Chlorination of Natural and Synthetic Polyisoprenes

Abstract

Abstract The extent of additive and substitutive chlorination was determined for emulsion polyisoprene, Hevea and gutta-percha chlorinated to different chlorine contents in darkness and in a nitrogen atmosphere. The maximum amount of additive chlorine that can be introduced into the polymer is 1.21 atoms of chlorine per isoprene unit for synthetic emulsion polyisoprene, 1.14 for Hevea. The chlorination involves a cyclization reaction by pairs, leading to isolation of reactive units. The loss in unsaturation due to cyclization has been calculated statistically by the method of Flory and Wall, and has been shown to be consistent with the total amount of additive chlorine in the fully chlorinated product. Cyclization takes place in the early stages of the chlorination simultaneously with the initial substitution. The theoretical limit of cyclization calculated statistically is 86.5 per cent of all isoprene units cyclized for natural rubber and 72 per cent for polyisoprene containing 10 per cent vinyl side chains. Iodine chloride titration gives fair results for samples of low chlorine content, but is not applicable to polymers containing much more than 35 per cent of chlorine. Knowing the degree of cyclization, the unsaturation can, however, be calculated for any polymer from the extent of additive reaction. Substitutive chlorination is catalyzed by oxygen and peroxide, additive chlorination by ultraviolet light. Cyclization is partially prevented by the increase in additive reaction. Under the combined effect of peroxides and ultraviolet radiation, chlorination can be carried to very high degrees.