Syntheses Of Sequence-controlled Copolyesters Catalyzed By Metal Complexes

Biodegradable polyesters,represented by poly（α-hydroxy acids）（PAHAs）,have attracted much attention due to their excellent biodegradability,biocompatibility,and renewability.Due to its outstanding performance in controlling molecular weight,ring-opening polymerization（ROP）of cyclic monomers is currently the most promising method for preparing degradable polyesters.Many metal complex catalysts/initiators have been used to prepare PAHAs so far.With the continuous increase in production and living needs,the requirements for the performance of biodegradable polyesters are also gradually increasing.In addition to the challenging task of discovering new industrialized monomers,using a limited number of monomers to form new copolymers is another effective solution for addressing the limited variety of polyesters and controlling their properties.Further precise tuning of copolymer properties can be achieved by altering the sequence structure of multiple monomers along the copolymer backbone.Therefore,controlling the different sequence structures of a limited type of monomers in biodegradable polyester copolymers will undoubtedly provide more opportunities for developing new types of biodegradable polyesters and precisely regulating their properties.However,due to the lack of polymerization systems that exhibit catalytic activity and high stereoselectivity,regioselectivity,or chemoselectivity for the ROP of multiple cyclic monomers,synthesizing sequence-controlled biodegradable copolymers remains a challenging issue through ring-opening copolymerization.Based on the above reasons,in this thesis,the following four aspects of work based on investigating and summarizing relevant literature were conducted.And this thesis consists of the following five parts:Part 1:Brief introduction of the synthesis methods and mechanisms of sequence-controlled degradable copolyesters and summarization the research progress of sequence-controlled degradable copolyesters synthesis reported so far.Part 2:The stereoselective homopolymerization and alternating copolymerization of lactic acid-derived O-carboxyanhydride（Lac OCA）and malic acid O-carboxyanhydride（Mal OCA）using an amine tris（phenolate）hafnium alkoxide complex as an initiator:firstly,the syndioselectivity for the ROP of rac-Lac OCA reached up to 0.91（P_r=0.91）,and the isotactic selectivity for the ROP of rac-Mal OCA reached up to 0.78（P_m=0.78）.The degree of alternation for the resulting poly（Lac OCA-alt-Mal OCA）was high up to 86%.The experimental results demonstrate that this ROP system,which exhibits high syndioselectivity only for Lac OCA polymerization,can also initiate the syndioselective alternating copolymerization between the two monomers with different stereo selectivities.Part 3:To prepare isotactic-alternating copolyester of lactic acid and mandelic acid via the regioselective ROP of methyl-6phenyl-1,4-dioxane-2,5-dione（MPDD）using a bulky zinc complex as an initiator:the zinc complex exhibited a high regioselectivity of 0.96(P_reg=0.96)in the ROP of enantiomerically pure MPDD;unlike random amorphous copolymers,the resulting alternating copolyester showed a glass transition temperature of 90.8 ^oC and a melting point of 152.4 ^oC;a stereocomplexation interaction between two opposite enantiomeric isotactic polymer chains was also discovered in the isotactic-alternating copolymer.Part 4:Building on the work in Part 3,we used a zinc complex with large steric hindrance to achieve a high regio-and stereo-selectivity in the homopolymerization of rac-MPDD(P_reg=0.98,P_r=0.97).Furthermore,by alternating copolymerization of MPDD and lactide（LA）,we prepared an ABAA-type sequence-controlled heterotactic copolyester of lactic acid and mandelic acid.We also demonstrated that the thermal properties of these sequence-controlled copolyesters could be easily adjusted by changing the incorporation ratio of the two monomer units,the regularity,and the monomer sequence structure.Part 5:The self-switchable ROP of O-carboxyanhydrides（OCAs）,3-phenyl-1,4-dioxane-2,5-dione（PDD）,and LA to synthesize sequence-controlled A_x（BC）_yA_z and A_x（BC）_yD_z type triblock PAHAs from three monomer mixtures initiator of an amine tris（phenolate）hafnium alkoxide complex.Depending on the stereoselectivity and regioselectivity of the catalytic system,the tacticity and region-regularity of each segment of the triblock copolymers can be precisely controlled.With the high degree of polymerization control,the catalysis can be selectively manipulated to form multi-block copolyesters with 15 blocks through sequential additions of the three-component mixture.Finally,all the work during the doctoral period was summarized and future directions for the research topic studied in the paper were proposed.