Trinuclear and Tetranuclear Magnesium Alkoxide Clusters as Highly Active Initiators for Ring-Opening Polymerization of l‑Lactide

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-<i>N</i>,<i>N</i>-dimethylaminoethanol/methoxyethanol, different phenols, and dibutylmagnesium. These complexes have been characterized using <sup>1</sup>H and <sup>13</sup>C 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 <b>2</b> 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 <b>3</b> and LA. However, for the tetranuclear magnesium complex <b>5</b>, the polymerization rate is first order for <b>5</b> and second order for LA.