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 L. Wistinghausen and B. A. Dogadkin and D. M. Pevzner (1951, unpublished) showed that MBT is consumed in vulcanization, but these authors, because of the crudity of their methods, did not obtain quantitative relationships.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. We have synthesized and investigated as vulcanization accelerators the derivatives of 2-mercaptobenzothiazole (MBT) in which the thiol hydrogen is replaced by the nonpolar methyl radical, as well as compounds in which methyl hydrogen of the methyl derivative is replaced by various functional groups. 2. It has been shown that methyl-2-thiolbenzothiazole is not an accelerator. The replacement in this compound of one of the methyl hydrogens by a polar hydroxyl group substantially enhances the activity (see, however, editors note in the text). The substitution of hydrogen by a carboxyl group does not increase vulcanizing activity. 3. We have determined that replacement of a methyl hydrogen by an amino radical increases sharply the accelerating activity. The structure obtained as a result of this reaction, benzothiazolyl-2-thiolmethyldiethylamine (BTMA), is of great practical interest as an accelerator. 4. The accelerator BTMA in stocks of natural and SKS rubber gives vulcanizates which are substantially superior in their properties to rubbers cured with MBT, and are practically equal to vulcanizates obtained with sulfenamide accelerators—sulfenamide BT and sulfenamide Z (Santocure). 5. The accelerator BTMA is much cheaper than sulfenamide BT since its production requires much less diethylamine. 6. It has been determined that, just as in the case of 2-mercaptobenzothiazole derivatives, for the derivatives of dimethyldithiocarbamic acid containing analogous functional groups the same results are obtained for the change in activity depending upon the chemical structure of the accelerator.

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The mechanism of the action of one of the most widely used vulcanization accelerators, 2-mercaptobenzothiazole (MBT), has not been sufficiently investigated. It was demonstrated by Wistinghausen, and also Dogadkin and Pevzner (1951) that MBT is consumed during vulcanization, but the quantitative relationships involved in this reaction could not be established due to the lack of a method for determining the disappearance of the MBT. For the present study we made use of MBT labeled with the radioisotope S35 in the thiazole ring. The synthesis of MBT was carried out in the following manner. First, MBT labeled in both sulfur atoms, was prepared from phenly isothiocyanate and elementary S35. The MBT thus obtained was then heated in sealed ampules with excess ordinary sulfur at 140° for 6 hours. Under these conditions, the interchange reaction between the elementary sulfur and the sulfhydryl sulfur leads to an equilibrium. There was a twofold drop in the activity of the synthesized MBT. That is the way in which the labeling of the benzothiazole radical alone was accomplished, so that the radioisotope S35 introduced into the MBT molecule was not exchanged with elementary sulfur under the vulcanization conditions that obtained. This now made possible simultaneous measurements of the rate of sulfur addition and the rate with which the accelerator is added to the rubber.

1. The accelerator activities of heterocyclic N-thiocarbamylsulfendialkylamides based on piperidine, morpholine, and piperazine were investigated; it was found that compounds of this type are highly active accelerators of the vulcanization of natural and styrene butadiene rubber stocks, inducing rapid crosslinking and giving rubbers of higher modulus than the usual vulcanization accelerators. 2. The vulcanization kinetics in presence of these heterocyclic N-thiocarbamylsulfendialkylamides depends to a considerable extent on the nature and number of hetero atoms in the accelerator molecule. Morpholine and piperazine derivatives have a slower effect at the initial stage of vulcanization than the corresponding piperidine derivatives, and confer greater resistance to premature vulcanization of the rubber stocks.