| Polymer materials are one of the important materials for industrial development,but most of them are petroleum-based sources and non-degradable,resulting in global environmental pollution.Bio-based materials have received widespread attention due to the renewability and degradability.Polylactide is one of the representatives of biobased degradable materials,but a single polylactide cannot meets the requirements of large-scale commercial applications,and mixing with other petroleum-based polymers can improves its properties,but it is not an environment-friendly method.Therefore,to improve the properties of polyesters,it is necessary to develop polyesters with different microstructures,components,and topologies.Linear polyesters with different microstructures and components can be efficiently obtained through controlled ring opening polymerization of monomers from different biological sources: for example,O-carboxylic anhydride(OCAs)with various substituents can be easily prepared from α-amino acids or α-hydroxycarboxylic acids,then a wide variety of polyesters can be obtained through ring opening polymerization(ROP)of OCAs,thereby regulating the polyester performances.Cyclic polyesters can be obtained under specific reaction conditions: turning the linear polyester into hoops or enlarging hoops from established templates.However,it is still a challenge to synthesize cyclic polyester using a living active polymerization method under a controllable model.Whether the linear polyester or cyclic polyester,the catalyst plays an important role in the process of controlled syntheses.At present,catalysts can be divided into two types: organic catalysts and metal catalysts.Among them,due to the adjustable metal center and the ligand,the metal catalysts show a better performance for the ringopening polymerization of different types of monomers.Therefore,in the process of developing polyester with different microstructures,components and topologies,it is important to design high-performance metal catalysts.In this thesis,different types of zinc,magnesium and yttrium metal catalysts with large steric hindrance were designed and synthesized,and the stereoselective ring opening polymerization of different types of monomers was carried out to obtain linear polyesters with different microstructures and components;The catalyst connecting the initiator group with the ligand was designed and synthesized,and the controlled synthesis of small and medium molecular weight cyclic polyester was achieved via the ring-opening polymerization of different types of monomers.Chapter 1: The stereoselective ring opening polymerization of lactide with magnesium,zinc and yttrium metal complexes,the ring opening polymerization of Ocarboxylic anhydride and the synthesis of cyclic polyester in recent years are introduced.Chapter 2: OCAs monomers with different substituted groups have attracted much attention due to the biodegradability,high activity,simple preparation process.And different kinds of polymers with controllable molecular weight and narrow molecular weight distribution can be obtained by the ROP of rac-OCAs.At present,the metal catalysts for stereoselective ring-opening polymerization of rac-OCAs have achieved some results,but catalysts with high-stereoselective ring-opening polymerization of rac-OCAs have not been widely studied,and rare earth metal catalysts for rac-OCAs have not yet been studied.Therefore,Salen-type yttrium metal complexes were designed and synthesized to achieve stereoselective ring-opening polymerization of rac-LacOCA and rac-PheOCA,the isoselectivity Pm reached to 0.86 and 0.91 at room temperature,respectively,which was the first case of the isoselective ring-opening polymerization of rac-OCAs monomers by rare earth metal complexes,and the resulting polyesters with molecular weight was controllable and the molecular weight distribution was narrow.Chapter 3: The controlled syntheses of cyclic polyester is a major problem in the current development.In this chapter,a series of yttrium,zinc,magnesium aminobisphenol complexes with initiating groups are designed and synthesized.Among them,the yttrium complex has a bridging binuclear structure,but due to the strong alkalinity of the complex,the monomer was racemized during the process of the ring opening polymerization(ROP)of L-lactide(L-LA),and the complex was inactivated.When the magnesium complex with 4-dimethylaminopyridine(DMAP)as aco-catalyst was utilized as an imitator for the ring opnening polymerization of L-LA,the molecular weights of obtained polymers are controllable and the molecular weight distributionsare narrow.On the basis of the controlled syntheses of linear polylactide,the cyclic polyester can be obtained via increasing temperature in one-pot reaction.However,this method is more suitable for the synthesis of low and medium molecular weight cyclic polylactide,when the ratio of monomer to catalyst increases to 100:1,the molecular weight distribution of obtained cyclic polyester becomes broad.Chapter 4: In order to further extend the type of cyclic polyester,the stereoselective polymerization of LacOCA and PheOCA and the synthesis of cyclic polyester were studied.In the presence of the co-catalyst 4-methyl-pyridine(4-Me-Py),aminobisphenoxide zinc complexes can catalyze the controllable polymerization of LLacOCA to obtain linear polymer.When temperature increases to 100 oC,cyclic polyester can be obtained after 6 days.Similar to the synthesis of cyclic polylactide,this system can be used to synthesize small and medium molecular weight cyclic polyesters.In the synthesis of big cyclic polyester,the polymer molecular weight distribution is wide.Due to the serious side reaction of transesterification during the polymerization of L-PheOCA monomer,the controllability of its polymerization reaction is poor,which directly leads to the weight of the resulting cyclic polyesters be lower,and only small ring polyester is obtained.The results show that the first step of one pot step polymerization is the precondition of controlled polymerization of cyclic polyester.At room temperature,rac-LacOCA was subjected to a linearly controllable stereoselective ring opening polymerization,and the isotactic stereoselectivity of the reaction can reach 0.75.Chapter 5: Summary of work and prospect of the topic. |
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