Study On The Copolymerization Of Carbon Dioxide And Epoxide Catalyzed By SalenMX

With increasing social industrialization,the concentration of CO₂in the atmosphere continues to rise,resulting in the greenhouse effect and a series of associated environmental disasters.However,CO₂is an inexpensive,nontoxic,rich and special renewable C1 building block.To date,wide attention has been paid to the chemical conversion of CO₂into high value-added energy,materials and chemical products its efficient usage.Among them,alternating copolymerization of CO₂and epoxides to synthesize biodegradable polycarbonate（PPC）is considered a promising green polymerization process.However,CO₂is a highly stable and low-reactivity molecule,so developing new catalysts that are efficient,inexpensive and easy to synthesize is crucial.Through efforts made by various researchers,several types of catalytic systems,such as metal porphyrins,β-diimine zinc,salen metal complexes and other homogeneous catalytic systems,diethyl zinc,zinc carboxylate and rare earth metal heterogeneous catalysts have been developed.Among them,the salen MX catalytic systems are commonly studied because they are easy to synthesize,the structure of ligands and complexes are easily modified,and they exhibit mild polymerization conditions,high activity and selectivity of PPC,and the narrow molecular weight distribution PDI.In this study,three types of mononuclear salen ligands were prepared by different salicylaldehyde derivatives with o-phenylenediamine,and dinuclear salen ligand was prepared by the reaction of 3,5-di-tert-butylsalicylaldehyde with 3,3ˊ-diaminobenzidine.With Co as the central atom,combined with different axial anions,a series of new monometallic and bimetallic salen MX catalysts were successfully synthesized.The ligands and complexes were characterized by elemental analysis（EA）,Fourier transform infrared spectroscopy（FT-IR）,ultraviolet（UV）spectrum analysis,and nuclear magnetic（NMR）techniques.A bifunctional catalytic system comprising the synthesized complexes and PPNX was used to catalyze the copolymerization of CO₂and propylene oxide（PO）,and it showed high catalytic activity.The effects of chemical structure,including salicylaldehyde substituents,metal active center and axial anions on copolymerization were systematically investigated,and the catalytic reaction system was further optimized,such as reaction temperature,CO₂pressure,reaction time,cocatalyst and substrate concentration.The results showed that the dinuclear complex4e/PPNTFA had the best catalytic effect under the reaction temperature of 25 ^oC,CO₂pressure of 3.0 MPa,n_[PO]:n_[4e]:n_[PPNTFA]=6000:1:2,and reaction time of 4 h.The TOF value of the copolymerization reaction was as high as 735 h^–1,the conversion of PO was 99%,the selectivity of PPC was 99%,the content of carbonate chain was 81.42%,and the content of end-to-end units was 94.47%.Gel permeation chromatography（GPC）test showed that the relative molecular weight of the polymer was M_n=295×10³g/mol,with a narrow molecular weight distribution PDI of 1.32.In binary copolymerization,the copolymerization of CO₂and PO is the most concerned.The obtained PPC is biodegradable and only produces CO₂and water after combustion,causing less environmental pollution.However,in practical applications,PPC is difficult to be industrialized owing to its low thermal and low mechanical properties.Therefore,improving the properties of PPC,yields a new direction for future research.Ternary copolymerization is one of effective methods to improve the properties of PPC.In this study,two types of the third monomers,DL-lactide（DL-LA）and endic anhydride（EA）were introduced to improve the thermal,mechanical,and degradation properties of CO₂-based polycarbonate.FT-IR,NMR,thermogravimetric（TG-DTG）,differential scanning calorimetry（DSC）,GPC and other techniques were used to characterize the synthesized terpolymer PPCLA and PPCEA.The results showed that terpolymer PPCLA and PPCEA have better thermal,mechanical and degradation properties compared with previously synthesized PPC.