Browsing by Subject "salen"
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Item Structural and mechanistic studies into the copolymerization of carbon dioxide and epoxides catalyzed by chromium salen complexes(Texas A&M University, 2006-08-16) Mackiewicz, Ryan MichaelThe ability to utilize cheaper starting materials in the synthesis of commercially important materials has been a goal of scientists since the advent of the chemical industry. The ideal situation would be one in which by combining the correct proportions of hydrogen, nitrogen, carbon and oxygen that virtually anything from simple sugars to complex polymers could be produced. Unfortunately, such processes are flights of fancy often reserved for movies and television shows. On a more realistic level, the utilization of simple molecules and a transition metal catalyst has been a process that industry has exploited for many years. The most easily identifiable process is that for polyolefin production, that employs homopolymerization of simple monomers such as ethylene and catalysts ranging from Ziegler-Natta to metallocene type catalysts. On a more difficult level copolymerization reactions require a delicate balance between two competing reactions and as a result these reactions have been much less successful. For over a decade now the Darensbourg Research Laboratories have focused on utilizing another simple molecule: carbon dioxide. Carbon dioxide is a cheap, inert, nontoxic starting material that appears to be an ideal monomer. Although simplistic, CO2 is also very stable and its utilization in polymerization reactions have proven to be quite complex. In order for us to facilitate these reactions we employ both a transition metal catalyst and a comonomer. Epoxides act as an effective comonomer because the thermodynamic energy gained from breaking the strained three membered epoxide ring overcomes the stability of CO2 and allows the copolymerization reaction to occur. We have demonstrated a great deal of success with this process, most of which will be mentioned throughout this report. The majority of this dissertation will detail our use of salen complexes to optimize this copolymerization process, in order to further the use of CO2 as a viable source of C1 feedstock. Herein, I will illustrate how we have obtained more than a 100 fold increase in the rate of polymer formation as well as detailed mechanistic data that will provide a basis for future catalyst design studies.Item Synthesis, reactivity, and coordination chemistry relevant to the copolymerization of CO2 and epoxides by first row transition metal schiff base complexes(2009-05-15) Frantz, Eric BenjaminExcepting agricultural based products, which themselves require a great deal of energy to produce, our supply of natural resources such as minerals, metal ore, fresh water, coal, oil and natural gas are all limited in supply. The depletion of these substances is imminent and this knowledge weighs heavily on humankind. The utilization of CO2 for the production of polycarbonates is one attempt at exploiting a profoundly abundant and renewable resource. The importance of research in this and similar fields justifies the detailed study of the chemicals and procedures involved with this chemistry. This current work concentrates on the fundamental study of transition metal Schiff base complexes that have shown a great deal of promise in their ability to catalyze the copolymerization of CO2 and epoxide to form aliphatic polycarbonates. A new chromium(III) Schiff base complex has been synthesized and evaluated for its ability to catalyze the formation of polymer. The ligand employed bears an N2O2 coordination sphere identical to the widely utilized chromium(III) and cobalt(III) salen catalysts. This complex was shown to be active towards the copolymerization of CO2 and cyclohexene oxide. Although the activity was less than that seen with chromium(III) salen complex, the study demonstrates that new ligand systems are available beyond salen and deserve further attention. A class of manganese(III) Schiff base complexes was also synthesized and evaluated as catalysts. Although crystallographic data has shown that these complexes are structural analogs to chromium(III) salens, the difference in metal center leads to a nearly complete elimination of catalytic activity. Such a marked difference has been taken advantage of by using this very low activity to study the ring-opening of epoxide in the initial step of the copolymerization both mechanistically and kinetically. It has also been utilized in an evaluation of the coordination chemistry of the polymerization process. This has led to some valuable conclusions about the nature and role of the metal center that previously have not been studied. Manganese(III) salen complexes were also synthesized and evaluated in an effort to compare these important ligands to other Schiff bases and confirm the findings mentioned above.