Research On The Ring-opening(Co) Polymerization And The Structural Control Of Salicylic O-Carboxyanhydrides Catalyzed By Organic Base-Thiourea(Urea)

The raw material source of Poly（salicylic acid）（PSA）is natural salicylic acid（SA）and its derivatives,which have good biocompatibility,low cytotoxicity,and rich pharmacological activity;it is widely used in the field of biomedicine.PSA has good degradability and thermal physical properties,making it an excellent alternative to petroleum-based chemicals.Currently,the main method for preparing poly（salicylate）materials is through ring-opening polymerization（ROP）using lactide as the monomer.However,due to the low activity of lactide monomers,higher polymerization reaction temperatures are usually required.Polymerization reactions using simple Lewis bases/alcohols or stannous octoate/alcohols as catalysts have low efficiency and poor controllability,resulting in low molecular weight products.In addition,the main chain structure of polymers prepared by ROP of lactide is a single alternating structure,and the sequence structure is not adjustable.Moreover,there is a lack of research on the ROP of salicylate-derived monomers and the ROP of salicylate monomers with other types of cyclic monomers.Due to the lack of effective synthesis methods for high molecular weight polymers and structural regulation methods for polymers,the performance improvement and application research of poly（salicylate）series materials have been greatly limited.ROP has the advantages of mild reaction conditions,low side reactions,and easy access to high molecular weight polymers.The monomers of salicylic acid-O-carboxylic acid anhydride（SAOCA）and its derivatives,which have high monomer activity and rich substituent structures,can undergo rapid and controllable polymerization under mild conditions,thus having the potential to prepare high molecular weight polymers.The Lewis acid-base pair composed of Lewis base（OB）/thiourea or urea（TU or U）is widely used in the active and controllable polymerization of cyclic ester monomers.The catalytic activity of OB/TU（U）can be controlled by the substituent structure of acid and base,so that it has the ability to regulate polymerization activity and chemical selectivity.Therefore,this dissertation hopes to develop a new approach to catalyze the ROP and copolymerization（ROCOP）sequence structure of SAOCA and its derivative monomers through monomer design,exploration of OB/TU catalyst combinations,and research on reaction conditions.Firstly,based on the OB catalyst,this article introduces the interaction between TU and OB to form acid-base reactions,and achieves controllable ring-opening polymerization of high SAOCA monomer equivalents by regulating the catalytic activity center through excessive TU.The acid-base combination of N-β-aminoethyl-γ-aminopropylmethyl dimethoxysilane（BEMP）/3,5-bis（trifluoromethyl）phenylcyclohexylthiourea（TU-2）can catalyze polymerization in the form of thiourea anion.By introducing excess TU-2 to regulate the chain growth rate and steric hindrance,the occurrence of polymerization-induced self-assembly was limited.Under the catalyst ratio of BEMP/TU-2=0.5/3,the SAOCA monomer was ring-opened polymerized in a mixed solvent of N,N’-dimethylformamide/tetrahydrofuran（DMF/THF=8/1）,and a high molecular weight PSA with a molecular weight of 152.2 k Da and a molecular weight distribution of?=1.34 was successfully prepared.Through exploring the applicability range of the OB/TU-2 catalytic system,the controllable ROP of various SAOCA derivative monomers was realized for the first time.The polymerization kinetics,polymerization mechanism,and key factors limiting polymer chain growth were deeply investigated.Secondly,the regulation of the copolymerization sequence structure of monomers with similar structures using organic catalysts remains a significant challenge.This article establishes a method for regulating the copolymerization sequence structure of SAOCA monomers using the OB/TU catalytic system,which further regulates the interaction between catalysts through the regulation of the p Ka values of acids and bases,and regulates the catalytic activity through the regulation of the steric hindrance of the catalytic active center.Based on the catalytic combination of 1,8-diazabicyclo[5.4.0]undec-7-ene（DBU）/TU,when the feeding ratio of DBU/TU is fixed at 0.5/3,the structure sequence of the copolymer obtained changes from random copolymerization to block structure as the acidity of TU changes from weak to strong.In addition,when the p K_aof TU is lower than that of DBU,the copolymerization product changes from random to block structure as the amount of TU increases.Under the condition of a feeding ratio of 200/200 for 5-methyl salicylate-O-carboxylic anhydride（5-Me SAOCA）and 4-fluorosalicylate-O-carboxylic anhydride（4-FSAOCA）,a copolymer with a molecular weight close to 50 k Da and a molecular weight distribution controlled at 1.10-1.21 was successfully synthesized from random to block salicylate copolyesters.Through research on reaction kinetics and catalyst activity regulation,a method for regulating the sequence structure of ROP from various SAOCA monomer combinations was constructed.Thirdly,4/5-methoxysalicylic acid（4/5-Me OSA）and 5-chloro/5-bromosalicylic acid（5-Cl/5-Br SA）have very rich and mild pharmacological activities.However,severe phase separation occurs during the homopolymerization of these monomers,making it difficult to characterize the products.In this Dissertation,a neutral molecule synergistic DBU/N,N’-bisisopropylthiourea（TU-1）catalyst was used for random copolymerization,avoiding the precipitation of products caused by strong hydrophobic effects and chain stacking due to the formation of continuous identical repeating units.By adjusting the feeding ratio of monomer combinations such as 4-methoxysalicylic acid-O-carboxylic anhydride（4-Me OSAOCA）,a random polymer with a molecular weight greater than 30 k Da and a molecular weight distribution of less than 1.25 was successfully obtained.The proportion of 4-Me OSA repeating units in the polymer reached 76%.A new approach to regulate the sequence structure of salicylic acid copolymers dominated by 4-Me OSA repeating units was developed.Furthermore,block copolymers are the basis for developing high-performance materials.By regulating the structure of ring-opening copolymerization of SAOCA and lactide（LA）,it is possible to combine the excellent properties of aliphatic polyesters.However,due to the weak nucleophilic character of the carbonate group at the end of PSA,it is difficult to copolymerize SAOCA/LA.This article utilizes the activation characteristics of OB on nucleophilic groups in the OB/U catalytic system,and achieves the regulation of the copolymerization sequence structure from block to random by regulating the activation and CO₂removal ability of OB on initiators and chain end groups.In this study,the combination of DBU/N,N’-bis（trifluoromethyl）phenylurea（U1）（DBU:U1=1.5:1.5）,which has the activity of activating monomers and initiators,was used as a catalyst to achieve the block copolymerization of SAOCA/LA.By using the characteristics of two monomers with significantly different reaction rates,the catalyst’s ability to regulate the multiblock structure was explored through batch feeding,and a heptablock copolymer with a molecular weight of up to46.2 k Da was prepared.By using 1,5-diazabicyclo[4.3.0]non-5-ene（DBN）/N-bis（trifluoromethylphenyl）-N’-phenylurea（U2）（DBN:U2=2:2）as a catalyst with stronger ability to enhance the nucleophilicity of initiators,random copolymerization of SAOCA/LA was realized.Through in-depth exploration of polymerization kinetics and catalytic mechanism,a method for regulating the sequence structure of SAOCA/LA ROP was established.The thermal properties of polymers with different molecular weights and copolymers with different sequence structures show that the properties of materials are directly related to their structure.The synthesis methods of high molecular weight and different sequence structure materials are of great significance for the preparation of materials with special structures and properties.Finally,the development of chemically recyclable polymers is an effective strategy to address the increasingly serious environmental pollution and sustainable development.However,there are few reports on the chemical recycling of salicylic acid-based polymers.In this Dissertation,3%mol of1,5,7-triazabicyclo（4.4.0）dec-5-ene（TBD）was used as a catalyst,acetonitrile as a solvent,and 80°C as the reaction temperature to achieve the efficient degradation and recycling of salicylic acid-based polymers.The efficient degradation and recovery of salicylic acid-based polymers have been achieved,with a recovery rate of over 70%.Moreover,research has shown that there are significant differences in the degradation rates of copolymers with different substituents and sequence structures under the same degradation conditions,indicating that the degradation performance of materials is also affected by their structure.The preparation of copolymers with different sequence structures is of great significance for achieving sustainable development strategies.