Identification of new zinc catalyst systems for the copolymerization of carbon dioxide and epoxides /
Zinc(II) complexes have long been recognized as active catalysts for the copolymerization of epoxides and carbon dioxide. Most of the first generation catalyst systems were heterogeneous and not readily characterized. Furthermore, the copolymerization reactions required exceedingly large catalyst lo...
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| Format: | Thesis Book |
| Language: | English |
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[Place of publication not identified] :
[publisher not identified] ;
1999.
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| Subjects: | |
| Online Access: | http://proxy.library.tamu.edu/login?url=http://proquest.umi.com/pqdweb?did=733676341&sid=1&Fmt=2&clientId=2945&RQT=309&VName=PQD |
| Summary: | Zinc(II) complexes have long been recognized as active catalysts for the copolymerization of epoxides and carbon dioxide. Most of the first generation catalyst systems were heterogeneous and not readily characterized. Furthermore, the copolymerization reactions required exceedingly large catalyst loadings and were not readily reproduced. Improved activities were realized when soluble zinc(II) phenoxide- and carboxylate derivatives were employed. The phenoxide complexes were fractionalized with sterically-encumbering substituents at the 2- and 6 positions of the aryl rings that precluded aggregation. X-ray crystallographic studies demonstrated that these complexes assumed distorted tetrahedral geometries with donor solvent molecules completing zinc's coordination sphere. The solvent molecules are nearly fully dissociated at ambient temperature in non-interacting solvents thus creating available sites for association of the ecocide, a critical component of coordination catalysts. Turnovers in these systems were consistently higher than those in the first generation systems and surpassed 1000 g polycyclohexene oxide carbonate / g Zn in some instances. Zinc phenoxide-phosphine complexes were synthesized in non-interacting solvents. One or two equivalents of the ancillary ligand were capable of binding depending on the size of the phosphine. Accordingly, crystal structures of 3-coordinate (trigonal planar) and 4-coordinate (tetrahedral) complexes were obtained. The solution behavior of these complexes was following by ³¹P NMR. Competition studies established a qualitative ordering of binding affinities as PPh₃ [] P(t-Bu)₃ << PhPMe₂ < PCy₃ < PhPMe₂ < PBu₃ < Pe₃ < Pme₃. Concerning catalysis, the phosphine ligates significantly enhance the reactivity of the zinc(II) center towards carbon dioxide as evidenced by the higher amounts of CO₂ incorporation in the presence of the base. Zinc crotonate was also identified to be a highly active copolymerization catalyst. The turnover frequencies produced with this catalyst are among the higher values documented. Though soluble, ³¹P NMR binding experiments suggested that zinc crotonate exists predominantly in potentially varying states of aggregation. By contrast, it is believed that the active fraction of the catalyst sample is monomeric in nature. |
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| Item Description: | Vita. "Major Subject: Chemistry". |
| Physical Description: | xi, 120 leaves : illustrations ; 28 cm. Issued also on microfiche from University Microfilm Inc. |
| Bibliography: | Includes bibliographical references (leaves 100-104). |