Study On The Copolymerization Of Carbon Dioxide And Epoxide Catalyzed By Transition Metal Complexes

Carbon dioxide is the main greenhouse gas,which is the most abundant resources of C1.Using carbon dioxide and epoxide to produce biodegradable polycarbonate is one of the hot spots.In view of the current research situation,the main problems of carbon dioxide and epoxide copolymerization catalytic systems are low catalytic efficiency,high catalyst cost,harsh reaction conditions,low copolymer yield and complicated catalyst separation.In recent years,the catalytic system used in the copolymerization of carbon dioxide?CO₂?with epoxides mainly includes SalenMX catalytic system,porphyrin catalytic system and rare earth catalytic system.Among them,the preparation method of SalenMX catalyst system is simple,the structure of the ligand and the complex are conducive to modification,the selectivity of the product is high,the content of the carbonate chain segment is high and the reaction conditions are relatively mild.Therefore,two mild catalytic systems are developed in this paper:SalenM^IIICl catalytic system and SalenM^IIITFA Catalytic system.SalenM^IIICl and SalenM^IIITFA catalytic systems were synthesized with 3,5-di-tert-butyl salicylaldehyde-o-phenylenediamine as ligand and Co,Cr,Mn,Fe and Ni as the central atoms,Mainly including SalenCo^IIICl,SalenCr^IIICl,SalenMn^IIICl,SalenFe^IIICl,SalenNi^IIICl,SalenCo^IIITFA,Salen Cr^IIITFA,SalenMn^IIITFA,SalenFe^IIITFA,SalenNi^IIITFA catalysts.The synthesized catalysts were characterized by elemental analysis,FT-IR,UV-vis and TG-DTG.Compared with the results of ligand test,The successful synthesis of five different metal complexes was confirmed.The performance of CO₂ and propylene oxide?PO?copolymerization on the synthesized SalenMX catalyst was tested.The product polycarbonate?PPC?was characterized by FT-IR,¹H NMR,¹³C NMR,TG-DTG.The PO conversion,product selectivity and catalytic efficiency of PPC copolymerization between CO₂ and PO catalyzed by SalenCo^IIICl,SalenCr^IIICl,SalenCo^IIITFA and SalenCr^IIITFA were also investigated.The results showed that the TOF value of SalenCo^IIITFA/PPNTFA catalyst reached 6.59×10³ g/mol·h under the pressure of2.5 MPa CO₂ at 25?.the PO conversion was 98.06%.The selectivity of the product was92.36%.The effects of polymerization temperature,CO₂ pressure,reaction time,amount of catalyst and promoter species on the copolymerization of CO₂ and PO were discussed with SalenCo^IIITFA as catalyst.The results showed that the best catalytic effect was achieved when the reaction temperature was 25?,Cat=[SalenCo^IIITFA]:[PPNTFA]:[PO]=1:1:1000 and CO₂ pressure was 2.5 MPa for 12 hours.TOF value was as high as 6.95×10³ g/mol·h,PO conversion was 99.56%and product selectivity was 96.32%.The obtained product was subjected to PPC analyzed by FT-IR,¹H NMR,¹³C NMR,TG and GPC.The test results showed that the content of carbonate chain segment of the polycarbonate was 98.31%,indicating that the polymer was substantially free of poly Ether segment.The regioselectivity was 93.6%of the product.Thermogravimetric analysis shows that the polymer PPC has a higher decomposition temperature and can be processed routinely over a wider temperature range without regard to thermal decomposition.By GPC test,the polymer relative molecular weight Mn=6700⁷100 g/mol,molecular weight distribution PDI=1.2¹.8.Through the investigation of the copolymerization of CO₂ with propylene oxide,3-chloro-1,2-propylene oxide and phenyl ethylene oxide,the catalytic activity of propylene oxide was found to be the best.The TOF PPC value was as high as 6.95×10³g/mol·h,PO total conversion rate as high as 99.56%,product selectivity of 96.32%.3-Chloro-1,2-propylene oxide and phenyloxirane reacted with CO₂ without polymerization products.The molecular frontier orbital analysis of the reaction substrate revealed that the difference in energy levels between the HOMO and LUMO orbitals of PO is smaller compared to3-chloro-1,2-propylene oxide.The smaller the HOMO-LUMO band gap,the easier the molecule to be excited,the better the catalytic effect.The steric hindrance of PO due to its steric hindrance,electron effect and material rigidity,and the PO as an electron donor also enhances the reactivity of the reaction substrate,so that the catalytic activity of PO reaction is better.Based on the speculation analysis of the reaction mechanism of SalenCo^IIITFA catalyzed copolymerization of CO₂ and epoxide,it was initially concluded that the reaction mechanism could be divided into chain initiation phase,chain growth phase and chain termination phase.