Synthesis, characterization, and applications of mono([mu]-alkoxo)bis(alkylaluminum) catalysts for epoxide polymerization
Recent advances in medicine, membranes for gas separations, and batteries have been made possible through associated advances in polyether materials. Further advances will be enabled by continued materials innovation enabled by advances in polymerization catalysis. Polyethers, derived from epoxides, represent a versatile class of functional polymeric materials with the potential for true compositional control of structure-property relationships in a macromolecular platform due to the high thermodynamic ring-strain driving force for polymerization. Despite this promise, there is currently no consensus polymerization technique available for epoxides that provides access to polyether materials with consistently controlled molecular weights, chain-end functionality, tolerance to monomer functionality, and is available to the non-specialist. The Vandenberg catalyst is a well-established, empirically developed, industrial catalyst solution for polyether-based elastomers derived from epoxides. Investigation of compositionally related complexes led to the discovery of well-defined mono(µ-alkoxo)bis(alkylaluminum) (MOB) complexes which function as effective catalyst/initiator for epoxide polymerizations, capable of producing > 10 kg/mol polyethers in minutes depending on MOB composition and epoxide monomer. Variation of alkylaluminum used in the MOB synthesis led to a four-fold variation in polymerization rate, whereas variation of N,N′-alkylamino-ethanol structure led to a 400-fold variation in polymerization rate of epichlorohydrin. The MOB structure appeared to rearrange and dimerize upon addition of polar compounds, E.g., epoxide monomers, revealing an aluminum alkoxide as the site of monomer addition and a N–Al Lewis pair functioning as catalyst. With new epoxide polymerization catalyst in hand, a series of investigations into new polymer structure and compositions was possible. Chain-extension polymerizations were conducted from poly(ethylene oxide) macroinitiators to create block polymer materials, whose architecture and solution self-assembly were investigated. Atactic polyethers exhibiting semi-crystallinity mediated by long alkyl side chains were synthesized and studied as phase-change materials. Finally, our synthetic advances enabled a fundamental study of polymer electrolyte structure-property relationships by varying dielectric constants among a series of homologous polymer structures revealing the important role of polarity in dictating ion transport properties. These studies highlight the enabling role of new polymerization catalysis in materials science and engineering.