Mechanism of Stereocontrol in Polymerization of rac-Lactide by Aluminum Complexes Supported by Indolide-Imine Ligands

Luke, A. M.; Peterson, A.; Chiniforoush, S.; Mandal, M.; Popowski, Y.; Sajjad, H.; Bouchey, C.; Shopov, D. Y.; Graziano, B.; Yao, L. J.; Cramer, C. J.; Reineke, T. M.; Tolman, W. B.
Macromolecules 2020, 53, 1809 (doi:10.1021/acs.macromol.0c00092).

The solid state and fluxional behaviors in solution of complexes L²AlOBn and L⁷AlOBn (Bn = benzyl) supported by an analog of salen incorporating indolide arms connected via their 2- and 7-positions were defined by experiment and theory. The complexes catalyze the stereoselective conversion of rac-lactide (LA) to isotactically enriched polylactide. A key aspect of the stereocontrol was examined through study of the initiation reactions via NMR spectroscopy, X-ray crystal structures of the ring-opened products, and theory. The results include the first unambiguous structural definition of stereocontrol in ring-opening of LA by a metal-alkoxide complex and the finding that definition of the stereochemistry of initiation by the studied system is governed thermodynamically rather than kinetically.