Ring-opening Polymerization Of Lactide Catalyzed By Low-toxicity Metal Complexes

Polylactide(PLA)is a biodegradable material,which is mainly prepared by the ring-opening polymerization of lactide(LA).Metal complexes have become the most widely used catalysts in the ring-opening polymerization of lactide due to their advantages of easy structural modification,high catalytic activity and stereoselectivity.However,metal complex catalysts often cause harmful metal residues,which will limit their applications in medicine,pharmaceutics and other fields.NNO tridentate ligands L1-L2 and NNOO tetradentate ligands L3-L5 were synthesized in this paper,and the ligands were analyzed by mass spectrometry and 1H NMR.Considering the residue of harmful metals,low toxicity magnesium and zinc are used as central metals.Under the conditions of room temperature,tetrahydrofuran and standard schlenk techniques,five zinc and magnesium complexes C1-C5 were synthesized by the reaction of the ligands(L1-L5)with magnesium chloride and zinc chloride,respectively.The complexes were characterized by mass spectrometry and 1H NMR.The zinc and magnesium complexes were used to catalyze the ring-opening polymerization of lactide and their catalytic properties were explored.The effects of the amount of catalyst and reaction temperature on the ring-opening polymerization of lactide were studied.The complexes are all effective catalysts.Complexes C3 and C5 exhibit high monomer conversion and narrow molecular weight distribution,showing good catalytic performance.When the concentration of lactide is 1 mol/L,and the concentration ratio of catalyst to initiator and the concentration ratio of catalyst to lactide are 1:1 and 1:100,with benzyl alcohol as initiator,the catalytic activity of the metal complexes is the highest at 130 ? in toluene.The polymerization catalyzed by the metal complexes follows the first order kinetics law.The microstructure of the polymerization products was analyzed by 1H NMR and MS.The results imply that benzyl alcohol was the initiator of polymerization,and the ring-opening polymerization of lactide follows the coordination-insertion mechanism.