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

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 1 H NMR spectroscopy disclosed the isotactic-biased stereocontrol of all synthesized complexes, except 5 . Isotactic stereoblock polylactide with a high P m 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.