A DFT study of the hydrogen atom abstraction from 2,4,6-trimethylheptane: A model of peroxidic degradation for syndio polypropylene

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.