Rare-Earth Metal Complexes Bearing Unsymmetrical Diarylamido Ligands for Ring-Opening Polymerization of rac-Lactide

A family of rare-earth metal complexes bearing unsymmetrical diarylamido-[PNNox] pincer ligands with diverse chiral substituents were synthesized by utilizing direct alkane elimination. NMR and X-ray diffraction analyses revealed that the scandium and lutetium complexes were mononuclear, while the yttrium complexes were binuclear via the alkoxo bridges. All rare-earth metal complexes showed high catalytic activity for ring-opening polymerization (ROP) of rac-lactide, affording heteroselective polylactide (Pr up to 0.78). The yttrium complexes exhibited higher catalytic activity and higher heteroselectivity than the scandium and lutetium complexes. Especially, slight influence of the chiral substituents on the activity and heteroselectivity was observed in the catalytic systems. The kinetic study revealed that PMe–Y catalyzed the ROP of rac-lactide in a controlled manner and the polymerization proceeded with first-order dependence on both monomer and initiator concentrations.