Browsing by Subject "polycarbonate"
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Item Advances in Synthesis of Co- and Ter- Polycarbonates and Polyesters from Non-Petroleum Feedstocks and Kinetic Studies of Ligand Substitution from Manganese Half-Sandwich Complexes(2012-07-16) Poland, Ross RiversThis dissertation is written in two parts. The first pertains to polycarbonate and polyester synthesis using relatively benign processes. The synthesis of polycarbonates from the coupling of CO2 and epoxides catalyzed by transition metal catalysts has long been studied in the DJD group. The benefits of this process are that it utilizes comparatively benign reagents, can be performed using no extraneous solvent, and is 100% atom efficient. A method potentially useful for achieving more desirable polycarbonate properties is to produce an epoxide A/epoxide B/CO2 terpolymer, thus allowing more fine "tuning" of properties to what one may desire while simultaneously influencing relative epoxide reactivity to potentially increase catalytic turnovers. Specifically, the coupling of propylene oxide and cyclohexene oxide with CO2 to yield a random copolymer with tunable properties has been studied via a Fineman-Ross analysis. Propylene oxide was found to be incorporated into the resultant polymer chain with anywhere from 4-10 times the preference of cylcohexene oxide. Although it has been reported as early as 1969, the copolymerization of epoxides and cyclic anhydrides catalyzed by transition metal complexes to yield polyesters via a chain-growth mechanism has recently gained much attention. This robust method of polyester synthesis can utilize rather inexpensive reagents to synthesize an array of polyester products which have a wide range of Tg values (-30 degrees C ? 90 degreesC), achievable through simple monomer selection. The second part of this dissertation deals with the kinetic study of ligand substitution from manganese carbonyl metal fragments. Some time ago it was postulated that complexes of the (Cp)M(CO)2L variety undergo ligand substitution via a associative mechanism allowed by a haptotropic eta5-eta3 shift in the eta5 ligand. Through kinetic studies and theoretical modeling, an approximate activation energy barrier of ~34 kJ/mol has been calculated for the ring slip of (2,5-dimethylpyrrole)Mn to occur. Additionally, further kinetic studies were performed in which Tp, a ligand electronically similar to Cp, was compared to MnCp complexes.Item DEVELOPMENT AND MECHANISTIC STUDIES OF THE CHROMIUM TETRAMETHYLTETRAAZAANNULENE CATALYST SYSTEM FOR THE COPOLYMERIZATION OF CARBON DIOXIDE AND EPOXIDES(2010-07-14) Fitch, ShawnA prominent goal of scientists is to develop products and processes to meet the ever-growing needs of society. Today's needs include products that are economical, specialized, and made through processes with minimal impact on the environment. One such product that serves an important and widespread need is poly(bisphenol A carbonate) for its physical properties and ease of synthesis and processing. However, this polymer does not meet the growing need of being environmentally benign as production involves carcinogenic, chlorinated solvents and toxic monomers that can leach out from the polymer product. An answer to this new demand is the development of a different process for the production of polycarbonate plastics utilizing carbon dioxide and epoxides. Carbon dioxide is an attractive monomer that is cheap and nontoxic, and its utilization signifies an important contribution to counteract global greenhouse emissions. The stability of carbon dioxide has posed a significant and complex challenge towards its utilization. Epoxides are attractive since they are synthesized from a wide variety of olefins, both naturally occurring and those derived from petroleum. The exploration of catalysts to facilitate the coupling of epoxides to carbon dioxide to afford polycarbonates has been under investigation in the Darensbourg lab for fifteen years, and has lead to the development of several successful systems such as zinc bisphenoxides and chromium salens. This dissertation focuses on the development of another successful catalyst system, chromium tetramethyltetraazaannulene, and further elucidation of the mechanism by which polycarbonates are formed. Herein, aspects of the copolymerization process using this system will be discussed in detail, such as cocatalyst and pressure dependence, catalyst derivatization, and kinetic and mechanistic investigations. The end result of these investigations is the development of the most active chromium-based catalyst for the copolymerization of cyclohexene oxide and carbon dioxide and a better understanding of how the copolymer product is produced.Item Metal catalyzed copolymerization processes involving carbon oxides as substrates(Texas A&M University, 2005-11-01) Phelps, Andrea LeeStudies concerning two different copolymerization processes are detailed in this dissertation: propylene oxide/CO2 coupling to afford poly(propylene carbonate) and Nbutylaziridine/ CO coupling to afford poly-??-butylalanoid. The copolymerization of propylene oxide and CO2 to form the industrially useful poly(propylene carbonate) has been investigated employing chromium(salen)N3 complexes as catalysts. Unfortunately the reaction could not be studied in real time via in situ infrared spectroscopy, thereby obtaining detailed kinetic data, because of the copolymer-limited solubility in most solvents. Studies employing batch reactor runs concentrating on varying the cocatalyst, the equivalents of cocatalysts, and the steric and electronic structure of the catalyst through modification of the salen ligand were undertaken. It was discovered that the optimal catalyst for copolymer selectivity vs. the monomeric propylene carbonate was one that contained a salen ligand with an electron withdrawing phenylene backbone and electron donating tert-butyl groups in the phenolate rings. This catalyst was used to investigate the effect of altering the nature of the cocatalyst and its concentration. The coupling of carbon monoxide and aziridines has been shown to be selective for comonomer-alternating enchainment in the presence of PhCH2C(O)Co(CO)4 to afford poly-??-peptoids. The mechanistic aspects of the reaction of CO and Nbutylaziridine by means of in situ infrared spectroscopy employing CH3C(O)Co(CO)3L (L = PPh3 and P(o-tolyl)3) as precatalysts was investigated. It was found the PPh3 precatalyst exists in solution under catalytic conditions as an equilibrium mixture of CH3C(O)Co(CO)3PPh3 and CH3C(O)Co(CO)4, and affords both poly-??-butylalanoid and the corresponding lactam as a side-product. By way of contrast, the P(o-tolyl)3 precatalyst which possesses the sterically bulky and labile phosphine ligand, affords only the acyl cobalt tetracarbonyl species in solution during catalysis with the selective production of the copolymer. Kinetic studies conducted with CH3C(O)Co(CO)3P(otolyl) 3 showed the coupling reaction to have a first order dependence on catalyst, a first order dependence on N-butylaziridine, and only a slight dependence on the concentration of CO over the pressure range 17-69 bar. The working mechanistic model for the copolymerization reaction involves first aziridine insertion into the cobalt-acyl bond, rate determining ring opening by the cobaltate species, followed by the migratory CO insertion.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.