Statistical kinetics of Hevea rubber cyclization

Abstract

Rubber cyclization provides a useful illustration of statistical effects intervening in the reactions of long-chain polymers. Analogously to the known cyclizations of simple diisoprenic terpenes, each individual cyclization act links two adjacent isoprene units in the rubber chain into a six-membered ring. However, some of the units remain stranded between neighbouring rings and escape cyclization for want of a partner. Thus the final polymer has a co-polymer structure of 86.5% of di-isoprenic rings and 13.5% of uncyclized monisoprenic units. This result is derived from a statistical theory of Flory, and has previously found accurate experimental verification. The bearing of this statistical effect on the reaction kinetics has now been studied for the first time. Dilatemetric experiments on rubber emulsions in sulphuric acid have shown that the cyclization reaction conforms in the main with a statistical cyclization law much better than with a first-order equation uncorrected for the survival of stranded units. High accuracy was required for this experimental distinction, whose importance lies in the implicit proof that the polymer units participate in the rate-determining transition complex, in accordance with the accepted proton transfer mechanism. A kinetic proof has incidentally been furnished for the fact that the rubber polymer contains long chains of identical units. When handled by an emulsion technique, this polymer can provide a useful ‘model compound’ for its simpler terpene analogues. Certain rate abnormalities have been observed including an induction period of reduced rate, probably connected with diffusion effects. A small volume change subsequent to the cyclization reaction may reflect a prototropic double-bond shift. The activation energy and entropy are found to be lower than in previous work, an effect traced in part to a curvature of the Arrhenius plot. The known dependence of cyclization rate on Hammett’s acidity function H 0 is quantitatively confirmed.