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

Abstract

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