The mechanism of the action of 2-mercaptobenzothiazole, MBT, one of the most commonly employed accelerators of vulcanization, remains unclear. L. Wistinghausen and B. A. Dogadkin and D. M. Pevzner (1951, unpublished) showed that MBT is consumed in vulcanization, but these authors, because of the crudity of their methods, did not obtain quantitative relationships. In the present work there has been employed a preparation of the MBT which was tagged in the thiazole ring by the radioisotope S35. The synthesis of this compound was carried out in the following manner. MBT, tagged in both of its sulfur atoms, was first synthesized from phenyl isothiocyanate and elementary S35. The resulting MBT was then heated in a sealed ampule with an excess of inactive elementary sulfur for six hours at a temperature of 140°. Under these conditions the exchange reaction between the elementary sulfur and the sulfur of the mercapto group of the MBT reaches equilibrium. The activity of the resulting MBT fell to one-half of the original value. In this way the benzothiazole radical was tagged, by the introduction into the molecule of MBT of the radio-isotope S35 which does not exchange with elementary sulfur, under the conditions of vulcanization, and it thus became possible to measure simultaneously the velocity of the addition of the sulfur and the velocity of addition of the accelerator to the rubber.

1. 2,2′-dibenzothiazolyl disulfide (MBTS) labeled in the thiazole ring with the S36 isotope was synthesized. 2. During vulcanization of purified sodium butadiene rubber with MBTS the accelerator is observed to combine with the rubber simultaneously with the sulfur reaction. When all of the sulfur is combined the bonding of the accelerator is sharply retarded in spite of the fact that there is a significant amount of free MBTS left in the reacting mixture. 3. There is a linear relationship between the bound sulfur and the combined accelerator. 4. The rate constant for the combining of MBTS with the rubber increases linearly with the concentration of the accelerator. The limiting quantity of combined MBTS is about 75% of that introduced and does not depend on the concentration of the accelerator. 5. During sulfurless vulcanization with MBTS labeled with S35 in the disulfide bridge the quantity of combined accelerator determined chemically exceeds that determined radiomctrically by more than two times which indicates that there is an unsymmetrical scission of the MBTS. 6. During vulcanization with sulfur in the presence of MBTS labeled in the thiazole ring the ratio between the activity of the extract and the vulcanizate is significantly greater than unity. After all of the sulfur is combined this ratio becomes less than unity. An explanation of this phenomenon is reported.

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