Synthesis And Characterization Of Alkali-metal Complexes And Their Catalysis In The Ring-opening Polymerization Of Rac-lactide

Aliphatic polyesters have found use in high value biomedical applications because of their biocompatible,biodegradable,and permeable properties.For their properties,aliphatic polyesters can be applied in packaging,agricultural materials,drug delivery,medical devices,and so on.The ring-opening polymerization of cyclic esters catalyzed by metal complexes is one of the most effective methods for the synthesis of the polymers with controlled molecular weight and low polydispersity.Polylactide is an important aliphatic polyester and its physical and chemical properties highly associate with its stereoregularity.Hence stereocontrolled synthesis of polylactide received great attention.Recently,alkali-metal catalysts for the ring-opening polymerization of rac-lactide attracted considerable interest.Among the catalysts the alkali-metal complexes with a sandwich-like structure exhibit high activities and lead to polymers with good iso-selectivities.In this thesis,we present synthesis and characterization of a series of alkali-metal complexes and their catalysis towards the ring-opening polymerization of rac-lactide.The main contents are as follows:Chapter 1 summerizes progress in the ROP of cyclic esters catalyzed by various metal complexes,especially alkali-metal complexes.Chapter 2 describes synthesis and characterization of crown ether complexes of sodium and potassium(R-)and rac-1,1’-binaphthyl-2-olates and their catalysis in the controlled ROP of rac-LA.In the presence of BnOH,all the complexes are highly active to catalyze the ROP of rac-LA,leading to polylactide with good molecular weight control,narrow molecular weight distribution and good isoselectivity.It was also demonstrated that the complexes with larger hindered 1,1’-binaphthyl-2-olate ligands led to higher isoselectivity of PLA.Suprisingly,the complexes bearing(R-)and rac-1,1’-binaphthyl-2-olate ligands resulted in polymers with the same Pm values.Chapter 3 introduces synthesis and characterization of crown ether complexes of potassium quinolin-8-olates and their catalysis toward the ROP of rac-LA.The electron effects of the ligands markedly affect catalytic activity of the complexes.Electron-withdrawing substituted groups on the quinolyl ring caused reduction of catalytic activity or even inactivation of the complexes.Although quinolin-8-olate ligands have small steric hindrance,the complex 3.1 still led to stereoselective polymerization in toluene,the Pm ranging from 0.66 to 0.75 when the catalytic polymerization was carried out at room temperature or 0 ℃.Chapter 4 presents synthesis and characterization of crown ether complexes of sodium and potassium iminophenoxides and their catalysis in the ROP of rac-LA.The structure shows that the phenoxy ligand coordinates to the central metal in a monodentate mode,rather than a 0,N-chelate mode.All the complexes are highly active to catalyze the ROP of rac-LA at room temperature.The potassium complex without any substituents except imino group on the aromatic rings,[(18-crown-6)KOC6H4{2-(PhN=CH)}](4.4),exhibited the best selectivity of isotacticity(Pm = 0.75)when the ROP was carried out at 0 ℃.Chapter 5 is about synthesis and characterization of sodium and potassium phenolate complexes with chiral crown ethers as the auxiliary ligands and their catalysis in the ROP of rac-LA.In the presence of BnOH,the complexes exhibited excellent catalytic activity and led to polymers with controlled molecular weights and narrow PDI.However,compared with the normal crown ether complexes reported previously,the chiral crown ether complexes did not lead to higher polymerization selectivity.In addition,effect of co-initiators on the polymerization of rac-LA was also investigated.