The yield and regioselectivity of H-atom abstraction by cumyloxy radicals from poly(isobutylene-co-isoprene) (IIR) are quantified and discussed in the context of cross-linking/degradation outcomes and vinyltriethoxysilane (VTEOS) graft yields. Studies of IIR materials with different isoprene contents show that H-atom abstraction from the allylic functionality provided by isoprene mers is responsible for the heightened H-atom transfer reactivity of IIR relative to poly(isobutylene). Differences in the reactivity of allylic and alkyl macroradical intermediates makes high isoprene IIR materials less prone to peroxide-initiated chain scission, but less responsive to VTEOS grafting formulations. Improved knowledge of H-atom transfer reactivity is extended to a new approach for IIR cross-linking involving acrylate-functionalized nitroxyl chemistry. (c) 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3102-3109