Bis(pyrrolidene) Schiff Base Aluminum Complexes as Isoselective-Biased Initiators for the Controlled Ring-Opening Polymerization of <i>rac</i>-Lactide: Experimental and Theoretical Studies

A series of bis­(pyrrolidene) Schiff base aluminum complexes (<b>1</b>–<b>7</b>) were synthesized and characterized by NMR spectroscopy and elemental analysis. All complexes were efficient initiators for the ring-opening polymerizations of l-LA and <i>rac</i>-LA in toluene at 70 °C. Kinetic studies revealed first-order kinetics in monomer and the rates of l-LA and <i>rac</i>-LA polymerizations decreased in the order of 1,2-benzylene (<b>4</b>) ≫ 1,3-propylene (<b>2</b>) > 2,2-dimethyl-1,3-propylene (<b>3</b>) > 1,4-butylene (<b>5</b>) > <i>rac</i>-1,2-cyclohexylene (<b>7</b>) > 1,2-ethylene (<b>1</b>) ≫ 1,2-phenylene (<b>6</b>). Microstructure analyses of the resulting polylactides by homonuclear decoupled ¹H NMR spectroscopy disclosed the isotactic-biased stereocontrol of all synthesized complexes, except <b>5</b>. Isotactic stereoblock polylactide with a high <i>P</i>_m value of 0.80 was produced by <b>3</b>. A systematic DFT study on the <i>rac</i>-lactide ring-opening mechanism initiated by the initiators synthesized in this study revealed the correlation between the structure of backbone linker and the polymerization activity and stereoselectivity