The Structure of Angular Polycycloisoprene

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Abstract
Lewis and Brö nsted acids, heat, silent electric discharge and cathode rays cause progressing isomerization of synthetic and natural cis-1,4-polyisoprene to yield a polymeric product which has the empirical formula, (C5H8)n identical with that of the initial rubber hydrocarbon. The reaction, commonly carried out in benzene with stannic chloride as catalyst, is accompanied by an increase in specific gravity, refractive index and polymer melt temperature and a decrease in unsaturation, intrinsic viscosity and molecular weight of the material. The resulting polymer, readily soluble in a variety of solvents, is known as cyclized rubber, cyclo-caoutchouc, or “Pliolite-NR”, manufactured by the Goodyear Tire and Rubber Company. Staudinger recognized as early as 1926 that the reaction proceeds by isomerization and intramolecular cyclization. D'lanni postulated formation of cyclohexane rings within the polymer chain, while Veersen extended D'lanni's proposal and suggested random protonation of bonds leading to intralinear, mono-, di-, and polycyclic ring polymers. Ideally, complete and stereoselective cyclization would lead to angular polycycloisoprene. Comprehensive reviews concerning the chemistry of the cyclization have been published by Reese and by Bloomfield. This communication provides the first chemical structure proof of the cyclized product. It is based on the cyclization of cis-1,4- and of trans-1,4-polyisoprene. The resulting products were identical and their properties in agreement with literature data. Hence cyclization must have proceeded through a common intermediate.

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