Synthesis and Structural Characterization of Magnesium Ketiminate Complexes:  Efficient Initiators for the Ring-Opening Polymerization of l-Lactide

A series of dinuclear magnesium complexes of the type [LMg(μ-OBn)] 2 (where L = NNO-tridentate ketiminate ligand) have been prepared. Their activities toward the ring-opening polymerization of l -lactide have been investigated. Experimental results indicate that the reactivity of [LMg(μ -OBn)] 2 is dramatically affected by the steric and electronic effect of the substituents on the ketiminate ligand. On the basis of the melting point depression and variable-temperature 1 H NMR studies, it is believed that these magnesium complexes exist as an equilibrium between a monomeric and a dimeric species in solution in which the monomeric species is the most active species during polymerization. The polymerization probably proceeds with the coordination of the lactide to the monomeric species giving a five-coordinated magnesium complex as an intermediate.