Ring-opening polymerization of rac-lactide, copolymerization of rac-lactide and ε-caprolactone by zinc complexes bearing pyridyl-based tridentate amino-phenolate ligands

A series of zinc complexes supported by tridentate aminophenolate ligands bearing a substituted pyridyl ring, LZn[N(SiMe3)(2)] = {2-[(6-R-3-pyrid-2-yl)CH2N(R-4)CH2]-4-R-2-6-R-1-C6H2O}, 1: R-1 = Trityl, R-2 = Me, R-3 = Me, R-4 = Bn; 2: R-1 = Trityl, R-2 = Me, R-3 = Me, R-4 = (n)Hex; 3: R-1 = Trityl, R-2 = Me, R-3 = Me, R-4 = Cy; 4: R-1 = Trityl, R-2 = Me, R-3 = Ph, R-4 = Cy; 5: R-l = Trityl, R-2 = Me, R-3 = Bu-t, R-4 = Cy; 6: R-1= R-2 = Bu-t, R-3 = Ph, R-4 = Cy), were synthesized via the reactions of Zn[N(SiMe3)(2)](2) and the corresponding aminophenols. The X-ray diffraction analysis of typical complex 2 confirmed its monomeric nature, where the metal centre is tetra-coordinated by three donors of the aminophenolate ligand and one silylamido group in a distorted tetrahedral geometry. All these zinc complexes were moderately to highly active initiators toward the ring-opening polymerization (ROP) of rac-lactide (rac-LA). The substituents on the ortho-positions of the phenolate and pyridyl rings as well as on the skeleton nitrogen atom of the ligand show significant influences on the catalytic activities and the stereoselectivities of the corresponding complexes toward the polymerization of rac-LA, leading to the production of atactic polylactides (PLAs) via complex 5 (P-m = 0.53) and moderate isotatic bias PLAs via complexes 1-4, 6 (P-m = 0.63-0.66). In addition, complexes 2 and 4 could efficiently initiate the copolymerization of rac-LA and epsilon-CL at 70 degrees C to afford random copolymers with high molecular weights and moderate polydispersities.