Mechanism zirconium catalyst system

It has been found that isotactic polypropylene produced by unbridged biscyclopen-tadienyl zirconium catalysts is formed through a mixed chain end control and enan-tiomorphic control.352 Molecular mechanics calculations led to the development of a model that takes into account nonbonded interactions to rationalize observed selectivities and predict properties of new catalyst systems.353... 

Cerium-zirconium mixed metal oxides are used in conjunction with platinum group metals to reduce and eliminate pollutants in automotive emissions control catalyst systems. The ceria-zirconia promoter materials regulate the partial pressure of oxygen near the catalyst surface, thereby facilitating catalytic oxidation and reduction of gas phase pollutants. However, ceria-zirconia is particularly susceptible to chemical and physical deactivation through sulfur dioxide adsorption. The interaction of sulfur dioxide with ceria-zirconia model catalysts has been studied with Auger spectroscopy to develop fundamental information regarding the sulfur dioxide deactivation mechanism. 

The general model and mechanism that was discussed in Sect. 2.4 and proposed for the mechanism of the syndiospecific polymerization of propylene with the Cs symmetric metallocene catalyst system 1/MAO vindicate the similar microtacticity and stereoregularity of the syndiotactic polymers perfectly, but does not in any way account for or rationalize, the polymerization behavior of catalyst system diphenyl-methylidene-p-(cyclopentadienyl-fluorenyl)zirconium dichloride 6/MAO (7/MAO) with respect to the dramatic increases in the molecular weights of the resulting s-PP polymers. 

It should be noted that cationic titanium and zirconium catalysts, which are isoelectronic to neutral group 3 metal complexes, cyclize only aminoalkenes with a secondary amino group, whereas primary amines are urueactive [61, 62]. It has been proposed that the lanthanide-like insertion mechanism is operating in these systems, which is in agreement with DFT calculations [63]. 

The disproportionation activity in the supported species is parallel to the increased activity of ethylene polymerization on supported catalysts. Many of the steps in the reaction may be identical for example, the initial coordination of olefin to the metal center will be common to both systems. Indeed, some of these catalysts are also ethylene polymerization catalysts (see Table IV) although their activities are much less than the corresponding zirconium derivatives. A possible intermediate common to both disproportionation and polymerization could be the hydrocarbyl-olefin species (Structure I). Olefin disproportionation would result if the metal favored /3-hydrogen elimination to give the diolefin intermediate (Structure II) which is thought to be necessary for olefin disproportionation. Thus, the similarity between the mechanism and activation of olefin disproportionation and polymerization is suggested. 

Recently, other pathways where the C—C bond is postulated to form via metalla-cyclopentanes (Scheme 3) have been established for the catalytic dimerization of ethylene and higher a-olefins by zirconium , tantalum, and nickel complexes. At least for the tantalum systems, the mechanisms of conversion of the metallacyclopentane to the product olefin differ significantly with the specific catalyst. With some variations, the metal-... 

The metal acetylacetonates and their pyridine complexes were used as catalysts in the epoxy-novolac systems [310-320]. These compounds improved the mechanical strength of the polymers [310,312], but the titanium and zirconium chelates also advanced the adhesive strength and the water resistance [319]. The investigation of the effect of the acetylacetonates on the interaction of the epoxy oligomers with the phenolformaldehyde resins showed that the catalytic activity of the metal cations diminishes in the series [Eq. (2)]... 

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