Schiff Base Aluminum And Zinc Complexes Ring Open Polymerization Of Lactide And ?-caprolactone

Polylactic acid(PLA)and poly(?-caprolactone)(PCL)have attracted more and more attention as the leading biodegradable synthetic polyesters.Compared with the direct polymerization,ring opening polymerization(ROP)of lactide(LA)could obtain high molecular weight products of PLA or PCL.For the polymerization of LA,the behavior of isomers is important to the physical,mechanical,rheological and degradation properties of the PLA.What Metal-based complexes are the most effective methods for producing PLA could well control PLA structure and properties,including molecular weight,composition,microstructure and monomer combination.Especially stereoselective ROP,the stereoregularity of PLA could significantly affect its physical and chemical properties.Based on our previous work,at first part,a series of copolymers of LA and ?-CL with different monomer feed ratios were achieved using three kinds of bimetallic Schiff aluminum complexes as catalysts.The ratios of LA and ?-CL units in different copolymers and the average segments length were determined by NMR analysis.The comparative kinetic study of L-LA/?-CL and rac-LA/?-CL copolymerization systems showed that the polymerization rate of LA was faster than ?-CL,and L-LA showed polymerization rate slightly faster than rac-LA.It was inferred that the copolymers achieved by these complexes were gradient copolymers with gradual change in distribution of LA and ?-CL units.The thermal properties of these copolymers were characterized by DSC analysis,which showed that the glass transition temperature(T_g)of these copolymers changed regularly according to the proportion change of two structural units.In view of the fact that the metal complex is difficult to be separated from the polymerization product,which restricts the application of the materials.People prefer to use the non-toxic metal(zinc)complex to catalyze the polymerization of the cyclic ester.The second part,based on the enolic coordination structure,we successfully design and synthesize a series of enolic zinc complexes as initiator for the polymerization of lactide and ?-caprolactone.The results showed that enolic zinc complexes exhibited higher activity in the polymerization of lactide.The presence of electron-withdrawing substituents and the growth of the amine bridge facilitating the coordination insertion of the monomer could significantly improve the activity of the catalyst.The complex 4a exhibits the highest catalytic activity.The addition of isopropanol to the catalyst system did not have an effect on the catalytic reaction.The end group and NMR analysis showed that the isopropanol did not participate in the coordination with the complex.