Abstract
From the time, more than, a century ago, that isoprene was isolated by pyrolysis of natural rubber, scientists around the world have attempted to reverse the process, namely, to prepare rubber from isoprene. Bouchardat, in 1879, reported what may have been the first preparation of a synthetic rubbery material. It was done by treating isoprene with hydrochloric acid. This report signalled the start of a vigorous search for a system capable of converting isoprene into a useful rubber. The quest, though not fully successful, led to the discovery of efficient catalysts, such as the alkali metals, for use in anhydrous polymerizations and catalysts for aqueous emulsion polymerizations. Yet the preparation of an all cis-1,4-polyisoprene, a duplicate of natural rubber, continued to elude the most extensive efforts of scientists for many more years. It was not until the mid 1950's that two independent catalyst systems, each capable of polymerizing isoprene to a cis-1,4-polymer, were disclosed. One such catalyst came briefly on the heels of the famous discovery, by Nobel Laureate Karl Ziegler, of transition metal halide coordination catalysts for the low pressure polymerization of ethylene. Home and coworkers, employing a Ziegler-type catalyst prepared from trialkylaluminum and titanium tetra-chloride, polymerized isoprene to an essentially all cis-1,4-polyisoprene. The other catalyst, based on lithium metal, was discovered by a Firestone Tire and Rubber research team. The discovery was part of a broad study on structures and molecular weights of polyisoprenes prepared with alkali metals. The polymerization of isoprene in the presence of alkali metals was the subject of other extensive investigations. A polyisoprene with 90% 1,4 units was reportedly synthesized with the aid of lithium catalysts as early as 1949. Once the capability of converting isoprene to a high cis-1,4-polyisoprene was achieved, it was only required that isoprene be available at low cost for synthetic cis-1,4-polyisoprene to become a commercial reality. Thus, while the search for an economical isoprene source was underway, teams of scientists tackled an array of new problems related to stereoregular solution polymerization. The requirements for high purity, the optimization of the polymerization conditions, and the complex finishing processes, all put a great demand on the ingenuity and skill of research and development staffs. It was therefore rather remarkable that, in 1960, just a few years after the discovery of the stereospecific catalysts, the first commercial plant for the production of cis-polyisoprene with a lithium catalyst was on stream. By 1967 two other commercial polyisoprenes were being produced in the United States with catalysts based on trialkylaluminum-titanium tetrachloride. Additional production facilities went on stream in other countries. It was estimated that 670 000 long tons of cis-polyisoprene was produced in 1975. In more recent years oil shortages, inflation and related economic and political factors have curtailed U. S. production. A vast amount of literature has sprung up in the wake of this stunning commercial growth. Several comprehensive reviews have covered developments in this field. A symposium held in Moscow in 1972, exclusively dedicated to polyisoprene rubber, revealed the intensity and breadth of new work on synthetic polyisoprenes. The Eastern nations have been very active in polyisoprene research. A rough count of recent titles indicates that about two-thirds originate in the USSR and contiguous countries. We believe this review adequately covers the polyisoprene literature through 1977. It is not exhaustive because of the volume of publications and because many references appear in Chemical Abstracts as “Title only translated”.
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