Polylactide has attracted much interest as a potential renewable substitute for petroleum-based plastics. The stereoselective ring-opening transesterification polymerization (ROTEP) reaction of lactide (LA) is an effective tool to control the properties of polylactide by tuning the microstructures formed. Using combined experiment and theory, we are evaluating the mechanism underlying stereocontrol of the ROTEP of meso-LA by a bipyrrolidine-based Al-alkoxy salan catalyst. Density functional theory calculations show that the (R,R)-catalyst shows kinetic preference to stereoselectively ring-open meso-LA at the carbonyl unit adjacent to the (R) stereocenter. The “match” initiation product for the ring-opened meso-LA, which has an (S) configuration at the stereocenter proximal to Al, is more thermodynamically stable than the product with an (R) proximal stereocenter. The highly active and stereoselective "match" complex is stabilized by ligand-monomer intramolecular interactions. These results are consistent with the dominant initiation product for meso-LA observed in experiment. Our results support the proposed “dual-stereocontrol” mechanism in the stereoselective ROTEP of meso-LA, contributing to the thermodynamic preference for the “match” product.

Stereoselective ring opening polymerization of lactide using chiral Al salan catalysts

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