Preparation Of Lithium And Lanthanum Complexes Containing An Unsymmetric N₂O₂ Ligand And Their Catalytic Properties In Stereoselective Polymerization Of Rac-Lactide

Polylactide (PLA) is a sustainable aliphatic polyester composed of lactide (LA) repeat units. The lactide is obtained from starch harvested from com or sugar beet.which are green and renewable sources. Intensive attention has been attracted for preparation of PLA as it is biodegradable and can be hydrolyzed to lactic acid in the environment. The physical and processing properties of PLA depend on the stereostructure of the polymer. Atactic PLA is an amorphous material, whilst an equivalent mixture of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) forms a crystalline stereocomplex with Tm as high as 230℃Preparation of crystalline poly(rac-LA) from the direct ring-opening polymerization of rae-LA by stereoselective catalysts is a convenient, practicle. and effective method and also a challenging research subject. Many efforts have been made to develop such stereoselective catalysts, while the reports on efficient catalysts are limited to Salen and Saian aluminium complexes, which are not good catalysts for industrial application because they are quite sensitive to humidity and with certain toxicity.Four unsymmetric N2O2 ligands L1-L4 were prepared from non-chiral compounds. The structures of L1-L4 were characterized by MS and'H NMR spectroscopy as well as elemental analysis. In addition, the molecular structures of L2 and L3 were determined by single crystal X-ray analyses. In the crystalline state, the N, N, O.0 atoms in L2 and L3 tend to orient to the same center due to the intramolecular interaction, which makes the inversion of the three groups in the tertiary N atom limited. The'H NMR spectra show that the methylene groups of L2 in CDCl3 can rotate freely, while L3 keeps its chiral N structure in solution.The reactions of LI and L2 with nBuLi in THF give 5 (LILi) and 6 (L2Li) in high yields, respectively. The structures of 5 and 6 were characterized by MS and1H NMR spectroscopy. Complexes 5 and 6 are active in polymerization of rac-LA,giving heterotactic-rich polylactide (Pr= 0.65, at 100℃; Pr= 0.70, at 70℃) with Mw/Mn= 1.70. Addition of phenylmetanol can improve the activity of the reaction but decrease the stereoselectivity and the molecular weight.The reactions of L1 and L2 with anhydrous LaCl; in THF afford complexes 9 and 10, respectively. The chelating coordination of the unsymmetric N2O2 ligands to a lanthanum ion gives racemic complexes of 9 and 10, in which the chiral N atom is created by fixing the spatial conformation of the tertiary N atom of the ligand. The structures of 9 and 10 were characterized by MS and'H NMR spectroscopy. The catalytic properties of 9 and 10 were studied for stereocontrol polymerization of rac-LA. It was found that the lanthanum complexes displayed a good asymmetric induction in stereoselective ring-opening polymerization of rac-LA. The isotactic-rich polylactide obtained from polymerization of rac-LA initiated by 9 and 10 has Pm values up to 0.81 and the polymerization reaction is well controlled (Mw/Mn= 1.06-1.10).