Browsing by Subject "polymerization"
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Item A new structural subclass of constrained geometry catalysts for the polymerization of olefins(Texas A&M University, 2006-04-12) Irwin, Levi JacobThe sterically expanded octamethyloctahydrodibenzofluorene moiety, C29H38 (Oct), has been incorporated into ansa-metallocenes and constrained geometry catalysts (CGC's). Utilization of this sterically expanded version of fluorene has resulted in solidstate anomalies for both systems. The ansa-metallocenes Me2C(n5-C5H4) (n5-C29H36)MCl2, M = Zr, Hf and Me2C(n5-C5H4)(n5-C29H36)ZrBn2 demonstrate crystal motifs expected for ansametallocenes while Me2C(n5-C5H4)(n5-C29H36)MMe2, M=Zr, Hf exhibit diffuse diffraction, a phenomenon that is extremely unusual for organometallic complexes. This crystalline anomaly is the result of a disorder restricted to two dimensions caused by the rare pillared motif of the system. The best solution for this system consists of parallel and anti-parallel pillars present in a 60:40 ratio. The solid state anomaly observed for the Oct-CGC's occurs on a molecular level. The parent Oct-CGC, Me2Si(1-C29H36)(1-N-tBu)ZrCl2?OEt2, demonstrates an unprecedented n1 ligation to the fluorenyl-based ring. Systematic derivatization of this system via halide substitution, alkylation, and exchange of Zr for Hf has revealed that the n1 ligation persists for systems with small substituents on the metal center capable of retaining a coordinated ether. It is hypothesized that the unusual structure of this new Oct-CGC results in Me2Si(1-C29H36)(1-N-tBu)ZrCl2?OEt2/MAO (MAO = methylaluminoxane) being six times more active in the homopolymerization of 1-octene than ethylene. When compared to the prototypical Ti-CGC Me2Si(n5-C5Me4)(n1-N-tBu)TiCl2/MAO, the Oct-CGC is 85 times more reactive in the homopolymerization of 1-octene and 52 times more active in the copolymerization of 1-octene and 4-methyl-1-pentene. The high reactivity of the Oct-CGC towards alpha-olefins results in the observation of an unyielding comonomer effect in the copolymerization of these olefins with ethylene. In addition, the Oct-CGC is perhaps the most syndioselective catalyst known. With an enantiofacial selectivity of 99.7% and a remarkably high activity towards alpha-olefins, the Oct-CGC is capable of producing the highest melting syndiotactic polypropylene (Tm = 165oC, annealed = 174oC) reported thus far. The high activity and syndioselectivity of the Oct-CGC can be extended to the production of syndiotactic poly(4-methyl-1-pentene) with the highest melting point thus far reported (Tm = 215oC).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.