This paper presents the results of a thoroughly unorthodox approach to the synthetic rubber problem. In developing their new butyl rubber, the Esso Laboratories have turned to simple olefins rather than diolefins or more complicated chemical derivatives as the main raw material.

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The present distinction between the thoughts expressed by the terms synthetic rubber and rubber synthesis is indicative of the changes in viewpoint on which current progress in the duplication of rubberlike properties is based. For almost fifty years after the discovery by Williams that isoprene is the fundamental structural unit of rubber, research was directed toward rubber synthesis. Bouchardat, Tilden, Wallach, Hoffman, Mathews, and Harries all made valuable contributions to the chemistry of rubberlike products from isoprene. That none of them accomplished the purpose of synthesizing rubber is an accepted fact. In the preparation of a rubberlike material from dimethylbutadiene, Kondakow departed from classical attempts at rubber synthesis, and laid the foundation for the first commercial “synthetic rubber”, the German wartime “methyl rubber”. The tradition that rubber properties were related to high unsaturation persisted, and has led to numerous theories of elastic behavior based on geometric structures resulting from unsaturated linkages. Whitby and Staudinger questioned the importance of unsaturation in determining rubberlike behavior, but encountered some difficulty in finding adequate support for this view, owing to the poor elastic properties of the practical examples available to them. Our studies on polymers of the simple monoölefins, which have been in progress for the past ten years, have led to some interesting conclusions in regard to this point. It was shown that high-molecular-weight linear polymers derived from simple olefins, such as the butylenes, possess a majority of the properties characteristic of rubber. Only the susceptibility to chemical action can definitely be attributed to the unsaturation of the natural rubber molecule. Physical properties which appear not to be intrinsically dependent on the carbon-double bond-carbon configuration are: tensile strength, elasticity, rebound, elastic memory, x-ray structure, mechanical orientation, electrical properties, and fractional solubility. It was, however, conclusively demonstrated that the saturated polymers are not capable of vulcanization in conventional manner.

Isobutylene and styrene were copolymerized at low temperatures, in methyl chloride solution, in the presence of aluminum chloride. The temperature was varied from −30 to −94°C., and the monomer/solvent ratio, the ratio of monomers, and the catalyst concentration were varied over a considerable range. The copolymerization equation was found to be applicable to the data, and the reactivity ratios were determined for various experimental conditions. For an “open” system in which the volume steadily increases during the course of polymerization, it is shown that the copolymerization equation is formally identical to that of Mayo and Lewis, except that concentrations are replaced by amounts of the reacting species. The temperature coefficients of the reactivity ratios in the present system are compared with those for free‐radical systems; some important differences are noted. The reactivity ratios of isobutylene and styrene in the present system are greatly increased under conditions of turbulent agitation. A procedure for the compositional fractionation of the copolymers is described. The results are interpreted in terms of the copolymer chain structure; they appear to admit the postulate that the reactivity ratios and corresponding propagation rate constants are functions of chain length.