Studies of the Vulcanization of High Elastic Polymers. IV. Investigations with Model Compounds
Authors: Scheele Walter , Lorenz Otto 
Published: February 2011

If we disregard for the moment the phenomena which occur in the interaction of thiuram disulfide with geraniol under the influence of air and confine our consideration to the results which can be deduced from the study of this reaction in a stream of nitrogen, we are led to the conclusion that geraniol, which was used as a model compound, behaves with respect to its interaction with tetraethylthiuram disulfide, not only qualitatively but also quantitatively, exactly like rubber, and that it is not to be assumed that other tetraalkylthiuram disulfides as well as other compounds which are analogous to geraniol will behave differently. Thus the results can be evaluated as a proof that the vulcanization of natural rubber by thiuram disulfides depends on nothing other than a definite and always similar interaction with the allyl groups of the polyisoprene chain. The results of the investigation described in this paper can be regarded, moreover, as support for the correctness of our procedure in the clarification of the vulcanization mechanisms, and they stress the importance of analytical-chemical methods, which have certainly not been pursued in the past with the necessary intensity and insight. These investigations are being continued. It has already been found that the vulcanization of natural rubber with thiuram monosulfides and sulfur leads to the same results as vulcanization with thiuram disulfides. This has been conjectured, to be sure, by a number of workers. However, it was never really demonstrated experimentally. We shall report on this in the near future. The experimental results can now be summarized as follows : 1. The reaction between thiuram disulfide and geraniol (demonstrated by the example of tetraethylthiuram disulfide) takes place qualitatively and quantitatively like the interaction between thiuram disulfide and natural rubber. In the vulcanization of rubber by thiuram disulfides, therefore, there is involved a reaction of the thiuram disulfide with the allyl groups, leading to a definite conversion. 2. It was found that in the interaction of geraniol with thiuram disulfide under nitrogen and in the presence of zinc oxide, the limiting value of zinc dithiocarbamate amounts to 66 mole-per cent of the original thiuram disulfide, independent of the temperature. This was the result found in the study of the reaction of the thiuram disulfides with rubber. 3. We were able to show that the limiting value of the dithiocarbamate yield in the interaction of tetraethylthiuram disulfide with geraniol is independent of the thiuram disulfide concentration. This is likewise true for the reaction of the thiuram disulfides with natural rubber. From this it follows that the interaction of thiuram disulfides with allyl groupings, as they occur in the polyisoprene chain, is apparently a stoichiometric one. For this reason we can no longer retain the assumption that the limiting value of the yield of dithiocarbamate is the result of a sterically hindered reaction. 4. The change of concentration of zinc dithiocarbamate in the interaction of tetraethylthiuram disulfide with geraniol under nitrogen can be accounted for by postulating a reaction of the first order. The velocity constants were calculated and it was found that the resulting activation energy is somewhat greater than the one for the interaction with natural rubber. 5. Similarly the concentration of thiuram disulfide obeys a first-order reaction as it falls off to zero. The velocity constants were calculated. The activation energy obtained from the temperature dependence is in good agreement with that found for the interaction with rubber. 6. In the presence of oxygen, the reaction between thiuram disulfide and geraniol takes a quite different course. The rate of decrease of concentration of thiuram disulfide, which likewise follows a first-order reaction, is higher. The yield of zinc dithiocarbamate does not attain the constant value of 66 mole-per cent of the original thiuram disulfide. In the temperature range studied, the amount of dithiocarbamate found is smaller. The limiting value reached at the various temperatures is progressively smaller, the lower the temperature chosen. Whether by expanding the temperature range, one would eventually reach a minimum and a maximum limiting value has not been investigated.