Vulcanization of Elastomers. 47. Vulcanization of Natural Rubber and Polybutadiene with Benzoyl Peroxide

Abstract The present article reports experiments intended to elucidate the kinetics of benzoyl peroxide decompositions in natural rubber and polybutadiene, and of consequent reactions. Both in natural rubber and in polybutadiene and at all temperatures, peroxide decomposition is first order with respect to concentration as well as time. The decomposition rate is substantially higher in natural rubber than in polybutadiene, and independent of the constitution of the polymer chains of the polybutadiene (tacticity, vinyl side groups, and the like). A somewhat higher activation energy of peroxide decomposition was derived for polybutadiene than for natural rubber. Formation of benzoic acid and crosslinking both follow a first order time law; the rates for peroxide decomposition, formation of benzoic acid, and crosslinking are the same in natural rubber and polybutadiene. It seems probable that peroxide decomposition in polybutadiene is purely homolytic. Interaction between peroxide and natural rubber is assumed, resulting in an increase in the rate of decomposition. The benzoic acid yield is considerably higher in natural rubber than in polybutadiene, and diminishes in the latter with the increase in vinyl side groups. In natural rubber this yield is a function of temperature, but in polybutadiene it shows practically no temperature dependence. Benzoyloxy radicals are incorporated into the elastomers as benzoate groups, as confirmed by infrared spectroscopic determinations with cis-1,4 polybutadiene. An interpretation of the formal kinetics of the vulcanization of 1,5-polyenes is proposed, which, together with the experimental results, may be used as a basis for consideration of the reaction mechanism.