Mechanistic Insights into the Alternating Copolymerization of Epoxides and Cyclic Anhydrides Using a (Salph)AlCl and Iminium Salt Catalytic System
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Authors
Megan E. Fieser, Maria J. Sanford, Lauren A. Mitchell, Christine R. Dunbar, Mukunda Mandal, Nathan J. Van Zee, Devon M. Urness, Christopher J. Cramer*, Geoffrey W. Coates*, and William B. Tolman*
Citation
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. Mechanistic Insights into the Alternating Copolymerization of Epoxides and Cyclic Anhydrides Using a (Salph)AICI and Iminium Salt Catalytic System. J. Am. Chem. Soc. 2017, 139, 15222–15231. DOI: 10.1021/jacs.7b09079.
Express Summary
- Mechanistic studies of the copolymerization of 1-butene oxide and carbic anhydride using a (salph)AlCl/[PPN]Cl catalytic pair showed a first-order dependence of the polymerization rate on the epoxide and zero-order dependence on the cyclic anhydride.
- Model complexes showed that a mixed alkoxide/carboxylate aluminum intermediate preferentially opens cyclic anhydride over epoxide, and ring-opening of epoxide by an intermediate comprising multiple carboxylates was rate-determining.
- A mechanism involving two catalytic cycles is proposed where the copolymerization proceeds via intermediates with carboxylate ligation in common, avoiding a secondary cycle involving a bis-alkoxide species and explaining the lack of side reactions until the polymerization is complete.
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.
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