Mechanistic Insights into the Alternating Copolymerization of Epoxides and Cyclic Anhydrides Using a (Salph)AlCl and Iminium Salt Catalytic System

Megan E Fieser, Maria J. Sanford, Lauren A. Mitchell, Christine R. Dunbar, Mukunda Mandal, Nathan J. Van Zee, Devon M. Urness, Chris Cramer, Geoffrey W. Coates, William B Tolman

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Abstract

Mechanistic studies involving synergistic experiment and theory were performed on the perfectly alternating copolymerization of 1-butene oxide and carbic anhydride using a (salph)AlCl/[PPN]Cl catalytic pair. These studies showed a first-order dependence of the polymerization rate on the epoxide, a zero-order dependence on the cyclic anhydride, and a first-order dependence on the catalyst only if the two members of the catalytic pair are treated as a single unit. Studies of model complexes showed that a mixed alkoxide/carboxylate aluminum intermediate preferentially opens cyclic anhydride over epoxide. In addition, ring-opening of epoxide by an intermediate comprising multiple carboxylates was found to be rate-determining. On the basis of the experimental results and analysis by DFT calculations, a mechanism involving two catalytic cycles is proposed wherein the alternating copolymerization proceeds via intermediates that have carboxylate ligation in common, and a secondary cycle involving a bis-alkoxide species is avoided, thus explaining the lack of side reactions until the polymerization is complete.

Original languageEnglish (US)
Pages (from-to)15222-15231
Number of pages10
JournalJournal of the American Chemical Society
Volume139
Issue number42
DOIs
StatePublished - Oct 25 2017

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    Fieser, M. E., Sanford, M. J., Mitchell, L. A., Dunbar, C. R., Mandal, M., Van Zee, N. J., Urness, D. M., Cramer, C., Coates, G. W., & Tolman, W. B. (2017). Mechanistic Insights into the Alternating Copolymerization of Epoxides and Cyclic Anhydrides Using a (Salph)AlCl and Iminium Salt Catalytic System. Journal of the American Chemical Society, 139(42), 15222-15231. https://doi.org/10.1021/jacs.7b09079