Polymers derived from cyclic esters of biomass feedstock origin have long been considered an attractive substitute to traditional commodity polymers, as they can likewise be molecularly engineered to meet specific fabrication processes. However, efforts in this field to achieve well-defined material properties at scalable volumes and wide design flexibility is currently limited by poor mechanistic understanding. Towards this end, research in the Tolman group has focused on ε-caprolactone (CL) polymerization using salen aluminum alkoxide complexes, with ligand architecture specifically manipulated at the backbone linker and the ortho position of the phenolate motif. These structural variations were targeted to provide modular systems with which to systematically probe the influence of catalyst framework on routes of monomer binding and insertion. The kinetics of these complexes in CL polymerization as a function of temperature were studied under saturation conditions, where all complexes studied exhibited saturation kinetics but overall reaction rates were enhanced for those with longer linkers and with more sterically encumbered ortho substituents. Theoretical calculations further support this fundamental difference, while also identifying significantly similar transition-state geometries. A simplified computational method for evaluating ligand structure and predicting catalytic activity is proposed, based on the differences in energetics associated with requisite structural distortions from the resting catalyst to the transition-state structure. Studies across these catalytic systems using this framework distortion analysis described herein elucidate upon the mechanistic behavior of cyclic ester polymerization catalysts using non-toxic, inexpensive metals for the development of rational design rules.

Mechanistic studies of ε-caprolactone polymerization by (salen)AlOR complexes and a predictive model for cyclic ester polymerizations

253rd National Meeting (2017)

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