Catalysts Zirconium metallocene derivatives

Zirconocene and Half-Sandwich Zirconium Derivatives The development of a single-site heterogeneous catalyst for metallocene-based polymerization catalysis has also been explored extensively with zirconocene and half-sandwich zirconium derivatives [32, 75, 91, 92]. 

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

Metallocenes have been used for complexes which have sandwich structures with two cyclopentadienyl ligands since the discovery of ferrocene. Recently metal complexes having one cyclopentadienyl ligand have also been classified as a member of metallocene derivatives. One important discovery in this area is the olefin polymerization catalyzed by metallocene complexes of early transition metals such as zirconium and titanium. In particular, the structure of the metallocene catalyst has a remarkable effect on the structure of the polymers. This discovery has had a strong impact on the industry. 

One of the earliest examples of the use of zirconium in catalysis is the area of Ziegler-Natta polymerisation, the catalysts employed are typically based on a family of Cp2ZrCl2 metallocene complexes." With judicious choice of the cyclopentadienyl ligand either atactic (bridged Cp-derived systems),... 

Catalysts that yield highly syndiotactic polypropylene (86% racemic pentads) were also developed. One of them is /-propylene(ri -cyclopentadienyl-r fluorenyl)zirconium dichloride [291]. Initial disclosures of metallocene catalysts were followed by numerous publications in the literature that described similar materials for the polymerizations of either ethylene or propylene, or both, and for formation of various copolymers. Thus, for instance, Kaminsky et al. [292], reported preparaticm of a zirconium dichloride-type catalyst for copolymerization of cyclic olefins with ethylene. These cyclic olefins are cyclopentene, norbomene, and the hindered cyclopentadiene adducts of norbomene [292]. The catalytic system consists of a bridged indene derivative that is combined with methylaluminoxane ... 

On the basis of this experiment, Pino and coworkers were able to determine that catalysts derived from the (/ )-ethylenebis(tetrahydroindenyl)zirconium binaptholate preferentially selected the Re enantioface of propylene. These results led to a model for the transition state where the polymer chain is forced into an open region of the metallocene, thereby relaying the chirality of the metallocene to the incoming monomer through the orientation of the p-carbon of the aUcyl chain (Scheme IIA).43 Here, the role of the C2-symmetry of the catalyst site can be readily appreciated since as the polymer chain migrates to the coordinated olefin, the coordination site available for binding of the olefin alternates between two coordination sites (A -> B -> C). Because these two sites are related by a C2-symmetry axis, they are homotopic and therefore selective for the same olefin enantioface. The result is polymerization to yield an isotactic polyolefin. 

Remarkably, very active catalysts for the copolymerization of bicyclic and polycyclic olefins with a-olefins are derived from transition metal metallocenes and cyclic or linear almninoxanes [31]. Cycloolefins (41 5) are reported to undergo high yield polymerization with linear olefins using metallocenes of titanium, zirconium, vanadium, or chromium associated with almninoxanes [31a]. 

The name of metallocenes is derived from ferrocene, which is a complex compound of iron with dicyclopentadiene. The metallocene complexes are new types of Ziegler-Natta catalysts. The general formula of these catalysts is identified as Cp MeCl, where Cp denotes the cyclopentadiene ring or its derivatives. Me is metal atom, which the most often is zirconium (Zr), hafhium (Hf), titanium (Ti), scandium (Sc), thorium (Th) or another rare earth element. 

As the demand for new polymers increased, catalysts with two active sites per molecule were developed. It was found that tris-pentafluoropheityl-aluminium, (C6F5)3A1, could be used to replace the borate derivatives, and when used in conjunction with the typical transition metallocene catalyst precursors, a series of di-cationic derivatives of titanium and zirconium could be produced. Work in Dow showed that these di-cationic catalysts were up to 30 times more reactive than boron activated catalysts, while still producing polymers with the same molecular weight and melting point. ... 

Ziegler-Natta catalysts have long remained the only organometallic catalysts in macromolecular chemistry. The area was revolutionized by the discovery that zirconocene derivatives catalyze the polymerization of propene. At the same time, strong industrial interest has prompted a worldwide development in the field. The first polymers based on metallocene-type catalysts have recently appeared on the market under the names Hostacen (Hoechst AG), Exact (Exxon), and Affinity (Dow). The metals zirconium and hafnium appear to be the most important metals in synthetic preparations. 

Metallocene chemistry is presently a very promising field of inorganic chemistry. E.g., metallocene complexes of titanium and zirconium play an important role in the production of polymers. The role of ferrocene derivatives as effective nucleophilic catalysts in kinetic resolutions of many useful... 

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