Synthesis And Characterization Of Zinc And Aluminum Complexes And Their Catalysis In The Ring-opening Polymerization Of Cyclic Esters

Aliphatic polyesters such as poly(lactide)(PLA),poly(?-caprolactone)(PCL),Poly(hydroxybutyrate)(PHB),and their copolymers have attracted considerable attention due to their biocompatible,biodegradable,and permeable properties,which are important in biomedical and pharmaceutical applications.These polymers are generally synthesized by the ring-opening polymerization(ROP)of cyclic esters initiated by metal complexes.Single site catalysts have been widely studied because of their good activity and controllability to the ROP of cycle esters.Specific polymerization process can be explained by a chain-end-control mechanism or an enantiomorphic site-control mechanism.The polymerization rate,polymer molecular weight,polymer molecular weight distribution and polymerization selectivity can be precisely controlled by careful design of the ligands,selectivity of central metals and initiators.Chapter 1 introduces background of biodegradable aliphatic polyesters and the polymerization mechanism of cyclic esters.The stereoselective ROP of LA and catalyst systems for the ROP are described in detail.Chapter 2 describes the synthesis and characterization of zinc complexes supported by NHC-based ligands.In the presence of benzyl alcohol,all the zinc complexes catalyze the ROP of rac-LA efficiently.The polymerizations proceed in a living manner and give polymers with controlled molecular weights and narrow polydispersity indexes(PDI).Chapter 3 describes synthesis,characterization of the zinc complexes bearing NHC-based C,N,O-tridentate ligands and their catalysis towards the controlled ROP of rac-LA.Chapter 4 presents synthesis,characterization and catalytic studies of di-and tri-nuclear zinc complexes containing pyrrole ligands([2-{5-R'2NCH2(C4H2N)?]2-CR2).Steric hindrance of the R groups plays a key role in determining the composition of the complexes.When R are ethyl groups a trinuclear complex was formed.When R are aryl groups such as phenyl and 1,1'-biphenyl-2,2'-diyl groups,dinuclear complexes were generated.Trinuclear zinc complex is unstable,the active species that actually initiate the polymerization process are more complex and lead to poor polymerization control.Kinetic study showed that the polymerization followed neither first-order nor second-order kinetics.Under the same conditions,the catalytic activity and controllability of binuclear zinc complexs are better.The polymerization proceeds with first order dependence on monomer concentration.Chapter 5 describes synthesis and characterization of a series of aluminum complexes supported by phenanthroline-based N,N,O-tridentate ligands.In the presence of BnOH,the complexes are efficient catalysts for the ROP of?-caprolactone,rac-lactide and rac-butyrolactone.The catalytic activity of the complexes is affected by the substituents on the ligands.The reactions lead to PCL with good molecular weight control and narrow molecular weight distribution.Isotactic enriched polylactides were obtained by using these complexes while atactic poly(hydroxybutyrate)was got.Kinetic analysis revealed that the polymerization of rac-LA proceeded with first-order rate dependence on both monomer and catalyst concentration.The block polymers were also synthesized utilizing the "living"character of the polymerization catalyzed by the aluminum complexes.