Highly Active Magnesium Initiators for Ring-Opening Polymerization of rac-Lactide

A series of magnesium silyamido complexes supported by monoanionic aminophenolato ligands which display extraordinarily high activity for the ring-opening polymerization of rac-lactide, were prepared. Single crystals of 2 suitable for X-ray diffraction studies were obtained by slightly cooling the saturated toluene solution. Preliminary kinetic studies of the polymerization of rac-lactide initiated by complex 1 in the presence of 1 equiv of isopropanol in toluene indicated a first-order dependence on the monomer concentration. Less active complex 2 was chose to study the NMR scale polymerization of rac-lactide in the presence of isopropanol to obtain some insight into the active species. The polymerization started instantaneously, and active oligomers end-capped with isopropoxy group could be identified. In conclusion, monomeric magnesium silylamido complexes 1-3 showed excellent catalytic activities for the ring-opening polymerization of rac-lactide at ambient temperature.