Demystifying the Mechanism of Regio- and Isoselective Epoxide Polymerization Using the Vandenberg Catalyst

Abstract

A 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(μoxo)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 1 H 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 1 H NMR spectra of polymerization.

Lewis Pair Polymerization of Epoxides via Zwitterionic Species as a Route to High‐Molar‐Mass Polyethers

“…As documented, the olefinic bond of the monomer remains intact and allyl isomerization is not detectable;t he application of such functionalized olefins thus enables the preparation of polyether structures with very high molar mass and suitable sites for subsequent cross-linking or other derivatizations.As one such future use,P O-based elastomers for low-temperature applications can be imagined;i ndeed, such materials have been commercialized (service range: À60 to > 200 8 8C) for which high-M n polyether was employed. [41,42] Current research in our lab aims to revive this type of material.…”

The versatile, functional polyether, polyepichlorohydrin: History, synthesis, and applications

“…Reproduced with permission. 32 Copyright 2018, American Chemical Society use in other (co)polymerization contexts as well. In 2017, Chwatko and Lynd demonstrated the homopolymerization of lactones and the statistical copolymerization of lactones and epoxides, including ECH.…”

“…Epoxide polymerizations with Vandenberg's catalyst are marked by limited control over M n and Đ; Polymers are always high molecular weight (M n > 10 6 g/mol) and generally high dispersity (Đ > 2). 26,32 Therefore, low (M n < 10 4 g/mol) and intermediate (10 6 > M n > 10 4 ) M n polymers were unachievable. The reason for the uncontrolled M n is at least partially due to the lack of understanding of the initiating and catalytic species that make up the Vandenberg catalyst; Reacting water with tri-alkyl aluminum and acetyl acetone produces a large number of side reactions and so the catalytically active species was unknown.…”

“…26,33 Some work over the years has been invested in understanding the polymerization mechanism behind this culminating in a recent report by Ferrier, Mendoza-Cortes, and Lynd. 32 Due to these concerns and general interest in synthesizing a broader array of PECH (co) polymers, other synthetic methods have been developed, the best of which being revealed in the last decade or so.…”

Ultrahigh-Molecular-Weight Poly(propylene oxide): Preparation and Perspectives

“…Recently, by using calculations and experiments, Lynd and co-workers proposed a mode of operation that is based on a bis(-oxo)diethylaluminum species (Scheme 2a). 9 The Vandenberg Catalyst is described in their study as a single-site catalyst, profiting from its partially rigid backbone and a bimetallic cooperative interaction between the two Al centers.…”