Copolymerization of Propylene Oxide and Carbon Dioxide

Abstract

The carbon dioxide (CO2) is naturally present in earth’s natural carbon cycle. Human beings are adding more and more CO2 into the atmosphere and destroying natural sinks like forests that removes it. Increasing CO2 emission has led to change in temperature level which is affecting the growth rate of various plants and other species. Thus, the necessity to reduce CO2 emission has gained worldwide momentum and there are several international programs aimed at balancing CO2 emission through control on emission and its sequestration. One of the possible means to reduce CO2 involves its use as a raw material in synthetic chemistry which is of interest from the perspective of developing clean technologies for environmental protection. Although the process does not use large amounts of CO2 but still gives a greener means for resource utilization since CO2 is a nontoxic, non-flammable and inexpensive substance and is also present in abundance. In view of this there has been continued interest in CO2 activation using different catalysts and its further use as a viable carbon source. There are a number of reactions that can give value added products from CO2. Amongst the most important green reactions and the subject of present thesis is the catalytic conversion of CO2 into polycarbonates and cyclic carbonates by fixing it chemically with epoxides. The reaction products are value added commodities of immense economic importance and provide a methodology for preparation of polycarbonates under milder conditions unlike its traditional industrial preparation method which requires intense vigorous conditions. The aim of the present work has been to carryout copolymerization of propylene oxide and CO2 to synthesize poly(propylene carbonate) and cyclic carbonate using different catalyst/co-catalyst systems. This thesis embodies the subject matter resulting out of this study and is arranged in seven separate chapters. Chapter 1 discusses the background and introduction to the research problem. The chemistry of chemical fixation of propylene oxide and CO2 is discussed in Chapter 2. Opening of epoxide ring by nucleophilic attack and corresponding insertion of carbon dioxide molecule has also been discussed in this chapter. The available literature on copolymerization of propylene oxide and carbon dioxide (CO2) to poly(propylene carbonate) [PPC] and cyclic carbonate are summarized and critically assessed in Chapter 3. In view of the reports in literature, the present work has been planned to carry out copolymerization of vi propylene oxide and CO2 to poly(propylene carbonates) and cyclic carbonates in the presence of different Schiff base catalyst systems. The aim of the present work is to explore different achiral catalyst systems, investigate the relationship between effect of changing electron density around catalyst systems and corresponding polymerization and to optimize the reaction conditions with respect to pressure, temperature, monomer to initiator ratio, catalysts and reaction time. Chapter 4 presents the details of different materials that have been used in the reactions and equipment, along with the procedure for the synthesis of poly(propylene carbonate) and cyclic carbonate. Grade and source of materials used, procedure adopted for distillation of propylene oxide, details of equipment and experimental procedures and various characterization techniques used for the quantification and analysis of the product. Chapter 5 presents the details of the synthesis of different Schiff base ligands and their characterization. These have been used as catalysts/co-catalysts. Synthesis of cyclic carbonate and poly(propylene carbonate) and results of related studies are described in Chapter 6. The major conclusions derived on the basis of present work and scope of future work are summarized in Chapter 7. References cited are listed at the end and Appendix I includes the list of publications/presentations based on the work presented in the thesis and Appendix II includes the NMR spectra of the ligand precursors used for the synthesis of catalysts.