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

Fieser, M. E.; Sanford, M. J.; Mitchell, L. A.; Dunbar, C. R.; Mandal, M.; Van Zee, N. J.; Urness, D. M.; Cramer, C. J.; Coates, G. W.; Tolman, W. B.
J. Am. Chem. Soc. 2017, 139, 15222 (doi:10.1021/jacs.7b09079).

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.