Modern Views on the Chemistry of Vulcanization Changes. I. Nature of the Reaction between Sulfur and Olefins

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Abstract
At the present time it is taken for granted that in all kinds of linear high polymers, natural or artificial, true polymers or polycondensation products, the establishment of cross-links between adjacent thread molecules is in itself sufficient to transform the physical properties of the material, producing a state which can be collated with the vulcanized state in rubber. Yet in most of these instances—and particularly in the case of vulcanization of natural and synthetic rubbers by sulfur, by peroxides, and by other reagents—the idea that the vulcanized products with their characteristic toughness, swelling capacity, elasticity, etc., derive their special qualities from the mere formation (by one means or another) of connecting links between the original linear molecules still remains a matter of hypothesis. The sulfur-vulcanization process as applied to rubber presents many interesting and very elaborate features, especially those connected with its capacity for being accelerated by a host of similar and dissimilar organic substances, and for being retarded, reversed, delicately controlled, etc. Hence, it is of considerable interest to find out by experiment exactly what sulfur does to the rubber when the two are heated together (either alone or in the presence of different types of accessory reagents) and, thence, to discover whether the production of good vulcanized properties actually depends on the production of chemical cross-links, or on certain kinds of chemical cross-links, or on a judicious combination of cross-linking and mere sulfuration (the latter possibly assisting the establishment of van der Waals forces between molecules as an auxiliary factor) ; or whether, as some physical chemists have been inclined to allege, it results from a state of physical association between molecules which has little or nothing to do with cross-linking. The present series of communications describes recent experimental advances in this subject; it is necessary to explain at the outset that the method of experimental approach adopted has of necessity, owing to the formidable molecular magnitude of even unvulcanized rubber, been by way of the response to vulcanization treatment of comparatively simple olefinic compounds. This approach has thus involved in the first place a study of the chemistry of simple sulfur-olefin reactivity, which was previously never carried far.

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