Magnesium silylamido complexes 1～7 bearing non-symmetric β-diketiminate ligands were synthesized via the reactions of corresponding proligands HL 1～7 with one equivalent of Mg[N(SiMe 3 ) 2 ] 2 at 80 ℃ in toluene. All complexes were characterized by 1 H NMR, 13 C NMR and elemental analysis. The monomeric nature of complexes 1 and 5 in the solid state was further confirmed by X-ray diffraction studies. In both complexes, the metal center is tri-coordinated by the β-diketiminate ligand and one silylamido group. The very close bond lengths of two Mg-N bonds as well as close C-N distances of the chelate ring indicate significant delocalization. The apparent deviation of the magnesium center from the ligand backbone plane suggests a certain η n -coordination nature of the ligand to the metal center. These magnesium silylamido complexes showed good catalytic activities for the ring-opening polymerization of rac-lactide under ambient conditions, and could polymerize 300 equivalents of rac-lactide to high molecular weight polymers within short time in THF. The solvent effect played a critical role during the polymerization process. All complexes showed higher catalytic activity in THF than in toluene. Taking complex 2 as an example, a monomer conversion of 96% could be achieved within 10 min in THF, whereas a conversion of 80% could only be achieved within extended polymerization time of 210 min in toluene (Table 1, Entries 6 and 9, [LA] 0 /[Mg] 0 ＝100). Upon the addition of isopropanol, the activities of magnesium silylamido complexes 1～7 increased significantly. For instance, when the polymerization runs were carried in THF in the presence of isopropanol, the reaction time could be reduced to 10～20 min even with a high molar ratio of [LA] 0 /[Mg] 0 ＝300. Moreover, the type and location of the substituent(s) on the N-aryl group of the β-diketiminate ligand exerts a significant influence on the catalytic activity of the corresponding complex toward the polymerization of rac-lactide. In the absence of excess isopropanol, the introduction of sterically demanding substituent to the ortho-position of N-aryl ring leads to a decrease of the polymerization activity; but the influence of electron-withdrawing group is different in different solvents (toluene or THF). These magnesium complexes could produce heterotactic polymers in THF (P r ＝0.64～0.80) and atactic polymers in toluene (P r ＝0.45～0.58). Magnesium complexes 1～7 also displayed high catalytic activities for the ring-opening polymerization (ROP) of ε-caprolactone, among them complexes bearing β-diketiminate ligand with bulky ortho-substituted N-aryl rings showed higher activities. Generally, the ROPs of ε-caprolactone initiated by these magnesium silylamido complexes were not well-controlled, giving moderately distributed polymers (M w /M n ＝1.37～1.67). Additionally, diblock copolymers of L-lactide and ε-caprolactone were obtained by using 2 as the initiator via both sequential feeding of two monomers (in either order) and the one-pot method. The formation of ...