Reactivity of Polyolefins toward Cumyloxy Radical: Yields and Regioselectivity of Hydrogen Atom Transfer

Hydrogen atom abstraction from a series of homopolymers by cumyloxy radicals is examined under solvent-free conditions at temperatures that are relevant to radical-mediated polymer modifications. Abstraction efficiency data acquired for the thermolysis of dicumyl peroxide (DCP) within pure polymer samples establish an order of reactivity: poly(butadiene) (PBD) > poly(ethylene oxide) (PEO) > poly(ethylene) (PE) > poly(propylene) (PP) > poly(isobutylene) (PIB). The regioselectivity of hydrogen transfer from PE, PP, and PIB is assessed by model hydrocarbon experiments involving nitroxyl trapping of the alkyl radicals generated from pentane, 2,4-dimethylpentane, and 2,2,4,4-tetramethylpentane, respectively. Taken together, abstraction efficiency and regioselectivity data are discussed in terms of enthalpic and entropic contributions to H atom transfer rates, with particular emphasis on steric hindrance imposed by methyl substituents on secondary positions within PP and PIB. The utility of polymer oxidizability as a predictive measure of the reactivity of a polymer toward cumyloxyl and vinyltriethoxysilane graft modification is evaluated and discussed.