Radiation-induced crosslinking and cyclization in 1,2-polybutadiene

Abstract In order to get information on the radiolytic changes in 1,2‐polybutadiene (1,2‐PB) the sol and gel fractions, the conversion of double bonds, the structure and concentration of radicals, the formation of dienes and the formation of gaseous products were measured. In addition, the dose rate dependence and temperature dependence for the conversion of double bonds were determined. G values for double bond conversion depend on molecular weight and range from 20 to 200. G values for crosslinking are about 10. A mechanism for the double bond conversion is proposed which involves initiation by a transformation of the primary radical ion in the vinyl group into a carbonium ion and a radical. This is supported by ESR measurement. Reaction of the carbonium ion with a vinyl group in the same chain gives rise to cyclization, whereas reaction with a vinyl group in a neighboring chain results in crosslinking. A comparison of the G values for conversion of double bonds with the G values for crosslinking shows that the formation of cyclic rings exceeds the formation of crosslinks by a factor of about 10. The corresponding values in 1,4‐cis‐ and 1,4‐trans‐polybutadiene are much smaller [G(cl) ≃ 2; G(db) ≃ 7]. The pendent vinyl groups in 1,2‐polybutadiene therefore are more reactive than the vinylidene groups in 1,4‐polybutadienes.