American Chemical Society
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Hydrogen Atom Abstraction from Polyolefins: Experimental and Computational Studies of Model Systems

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journal contribution
posted on 2020-04-08, 15:06 authored by Graham E. Garrett, Derek A. Pratt, J. Scott Parent
The hydrogen atom transfer reactions that underlie peroxide-initiated modifications of polyolefin architectures and ambient-temperature air oxidations are examined through a combination of experiments and quantum chemical calculations on model hydrocarbons. The regioselectivity of H-atom from pentane, 2,4-dimethylpentane, and 2,2,4,4-tetramethylpentane to t-BuO generated from tert-butyl hyponitrite at 25 °C was quantified by trapping of the alkyl radicals with a UV-active nitroxide. The data show that hydrocarbon reactivity toward t-BuO does follow neither well-established trends in homolytic C–H bond dissociation energy (tertiary > secondary > primary) nor trends in the susceptibility of polyolefins to autoxidation. These experiments, together with quantum chemical calculations conducted at the CBS-QB3 level of theory, reveal the importance of entropic effects in these exergonic H-atom transfer processes. In contrast, the endergonic nature of H-atom abstraction by t-BuOO produces conventional hydrocarbon reactivity patterns, as enthalpic contributions to transition-state energies are dominant. These findings bring clarity to the structure/reactivity relationships of polyolefins in the field of radical chemistry.