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Demystifying the Mechanism of Regio- and Isoselective Epoxide Polymerization Using the Vandenberg Catalyst
journal contribution
posted on 2018-02-21, 00:00 authored by Robert
C. Ferrier, Srimanta Pakhira, Sarah E. Palmon, Christina G. Rodriguez, David J. Goldfeld, Oluwagbenga O. Iyiola, Malgorzata Chwatko, Jose L. Mendoza-Cortes, Nathaniel A. LyndA combined theoretical and experimental
investigation into the
structure and mechanism of the classical Vandenberg catalyst for the
isoselective polymerization of epoxides has led to a consistent mechanistic
proposal. The most likely reaction pathway was based on a bis(μ-oxo)di(aluminum)
(BOD) resting state that proceeded through a mono(μ-oxo)di(aluminum)
(MOD) transition state. The isoselectivity of the Vandenberg catalyst
was derived from the rigidity of the BOD structure and its bonding
to the ultimate and penultimate oxygen heteroatoms along the polyether
backbone. The energetic driving force for isoselectivity was the loss
of an energetically favorable secondary Al–O interaction during
enchainment of oppositely configured epoxides, providing a ca. 2 kcal/mol
driving force for the emergent isoselectivity. Experimental spectroscopic
and kinetic evidence based on model BOD and MOD complexes support
the new mechanistic framework developed using density functional theory
calculations. A purposefully synthesized BOD analogue of the proposed
Vandenberg structure produced a characteristically isotactically enriched
poly(allyl glycidyl ether) as produced by the classical Vandenberg
catalyst. In situ 1H NMR spectroscopy
of a Vandenberg-catalyzed polymerization of allyl glycidyl ether revealed
the activation enthalpy (ΔH‡ = 21 kcal/mol) and energetics of epoxide–aluminum coordination
(ΔH = −4.0 ± 1.0 kcal/mol, ΔS = −0.018 ± 0.004 kcal/(K mol)) by observation
of the shifting acetylacetonate signal located on the active site
of the Vandenberg catalyst in the 1H NMR spectra of polymerization.