On the regioselectivity of H-atom abstraction from model polyolefins by alkoxyl radicals

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Garrett, Graham E.
Polyolefin graft copolymerization , Hydrogen atom abstraction
Solvent-free peroxide-initiated polymer modifications are widely used to improve the physical and/or chemical properties of commodity plastics and elastomers. Although the reactions that underlie polymer grafting are known, our understanding of H-atom transfer reactions in this context is incomplete. Fundamental questions remain unanswered, such as the difference in reactivity between different polymers (polyethylene versus polypropylene and polyisobutylene) and differences in the regiochemical outcomes of grafting reactions upon them. Herein, experimental data pertaining to the H-atom transfers involved in polyolefin graft modifications were obtained to improve our fundamental understanding of these reactions by using radical-trapping techniques and quantum chemical calculations. In this project, experimental measurements of the efficiency of H-atom abstraction by t-butoxyl radicals from polyolefins, and suitable model compounds such as pentane, 2,4-dimethylpentane and 2,2,4,4-tetramethylpentane were determined. Insight is gained from alkyl-trapping experiments to quantify the relative reactivities of the primary, secondary and tertiary positions of the model compounds. Experimental data were compared to quantum chemical calculations, which revealed that entropic effects dictate the regioselectivity and preclude abstraction at the secondary position in favour of the less enthalpically-favourable primary abstraction site. MP2 and CBS-QB3 level calculations were able to reproduce experimental trends in model compound reactivity, while the highly common B3LYP density functional, used in other investigations on the subject, could not.
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