Kinetics of Cyclization of Hevea Latex

Abstract

Abstract Kinetic measurements on rubber cyclization now are possible by a latex technique; these were undertaken to elucidate the mechanism of the reaction. According to a patented process, dispersions of Hevea latex particles in 70 to 80 per cent sulfuric acid are reproducibly isomerized to a high molecular weight cyclized rubber. A diffusion gradient technique was adopted for measuring the density of the polymer after exposing it to cyclization for varying times. Unimolecular constants, k, spreading over four decades, were obtained and found enormously sensitive to the percentage, c, of sulfuric acid and absolute temperature, T; thus log10k=6+0.17c−7105/T. The constants, k, correlate satisfactorily with Hammett's acidity function, H0. A statistical theory by Flory shows that 13.5 per cent of the isoprene units originally present in rubber are left uncyclized by the reaction. This is in striking agreement with unsaturation measurements made by Fisher in 1927. The kinetic parameters deduced support a nonchain proton transfer mechanism, deprotonation being rate-determining. The proton affinity of rubber is estimated, and a comparison of the cyclization of rubber and other terpenes is presented. The structure proposed for cyclized rubber rings by D'Ianni and co-workers is supported by mechanistic analysis and physical properties. The method of kinetic measurements on stable emulsions in sulfuric acid is of general promise, and the cyclization of rubber polymer presents a useful model reaction for the smaller terpenes.