Synthesis And Characterization Of Bipyridine Bisphenolate Aluminum Complexes And Their Catalytic Synthesis Of Polyester

Polyester is a kind of representative environment-friendly polymer materials,which has attracted much attention due to its sustainability,degradability and biocompatibility.The ring-opening polymerization(ROP)of cyclic esters and the ringopening alternate copolymerization(ROAC)of anhydride/epoxide are synthetic methods of polyesters based on chain polymerization mechanism,which have advantages of atomic economy,controllable polymerization and low energy input.Until now,the research on the preparation of polyester by chain polymerization is developing rapidly,and various organometallic catalysts have been developed,which have important applications in the synthesis of polyester.Among them,aluminum complexes have been widely studied due to their low cost of metal source and relatively low biological toxicity.Using fast and reversible chain transfer reaction during the polymerization process can increase the number of polymer chains initiated by each catalyst,thus reduce the catalysts loading and their residuals trapped in the polymer without sacrifice of polymerization efficiency.Moreover,it can effectively regulated the molecular weight and chain structure of the resulting polymer.However,traditional catalysts have poor tolerance to protic impurities such as alcohol and organic acid,leading to the decrease of catalytic activity.Therefore,the development of efficient,controllable and stable aluminum catalysts has important research significance and academic value.Based on bipyridine backbone with high stability,a series of bipyridine bisphenol aluminum(Bpy Bph-Al)complexes were designed,synthesized.Combined with nucleophilic onium salts,they can efficiently catalyze ROP of lactides and ROAC of anhydride and epoxide,producing polyesters and block copolyesters with welldefined and various structures.The main research contents and results are as follows:(1)Bpy Bph-Al/onium salt binary systems can efficiently catalyze the ROP of lactide in excess of epoxide.It was revealed that the in situ generated six-coordinate aluminum(bis)alkoxides are the active species in this polymerization.The active species showed a strong tolerance to alcohol and organic acids,and maintained the high catalytic activity under the dosage of high chain transfer agent.By controlling axial groups,onium anions and chain transfer agents,the end groups of polymer can be effectively regulated.(2)Bpy Bph-Al binary systems can efficiently catalyze the ROAC of various anhydrides and epoxides,especially exhibit excellent catalytic performance for the copolymerization of maleic anhydride with low reactivity.The turn-over frequency is2-5 times higher than that of the traditional Schiff based-aluminum catalyst.The catalytic activity did not decrease obviously at high chain transfer agent concentration(100 equivalent)or low catalyst dosage(0.004 mol%).Notably,unpurified commercial maleic anhydride can be directly copolymerized with epoxides,and residual maleic acid can be used as a chain transfer agent to form unsaturated polyester diols.(3)Bpy Bph-Al binary systems can also efficiently promote the ROAC of thioanhydride/epoxides,producing polythiesters with high efficiency and controllability at relatively high temperature(80 °C).More importantly,based on the different reactivity of cyclic anhydride and thioanhydride toward alkoxide chain ends,this system can selectively catalyze the terpolymerization of cyclic anhydride/thioanhydride/epoxides to synthesis polyester-b-poly(ester-alt-thioester)triblock copolymers with diverse structures and well-defined sequences in one step.