Catalytic Effect Of Metal Complexes On Carbon Dioxide Conversion To Polycarbonate

Carbon dioxide is the main greenhouse gas,which is the most abundant resources of Cl.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,eight effective Salen-type catalytic systems are developed in this paper.Three mononuclear Salen ligand skeletons were prepared by reacting salicylaldehyde and o-phen.ylenediamine with different substituents.The binuclear Salen ligand backbone were prepared by reacting 3,5-ditertbuylsalicylaldehyde and 3,3'-diaminobenzidine.Eight complexes with Co as the central atom,combined with different axial anions successfully synthesized,which mainly including SalenCo???Cl?1?,Salen-2^tBuCo???Cl?2?,S alen-4^tBuCo???Cl?3?,Salen-4^tBuCo???DNP?4?,Salen-4^tBuCo???TFA?5?,Salen-8^tBu[Co???Cl]₂?6?,Salen-8^tBu[Co???DNP]₂?7?and Salen-8^tBu[Co???TFA]₂?8?.The performance of CO₂ and propylene oxide?PO?copolymerization on the synthesized catalyst systems were tested.The product polycarbonate?PPC?was characterized by FT-IR,1H-NMR,13C-NMR,TG-DTG.Meanwhile,the effects of reaction temperature,CO₂ pressure,reaction time,catalyst type and reaction substrate concentration on the catalytic system of the newly synthesized dual-nuclear complex Salen-8^tBu[Co???TFA]₂?8?/PPNTFA were investigated.The results showed that the reaction was carried out for 4 hours under the conditions of reaction temperature of 25 ?,n?PO?/n?catalyst?/n?cocatalyst?=3000:0.5:1,CO₂ pressure of 3.0 MPa and the product was obtained.The TOF value of PPC is as high as 1475.93 h-1,,the PO conversion rate is 99.82%,the product selectivity is 99.57%,the carbonate chain content of the product PPC is 89.66%and the unit content of the head-to-tail is 91.44%.By GPC test that the relative molecular mass of the polymer was Mn = 293 kg/mol,and the molecular weight distritbution was PDI=1.35.The above calculation model was optimized by the density functional theory calculation?DFT?method using the Guaessian 16 calculation program to construct the ideal copolymerization catalyst model for CO₂ and PO:Ligand skeleton The upper enantioselectivity is increased,and the greater the anion electronegativity of the axial ligand,the greater the catalytic activity of the complex.At the same time,combined with the results of catalytic reaction,it is proposed that the copolymerization reaction of CO₂ and PO catalyzed by Salen-type dinuclear metal complex is different from that of mononuclear catalytic system,explaining the reason why the catalytic activity of the dual-nuclear catalytic system is far stronger than that of single-nuclear catalytic system.