Investigations of the origin of stereocontrol in syndiospecific Ziegler-Natta polymerizations

In order to expand our understanding of the mechanism of stereocontrol in syndiospecific α-olefin polymerization, a family of Cs-symmetric, ansa-group 3 metallocenes was targeted as polymerization catalysts. The syntheses of new ansa-yttrocene and scandocene derivatives that employ the doubly [SiMe₂]- bridged ligand array (1,2-SiMe₂)₂{C₅H-3,5-(CHMe₂)₂} (where R = t- butyl, tBuThp; where R = i-propyl, iPrThp) are described. The structures of tBuThpY(µ-Cl)₂K(THF)₂, tBuThpSc(µ-Cl)₂K(Et₂O)₂, tBuThpYCH(SiMe₃)₂, Y₂{µ₂-(tBuThp)₂}(µ₂-H)₂, and tBuThpSc(µ-CH₃)₂ have been examined by single crystal X-ray diffraction methods. Ansa-yttrocenes and scandocenes that incorporate the singly [CPh₂]-bridged ligand array (CPh₂)(C₅H₄)(C₁₃H₈)(where C₅H₄ = Cp, cyclopentadienyl; where C₁₃H₈ = Flu, fluourenyl) have also been prepared. Select meallocene alkyl complexes are active single component catalysts for homopolymerization of propylene and 1-pentene. The scandocene tetramethylaluminate complexes generate polymers with the highes molecular weights of the series. Under all conditions examined atactic polymer microstructures are observed, suggesting a chain-end mechanism for stereocontrol.

A series of ansa-tantalocenes have been prepared as models for Ziegler-Natta polymerization catalysts. A singly bridged ansa-tantalocene trimethyl complex, Me₂Si(η⁵-C₅H₄)₂TaMe₃, has been prepared and used for the synthesis of a tantalocene ethylene-methyl complex. Addition of propylene to this ethylene-methyl adduct results in olefin exchange to give a mixture of endo and exo propylene isomers. Doubly-silylene bridged ansa-tantalocene complexes have been prepared with the tBuThp ligand; a tantalocene trimethyl complex and a tantalocene methylidene-methyl complex have been synthesized and characterized by X-ray diffraction. Thermolysis of the methylidene-methyl complex affords the corresponding ethylene-hydride complex. Addition of either propylene or styrene to this ethylene-hydride compound results in olefin exchange. In both cases, only one product isomer is observed. Studies of olefin exchange with ansa-tantalocene olefin-hydride and olefin-methyl complexes have provided information about the important steric influences for olefin coordination in Ziegler-Natta polymerization.

Properties of superconducting CU-rich composites containing V_3SI or V_3GA

Superconducting Cu-rich composites containing the A-15 compounds V₃Si or V₃Ga have been made by the "Tsuei" process, which consists of melting the constituent elements into ingots followed by subsequent cold working and heat treatment. The superconducting transition temperatures of the resulting composites have been measured. X-ray diffraction analyses have been performed to identify the phases in the alloys. The microstructures have been studied using both the optical metallograph and the scanning electron-microscope. For some composites containing V₃Ga, the critical current densities as functions of transverse magnetic field up to 60 kG, and as functions of temperature from 4.2°K to 12°K have been measured. It was found that the Tsuei process does not work for the composites containing V₃Si, but works satisfactorily for the composites containing V₃Ga. The reasons are discussed based on the results of microstructure studies, electrical resistivity measurements, and also the relevant binary phase diagrams. The relations between the measured properties and the various metallurgical factors such as the alloy compositions, the cross-section reduction ratios of the materials, and the heat treatment are discussed. The basic mechanism for the observed superconductivity in the materials is also discussed. In addition, it was found that the Tsuei composites are expected to have high inherent magneto-thermal stability based on the stability theory of superconducting composites.