Synthesis of Sodium Complexes Supported with NNO-Tridentate Schiff Base Ligands and Their Applications in the Ring-Opening Polymerization of L-Lactide

A series of sodium complexes bearing NNO-tridentate Schiff base ligands with an N-pendant arm were synthesized and used as catalysts for the ring-opening polymerization of L-lactide (L-LA). Electronic effects of ancillary ligands coordinated by sodium complexes substantially influence the catalysis, and ligands with electron-donating groups increase the catalytic activity of the sodium complexes for catalyzing L-LA polymerization. In particular, a sodium complex bearing a 4-methoxy group has the highest activity with conversion up to 95% within 30 s at 0 degrees C and a low polydispersity index of 1.13, whereas the 4-bromo group showed the poorest performance with regard to the catalytic rate of L-LA polymerization in the presence of benzyl alcohol (BnOH). H-1 NMR pulsed-gradient spin echo diffusion experiments and single-crystal X-ray analyses showed that sodium complexes [(LNa)-Na-H(THF)](2) and [L4-ClNa(THF)](2) were dinuclear species in both solution and the solid state. The kinetic results indicated a first-order dependence on each of [[L4-ClNa](2)], [L-LA], and [BnOH].