Preparation Of Lithium, Lanthanum And Yttrium Complexes Containing A Chiral Nitrogen Atom And Studies On Their Catalytic Properties In Stereoselective Polymerization Of Rac-lactide

Polylactide (PLA) is a biocompatible, biodegradable and sustainable aliphatic polyester, which is conveniently prepared by ring-opening polymerization (ROP) of lactide (LA). Many compounds have been reported to be effective catalysts in initiating the ROP of lactides. Among them, metal complexes were considered as optimal catalysts for the ROP of LA due to their excellent catalytic behavior and convenient fine-tuning of the stereoselectivity and polymerization reactivity by changing the metal center or the structures of ligands. As the stereoregularity of the polymers determines both the processing properties and the rate of biological degradation of PLA, there are strong demands for the development of new catalysts with high activity, good stereoselectivity for the ROP of rac-LA to produce stereocontrolled PLAs with high melting point, good processing properties and excellent mechanical strength.Three unsymmetric amine-pyridine-phenolate-phenol N2O2-ligands (H2L1-H2L3) containing a prochiral nitrogen atom and a symmetric ligand (H2L4) were prepared from nonchiral compounds. The reactions of H2L1-H2L4with nBuLi, LaCl3, and Y[N(SiMe3)2]3(THF)x, respectively, give eight metal complexes. The structures of ligands and complexes obtained were characterized by MS,'H and13C NMR spectroscopy. The results indicate that that these complexes are all mononuclear complexes. The prochiral nitrogen atom of the tertiary amine in the unsymmetric ligand becomes a chiral center because of the restriction on the conformation inversion of the tertiary amine upon coordination. Complexes1-7were thus formed as a pair of enantiomers. Complex8containing a symmetric N2O2-ligand is a nonchiral complex. The catalytic performances of these metal complexes were studied for the ROP of rac-LA.Lithium complexes1-3display good catalytic activities and stereoselectivity in the ROP of rac-LA, giving hetcrotactic-rich PLAs with the P, value up to0.78. The PLAs were obtained with molecular weights up to14260and polydispersities of1.68-1.95. Lithium complex8bearing a symmetric ligand does not display stereoselectivity in the ROP of rac-LA, giving atactic PLAs. Lanthanum complexes4,5and yttrium complexes6,7show good catalytic activities in the ROP of rac-LA, affording PLAs with predicted molecular weights and narrow molecular weight distributions (1.062-1.179). The observed Mn is linear correlation with the conversion of monomer, indicating a living polymerization system. On the contrary to the lithium complexes1-3, rare-earth metal complexes4-7give isotatic-rich PLAs with the Pm value up to0.81. These results show that the ROP of rac-LA is well controlled in these catalytic systems.