Cyclic esters derived from biomass show promise as monomers for alternative bio-degradable polymers that can be sustainably produced. Lactide is one such monomer, and enantioselective polymerization of rac-lactide is of special interest, because formation of stereo-complexes between polymer blocks having different chirality leads to favorable material properties. Aluminum Schiff-base compounds catalyze such enantioselective polymerization to varying degrees through Ring Opening Transesterification Polymerization (ROTEP). Aluminum indolide catalysts present a new variation on this theme. Single-crystal X-ray structures for two such catalysts reveal that they adopt chiral conformations in the solid state, and nuclear magnetic resonance (NMR) studies indicate that these enantiomeric conformations rapidly interconvert in solution. Such a situation presents two opportunities for enantioselective polymerization: chain-end control, where the most recently included monomer biases selection of the next monomer, and site control, where the conformational chirality of the catalyst, if sufficiently kinetically fixed, biases selection of the next monomer. To assess the degree to which each of these effects is operative for the two aluminum indolide catalysts, density functional theory calculations were undertaken, focusing on both catalyst fluctionality as well as the full mechanistic pathways for initiation and propagation of polymerization. Activation energies computed for catalyst chirality inversion agree well with experimental values derived from variable-temperature NMR studies, and an exhaustive survey of stationary points along initiations and polymerization pathways suggests that both chain-end control as well as site control can contribute to observed stereoselection.

Mechanism of polymerization of rac-lactide by aluminum indolide catalysts

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.