Supported Metallocene Catalysts by Surface Organometallic Chemistry. Synthesis, Characterization, and Reactivity in Ethylene Polymerization of Oxide-Supported Mono- and Biscyclopentadienyl Zirconium Alkyl Complexes: Establishment of Structure/Reactivity Relationships

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Abstract
The reactions of Cp*Zr(CH3)3, 1, and Cp2Zr(CH3)2, 2, with partially dehydroxylated silica, silica−alumina, and alumina surfaces have been carried out with careful identification of the resulting surface organometallic complexes in order to probe the relationship between catalyst structure and polymerization activity. The characterization of the supported complexes has been achieved in most cases by in situ infrared spectroscopy, surface microanalysis, qualitative and quantitative analysis of evolved gases during surface reactions with labeled surface, solid state 1H and 13C NMR using 13C-enriched compounds, and EXAFS. 1 and 2 react with silica(500) and silica−alumina(500) by simple protonolysis of one Zr−Me bond by surface silanols with formation of a single well-defined neutral compound. In the case of silica−alumina, a fraction of the supported complexes exhibits some interactions with electronically unsaturated surface aluminum sites. 1 and 2 also react with the hydroxyl groups of γ-alumina(500), leading to several surface structures. Correlation between EXAFS and 13C NMR data suggests, in short, two main surface structures having different environments for the methyl group: [Al]3−OZrCp*(CH3)2 and [Al]2−OZrCp*(CH3)(μ-CH3)−[Al] for the monoCp series and [Al]2−OZrCp2(CH3) and [Al]−OZrCp2(μ-CH3)−[Al] for the bisCp series. Ethylene polymerization has been carried out with all the supported complexes under various reaction conditions. Silica-supported catalysts in the absence of any cocatalyst exhibited no activity whatsoever for ethylene polymerization. When the oxide contained Lewis acidic sites, the resulting surface species were active. The activity, although improved by the presence of additional cocatalysts, remained very low by comparison with that of the homogeneous metallocene systems. This trend has been interpreted on the basis of various possible parameters, including the (p-π)−(d-π) back-donation of surface oxygen atoms to the zirconium center.