A series of magnesium benzylalkoxide complexes, [(LMg)-Mg-n(mu-OBn)](2) (1-14) supported by NNO-tridentate pyrazolonate ligands with various electron withdrawing-donating subsituents have been synthesized and characterized. X-ray crystal structural studies revealed that Complexes 1-3, 5, 7, 9, and 10 are dinuclear bridging through benzylalkoxy oxygen atoms with penta-coordinated metal centers. All of these complexes acted as efficient initiators for the ring-opening polymerization of L-lactide and rac-lactide. Based on kinetic studies, the activity of these metal complexes is significantly influenced by the electronic effect of the ancillary ligands with the electron-donating substituents at the phenyl rings enhancing the polymerization rate. In addition, the "living'' and "immortal'' character of 6 has paved a way to synthesize as much as 40-fold polymer chains of polylactides with a very narrow polydispersity index in the presence of a small amount of initiator. Among all of magnesium complexes, Complex 6 exhibits the highest stereoselectivity toward ring-opening polymerization of rac-lactide with Pr up to 88% in THF at 0 degrees C. (C) 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 696-707