| Carbon dioxide is the most fundamental carbon resource and is indispensable for the existence of all living organisms on Earth, including mankind, via photosynthesis by green plants. But excessive CO2 from industrial emission leads to the greenhouse effect. Therefore, recycling CO2 and chemical fixation of CO2 are receiving increased attention. On the other hand, chemical utilization of carbon dioxide has been rather limited. The synthesis of biodegradable aliphatic polycarbonates using carbon dioxide as raw materials is one of the most challenging works in carbon dioxide fixation field, among which, Poly(propylene carbonate) (PPC) from alternate copolymerization of carbon dioxide and propylene oxide (PO), have been given the most attention. Though transparent film having low gas permeability can be made from PPC, its thermal and mechanical properties are poor, one reason lies in that PPC is a weak polar hydrophobic polymer with weak molecular chain interaction, and the existence of many weak polar, flexible C-O-C blonds in the backbone leads to its amorphous state, enhancing molecular chain interaction of PPC is therefore badly needed, and one possible way is to control its chain structure.The synthesis of biodegradable aliphatic polycarbonates using carbon dioxide as raw materials is one of the most challenging works in carbon dioxide fixation field, among which, Poly(propylene carbonate) (PPC) from alternate copolymerization of carbon dioxide and propylene oxide (PO), have been given the most attention. Although there is growing effect to develop the effective cata1yst for the copolymerization of CO2 and epoxide, some problems still plague these catalyst systems, such as low molecular weight and polydispersity indices, inferior polymer selectivity, irregular region- and stereo- chemistry and so on. The metal Schiff base catalyst is an efficient catalyst for the alternating copolymerization of CO2 with epoxides. Some problems still plague these catalyst systems, such as low catalyst activity, inferior Polymer selectivity, irregular region- and stereo- chemistry and so on. Although the kind of catalysts is suited to control molecular weights and molecular weight distributions of the produced copolymer as well as to regulate selective formation of the polycarbonates. The X-ray crystal structure of the cobalt/Schiff base complex has not been reported. So it is difficult to explain the copolymerization mechanism.In this dissertation, One series of complexes (salenCoIII X) was synthesized, and the X-ray crystal structure of some complexes was presented. Based on the X-ray crystal structure and results of copolymerization of CO2 and propylene oxide, the copolymerization mechanism was examined further.The binary catalyst systems composed of SalenCoIII X and ionic ammonium salt as cocatalyst can efect1vely promote the asymmetric copolymerization of CO2 and racemic propylene oxide(rac-PO) to produce poly(propylene carbonate) (PPC) with excellent activity, >95% head-to-tail connectivity, and >99% carbonate linkages. The achievements will besummarized below:First, the recent advance in the catalytic asymmetric epoxidation and its corresponding structural studies were reviewed.One series salen complexes were designed, synthesized and charaterized by IR, NMR, element analysis etc. Among them, some complexes were also charaterized by X-ray diffraction analysis. The catalytic behaviors of these complexes for CO2/rac-PO alternating copolymerization were investigated. The influences of the ligand architecture, molecular configuration and electronic effects on the polymerization performances were demonstrated in detail.The axial X group of complex played a role in determining the order of the catalytic activity of these catalysts. The electronic effect of the axial X group in terms of greater advantages for producing linear polycarbonates by successive insertion of CO2/rac-PO towards the M–X bond as compared with the formation of cyclic carbonates by a back-biting reaction from the propagating alkoxide played important role. The electronic effect of the substituents R on the phenolates of the ligand also affected the catalytic activity of catalyst. While the PPC/PC selectivity as well as polymer head-to-tail linkages increases in the opposition order The bulky substituents R on the phenolates of the ligand influence the ring-opening position of the rac-PO incorporated next to the growing polymer chain and thus significantly influence the selectivity of PPC and polymer head-to-tail linkages. Both the axial group X of the complex and the substituents on the ligand do not have a clear effect on the resulting copolymer molecular weights.X-ray diffraction analysis confirmed that complex was triclinic species with a six-coordinated central cobalt octahedron in their solid. Complex 2 is triclinic with a six-coordinated cobalt atom in the center. CobaltIII center is coordinated by two phenolate oxygens, two imine nitrogens of the salen ligand and by the two oxygen atoms of 2,4-dinitrophenoxy, which occupy the axial positions of a distorted octahedral coordination geometry.The effect on reaction temperature, pressure, time, the molar ratio of cocatalyst to catalyst on fCO2 of copolymer, and effect of preparative method of catalyst on efficiency were thoroughly investigated. The results showed that the good copolymerzation conditions of CO2 and rac-PO should be generated at 2 MPa, 40-60°C. The influence of temperature on the copolymerization is also obvious. At a lower temperature, the catalyst was deactivated with carbonate linkages nearly 100%. At an increased temperature, the activities of catalyst were greatly enhanced with the same carbonate linkages. However, cyclic carbonates were additionally produced at elevated temperatures. Catalyst system kept their catalytic activity at low CO2 pressures. Even at 0.2 MPa, it was also active maintaining almost 50% of its optimal TOF. However, the increase of the pressure to 4 MPa resulted in a significant drop-off in reaction rate.Alternating copolymerization of carbon dioxide with epoxides to make aliphatic polycarbonates is one of the significant achievements for the chemical fixation of carbon dioxide. These polycarbonates not only exhibit attractive properties, but also display great potential as biodegradable polymeric materials, which have brought them extensive concerns as CO2-based materials. Great efforts have been dedicated directly toward the development of catalysts and mechanism of this copolymerization during the past forty years. By analyzing all of these investigations mentioned herein, such a noticeable conclusion can be drawn definitely, that is, activities and selectivity of the copolymerization catalyzed by tetradentate ligand complexes can be remarkably enhanced in the presence of cocatalysts, most of which are N-containing compounds.The molar ratio of monomer to catalyst as well as cocatalyst to catalyst was also studied here. It showed that higher molar ratio of monomer to catalyst increased the catalytic efficiency of catalyst. Additionally, the observed results also showed a pronounced dependence on the cocatalyst loading when keeping the concentration of catalyst constant. As indicated in the experimental results, the activities were substantially enhanced upon initially increasing the concentration of ionic ammonium salt. However, there was a dramatic decrease in the activities of catalyst as well as an increase in polyether linkages at a higher cocatalyst loading (more than 30 equiv). Reasonable explanations have been presented in this dissertation combining the proposed mechanism.High yield of turnover frequency and high molecular weight of 72.5 kg/mol were achieved at an appropriate combination of all variables. The structures of as-prepared products were characterized by the IR, 1H NMR, 13C NMR measurements. The linear carbonate linkage, highly regionselectivity and almost 100 % carbonate content of the resulting polycarbonate were obtained with the help of these effective catalyst systems under facile conditions. |
PDF Full Text Request