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Over the past two decades,polyesters such as polycaprolactone ( PCL),polylactide ( PLA) and their copolymers have attracted a great deal of attention because of their vast potential applications in many biomedical and pharmaceutical fields.Many metal complexes have been reported to be effective catalysts/initiators that catalyze ROP of lactones/lactides giving polymers with both high and controlled molecular weights.Previously,we have been reported that the magnesium alkoxides derived from the NNO-ketiminate ligands are good initiators toward the controlled polymerization of lactides and exhibit the high stereoselectivity with a Pr up to 87%.As a result,we report herein the synthesis and characterization of a series of magnesium complexes based on NNO-tridentate ketiminate ligands with pyrazole substituents of different electronic properties.The molecular structures of complexes [Mg2(OBn)2L2] reveal dimeric features.Furthermore,heteroleptic bimetallic magnesium complexes show high activity ( for example: [Mg2(OBn)2L12] within 20 min,30℃) and stereoselectivity ( Pr up to 85%) in the ring - opening polymerization of lactides producing high molecular weight polymers of narrow PDI ranging 1.06-1.15 with >90% conversion.