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

Abstract

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.

Isobutylene‐rich macromonomers: Dynamics and yields of peroxide‐initiated crosslinking

“…Hydrogen atom abstraction from polyisobutylene by cumyloxy radicals is regioselective for primary alkyl positions, due to steric hindrance of secondary positions imposed by adjacent quaternary carbons (Sch. ) . Termination of the resulting primary macroradicals by combination can generate crosslinks.…”

Effect of nitroxyl-based radicals on the melt radical grafting of maleic anhydride onto polyethylene in presence of a peroxide

“…the reactions non-selective. As initiator, dialkyl peroxides are most commonly used and the decomposition mechanism involves initial O-O bond homolysis to generate the corresponding alkoxy radicals that can add to the grafting monomer, which would lead to homopolymers as undesired by-products [20][21][22][23][24]. Alkyl radicals formed by decomposition of the primary radicals are known to preferably add to the monomer.…”

Post‐polymerization modification by nitroxide radical coupling

“…have defined the abstraction efficiency as the fraction of cumyloxy radicals that abstracts a hydrogen atom from the hydrocarbon as opposed to fragmenting into a methyl radical and ketone. For PP the abstraction efficiency was found to be 37%–39% whereas for PE it was estimated as 54% . This means that, considering PP as a matrix even if the functional TEMPO derivative is different, the amount of grafted C=C reactive groups is the same and each formulation has the potential to build the same polymer network by oligomerization.…”