Kinetics and Mechanism of Hydrochlorination of Synthetic and Natural Polyisoprene Latex

Abstract

Abstract The kinetics of hydrochlorination of synthetic polyisoprene emulsions, prepared by new and simple methods, are studied for comparison with the same reaction of natural latex. Nearly identical zero-order rate constants apply to both substrates at 300° K and 1 or 2 atmospheres of hydrogen chloride, despite differences in the detailed structure of the polymers. The conclusion that the substrates are not concerned in the rate control follows, since the rates of chain reactions which might give zero-order constants are very sensitive to the detailed structure of the polymers. Two initial rate abnormalities are observed. The first is shown to be due to a fast surface reaction, the second probably reflects the presence of a basic retarder in natural rubber particles only. Similar abnormalities of the cyclization reaction, reported earlier, are explained similarly. The effects of energetic factors and concentration on the hydrochlorination rate support visualization of the simple rate-determining step as: H+ + Cl− → (H+, Cl−) (ion pair), where the proton is probably solvated as H2C+. The critical transition state must be uncharged, because the reaction rate is unaffected by a change of dielectric constant due to the conversion of rubber hydrocarbon into rubber hydrochloride. Existence of the ion pair as reactive intermediate confirms Gordon and Taylor's recent stereochemical theories on hydrochlorination of natural rubber. The rapid surface reaction is ascribed to the ready formation of ion pairs or similar species in the aqueous phase, and the limited range (about 15 A) or the surface reaction to the slow rate of diffusion of unchanged rubber units to the surface.