Crosslinking efficiency in the reaction of dicumyl peroxide with dimethyloctadiene

The crosslinking efficiency of dicumyl peroxide was investigated with the use of 2,6‐dimethyl‐2,6‐octadiene as a model polyisoprene. High‐temperature gas chromatography was used to determine quantitatively the amount of crosslinked products as well as the decomposition products of the peroxide, α,α‐dimethylbenzyl alcohol and acetophenone. The crosslinked products were found to be almost exclusively a mixture of isomeric dimers when the molar ratio of dimethyloctadiene to dicumyl peroxide was 40:1 or higher. When larger amounts of peroxide were used, higher molecular weight crosslinked products were also formed. The dimethylbenzyl alcohol and acetophenone formed accounted for 90–95% of the peroxide decomposed. The results are consistent with a mechanism according to which the radicals formed by decomposition of the peroxide react exclusively with polyisoprenes by hydrogen abstraction. The polyisoprenyl radicals formed are then used up exclusively by combination. One molecule of decomposed dicumyl peroxide should then form one crosslink, and this was confirmed by experiment. Kinetic data showed that the crosslinking reaction followed a first‐order law. The activation energy was found to be 32 kcal./mole.