. The results of the characterization of the polybutadiene vulcanizates are not fully supported by the results of the model olefin studies. In the vulcanization of polybutadiene the initially formed crosslinks are polysulfidic. As vulcanization proceeds, the chemical complexity of the network increases. After long reaction times, however, no significant amount of monosulfidic crosslinks are present in the network and very little of the reacted sulfur is present in the form of zinc sulfide. Nitrogen analyses of the polybutadiene vulcanizates showed that a substantial fraction of the accelerator, equivalent to 80–90% of the available 2-thiobenzothiazole groups, become combined in the network during vulcanization. It is proposed that the combination of accelerator with polybutadiene prevents the desulfuration of dialkenyl polysulfides to dialkenyl monosulfides (the normally observed pathway of accelerated sulfuration of natural rubber) and allows vicinal crosslinking to proceed. Some support for this proposal is that vicinal crosslinks and a substantial amount of nitrogenous product are formed during the accelerated sulfuration of cyclohexene. The findings of Gregg and Katrenick on the MBTS accelerated sulfuration of cis-cis-1,5-cyclooctadiene are also consistent with this proposal. The nitrogen analyses of the polybutadiene vulcanizates indicate that very little of the accelerator is permanently combined in the network during the initial stages of network formation. Hence by comparison with the observed pattern of sulfuration of hex-3-ene, where it was shown that negligible amounts of nitrogenous product are present, it is proposed that dialkenyl (tetrafunctional) polysulfidic crosslinks are initially introduced into the polybutadiene network. The polysulfidic crosslinks then presumably undergo desulfuration reactions leading to dialkenyl crosslinks of reduced sulfur chain length until the desulfurating agent is, in effect, removed from the system by the 2-thio-benzothiazole groups becoming combined in the network. Once most of these groups have combined, after ca. 60 min. at 140° C, the desulfuration reactions are probably less important than the reactions leading to vicinal crosslinking, and it is likely that a well cured-polybutadiene vulcanizate contains a substantial fraction of vicinal crosslinks.

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