Polymerization of Lactide with Zinc and Magnesium β-Diiminate Complexes: Stereocontrol and Mechanism

A series of zinc(II) and magnesium(II) alkoxides based upon a β-diiminate ligand framework has been prepared. [(BDI-1)ZnOiPr]2 [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] exhibited the highest activity and stereoselectivity of the zinc complexes studied for the polymerization of rac- and meso-lactide to poly(lactic acid) (PLA). [(BDI-1)ZnOiPr]2 polymerized (S,S)-lactide to isotactic PLA without epimerization of the monomer, rac-lactide to heterotactic PLA (Pr = 0.94 at 0 °C), and meso-lactide to syndiotactic PLA (Pr = 0.76 at 0 °C). The polymerizations are living, as evidenced by the narrow polydispersities of the isolated polymers in addition to the linear nature of number average molecular weight versus conversion plots and monomer-to-catalyst ratios. The substituents on the β-diiminate ligand exert a significant influence upon the course of the polymerizations, affecting both the degree of stereoselectivity and the rate of polymerization. Kinetic studies with [(BDI-1)ZnOiPr]2 indicate that the polymerizations are first order with respect to monomer (rac-lactide) and 1.56 order in catalyst. Polymerization experiments with [(BDI-1)MgOiPr]2 revealed that this complex is extremely fast for the polymerization of rac-lactide, polymerizing 500 equiv in 96% yield in less than 5 min at 20 °C.