Active Sites in Soluble Ziegler Polymerization Catalysts Generated from Titanocene Halides and Organoaluminum Lewis Acids

Abstract

By means of X-ray single-crystal data the Ti-C and Ti-Cl bond lengths in trimethylsilylmethyltitanocene chloride (11) have been de termined. The operation of σ-hyperconjugation in this compound leads to an unusual shortening of the Ti-C bond. In solution the observed shifts in the 1 H NMR signals of the CH2 or the cyclopentadienyl (Cp) groups of 11 are attributed to varying degrees of coordinative solvation of the titanium center or hydrogen bonding at the chlorine center. Such inherent polarity of the Ti-Cl bond in 11 has been found to be sharply accentuated hy the introduction of a Lewis acid, AlCl2R. By monitoring such reaction mixtures by 1 H, 13 C, and 27 Al NMR spectroscopy, direct evidence is obtained for the gen eration of the alkyltitanocenium ion, Cp2RTi + , andAlCl4-. Such com binations of 11 and Lewis acid are effective catalysts for the polym erization of ethylene, and Me3Si fragments from 11 are found in the resulting polyethylene. These findings support the thesis that Cp2RTi + ions are the active polymerization sites in such Ziegler catalyst sys tems. THENATURE OF OLEFIN POLYMERIZATION CATALYSTS formed from com binations of titanium halides and alkylaluminum halides has remained the subject of controversy (I) since the epoch-making discoveries of Ziegler et al. (2) and of Natta et al. (3) some 35 years ago. Much of the difficulty in 0065-2393/92/0230-0575$06.00/0