Bifunctional Multi-Center Aluminum Porphyrin-Catalyzed Copolymerization Of Carbon Dioxide And Propylene Oxide

Carbon dioxide（CO₂）is the dominant greenhouse gas and also a rich reservoir of C1.The co-polymerization of CO₂ with epoxides to form polycarbonates is a highly promising method for the utilization of CO₂.In the research work that has been done,most of the highly active catalysts for CO₂ copolymerization（metal Salen,DMC,etc.）were based on heavy metal active center such as cobalt（Co）and chromium（Cr）.The polymer products do not meet the needs of food,medical and other fields.Moreover,when CO₂ copolymers are used as biodegradable materials,the residues of heavy metals can cause environmental pollution.This is not in line with the concept of green chemistry.Aluminum（Al）is a metal with good biocompatibility.The use of Al as the active center is promising for the synthesis of environmentally friendly catalysts.Previously reported Albased catalysts usually use porphyrin as ligands,and the low catalytic activity severely limits their wide application.The design of porphyrin structures to enhance catalytic activity is becoming a key area of research in green catalytic systems.In this paper,we aim at building synergistic effects of multiple active centers within the catalyst molecules.The multicenter porphyrin catalyst and bifunctional multicenter porphyrin catalysts with quaternary ammonium salts at the end of the chains of different polymerization degrees were designed and synthesized by RAFT polymerization.The characterization of the catalyst ligand structures was completed by 1H NMR.The bifunctional polycentric aluminum porphyrin catalysts were successfully prepared by reacting diethylaluminum chloride with the polycentric ligands and used in the copolymerization reaction of carbon dioxide with propylene oxide.Ring-opening homopolymerization of propylene oxide（PO）is essential to ensure its copolymerization with carbon dioxide.The bifunctional multicenter aluminum porphyrin catalyst was used for the ring-opening homopolymerization of propylene oxide showing excellent catalytic performance.The conversion frequency（TOF value）was up to 2900 h-1 and the number average molecular weight（Mn）of the product exceeds 200 kg/mol.The catalyst has been shown to enable non-chiral catalyst-mediated selective polymerization of racemic propylene oxide via "chain end control mechanism",resulting in mechanically strong,isotactic-poly（propylene oxide）with a tensile strength of 74 MPa and elongation at break of 750%.The bifunctional multicenter aluminum porphyrin catalyst for CO₂ and PO copolymerization gives a higher catalytic efficiency than the monocentric system.The unique structure of the anchored quaternary ammonium salt frees it from the dependence on co-catalyst and provides high catalytic activity when used alone,yielding polypropylene carbonate（PPC）with Mn of 4.3～25.0 kg/mol.The catalyst breaks away from the temperature and concentration limitations of conventional catalysts and maintains a polymer selectivity of over 98%and the TOF value of 1200 h-1 for the CO₂ and PO copolymerization reaction at 100℃.There was virtually no loss of reactivity at lower catalyst concentrations and TOF values were maintained a high level of 1000 h-1 at[PO]:[Al]of 60,000:1.In addition,a preliminary study was carried out to control the ratio of quaternary ammonium salts to metal centers to investigate the quantitative relationship between metal centers and co-catalysts for this type of catalyst.The relationship between the two-component synergistic effect and the number of metal centers was constructed:the unit quaternary ammonium salt with a 4 molar ratio of metal complexes showed the best synergistic effect and possessing the highest catalytic activity.On this basis,the catalytic activity increased with the number of metal centers.Finally,the results of the polymerization of single-center catalysts and polycentric catalysts were analyzed.The effects of intermolecular synergy with quaternary ammonium salt,intramolecular synergy effect of bifunctional polycentric structure on catalytic activity and the influence of relatively close polycentric structure on regional selectivity and stereotactic selectivity were studied.The mechanism of the polymerization reaction catalyzed by bifunctional multicenter aluminum porphyrin catalyst was further elaborated.In conclusion,a bifunctional multi-center aluminum（Ⅲ）porphyrin catalyst was designed and synthesis in this study.It has shown excellent catalytic performance in catalyzing the homopolymerization of PO and the copolymerization of CO₂ with PO by virtue of the intramolecular "bifunctional" and "multicenter" cooperative effects.The complexes are an inspiration for the construction of future bifunctional and multi-center catalysts.