Trinuclear and tetranuclear magnesium alkoxide clusters supported by bulky phenolates with triangular or rhombic structures were readily synthesized in acceptable yields via the reaction of 2-N,N-dimethylaminoethanol/methoxyethanol, different phenols, and dibutylmagnesium. These complexes have been characterized using H-1 and C-13 NMR, elemental analyses, and X-ray crystallography. The experimental results indicate that these clusters are efficient and excellent initiators for the ring-opening polymerizations (ROPs) of L-lactide (LA) and afford polylactides with desired molecular weights and narrow polydispersity indexes (PDIs). Complex 2 can even catalyze the ROP of 4000 equiv of L-lactide in 1 min in a controlled model. Kinetic studies indicate that the polymerization is first-order for both the trinuclear magnesium complex 3 and LA. However, for the tetranuclear magnesium complex 5, the polymerization rate is first order for 5 and second order for LA.