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For the reaction of 2,4-dimethylpentane or 2,4,6-trimethylheptane with t-butoxyl radicals – modelling the controlled-rheology peroxide degradation of polypropylene – only tertiary and primary radicals, with few or no secondary radicals have been identified by some authors. This is in disagreement with what is expected from the C–H bond strength involved in the formation of tertiary and primary radicals. In this paper, we present a complete DFT study (UB3LYP/6-311++G(3df,3pd)//UB3LYP/6-31G(d)) of the H-abstraction by t-butoxyl radical from 2,4,6-trimethylheptane 1, which allows qualitative assessment of the reactivity observed. Then combining the DFT calculated H-abstraction rate constants and a kinetic modelling (taking into account radical generation, radical decay and concentration evolution with time) allows quantitative assessment of the reactivity.

Alkoxyl radicals are often generated in order to perform regioselective hydrogen atom abstraction. However, their g-fragmentation generates alkyl radicals that are less efficient to abstract hydrogen atoms. We have performed density functional theory (DFT) calculations [UB3LYP/6-31+G(d,p)] followed by a natural bond orbital (NBO) analysis on a series of alkoxyl radicals. The beta-fragmentation of these radicals is discussed in terms of bond population and hyperconjugative interactions. (c) 2005 Wiley Periodicals, Inc.