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Reactivity and Mechanistic Studies of Stereocontrol for Ziegler-Natta Polymerization Utilizing Doubly-Silylene Bridged Group 3 and Group 4 Metalocenes 1
D. L. Zubris, D. Veghini, T. A . Herzog, and J. E . Bercaw
Downloaded by 80.82.77.83 on September 1, 2017 | http://pubs.acs.org Publication Date: December 29, 1999 | doi: 10.1021/bk-2000-0749.ch001
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, C A 91125 A family of zirconocene, yttrocene, and scandocene complexes have been prepared utilizing an easily modified doubly bridged ligand array, (1,2-SiMe ) {η -C Η -4-R}{η -C Η-3,5-(CΗΜe ) }, developed in our laboratories. Variations in ligand array substitution and metal center provide complexes that exhibit a range of stereoselectivities and activities in α-olefin polymerizations. A series of C - and C - symmetric metallocenes have been utilized for the preparation of polypropylene and polypentene under various polymerization conditions. Analysis of the resulting polymer microstructures has provided valuable information about the factors that dictate syndiospecificity, as well as common error forming mechanisms for these systems. 5
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The use of metallocenes as catalysts for the polymerization of ethylene and ocolefins has received increasing attention due to the striking activities and stereoselectivities they can effect (/). These catalyst systems are either neutral species of the form C p ^ M - R ( M = group 3 transition metal or lanthanide; Cp' = (rjS-CsHs) or substituted cyclopentadienyls) or cationic species of the form [Cp'2M-R]+X- ( M = group 4 transition metal; X - = non-coordinating anion). Efforts have been directed towards elucidating the active species and mechanism of polymerization to help determine the source of the remarkable regio- and enantioselectivities found with these systems. Slight variations in metallocene structure provide access to a variety of polymer microstructures, molecular weights, and melting temperatures; some of these microstructures are illustrated in Figure 1. Catalyst symmetry and the microstructure of the resulting poly-a-olefm are related in a rational manner. C V symmetric metallocenes, such as Brintzinger's chiral
