Determination of the molecular weight distribution of polyisoprenes from the sol dependence on crosslinking

A polyisoprene with a narrow molecular weight distribution was prepared with a polyisoprenyl lithium catalyst by the “seeding” technique. Dicumyl peroxide was shown to be a quantitative crosslinking agent for this polyisoprene. The dependence of the sol fraction on the amount of crosslinking introduced by dicumyl peroxide vulcanization was determined and found to give excellent agreement with gel network formation theory. Two procedures for estimating the breadth of the molecular weight distribution from the sol dependence on crosslinking yield the value of 1.05 ± 0.05 for the ratio of the weight‐average to number‐average molecular weight for this polyisoprene. These results demonstrate that it is possible to determine the molecular weight and molecular weight distribution of polyisoprenes, containing cis ‐1,4, trans ‐1,4, or moderate amounts of 3,4 addition structures, from the sol dependence on the amount of dicumyl peroxide used in crosslinking. Temporary or permanent molecular chain scission that might accompany the crosslinking by dicumyl peroxide was negligible, as there was no evidence of broadening of the molecular weight distribution due to scission. For the most likely set of recombination reactions that might occur it is estimated that less than 0.3% of the dicumyl peroxide is used in reactions leading to temporary molecular chain scission.