Ziegler-type complexes, catalytic

Although Ziegler-type catalysts have been widely investigated for the homogeneous hydrogenation of polymers, their catalytic mechanism remains unknown. One possible reason for this may be the complexity of the coordination catalysis and the instability of the catalysts. Metallocene catalysts are highly sensitive to impurities, and consequently it is very difficult to obtain reproducible experimental data providing reliable kinetic and mechanistic information. 

One of the most remarkable aspect on the bis-Cp titanium derivative chemistry has been the production of new and unprecedented variety of polyolefins. The use of this type of complexes as Ziegler-Natta pre-catalyts for the olefin polymerization has opened new possibilities to produce polyolefins with different properties, and significant effort has been devoted to the design of new bis-Cp catalyst structures. This section summarizes simple aspects related to the polymerization of ct-olefins catalyzed by bis-Cp titanium complexes containing a cr-Ti-C bond. A more comprehensive review of the catalytic applications of titanium complexes in the a-olefin polymerization processes is covered in Chapter 4.09. 

The class of monocyclopentadienylamido (CpA) titanium complexes has attracted the interest for the polymerization of a-olefins with bulky side groups. This arises since conventional Ziegler-Natta catalysts are less effective in starting the copolymerization of ethene with 4-methyl-l-pentene. Homogeneous catalysts of the zirconium cyclopentadienyl type (Cp2M) with methylaluminoxane exhibit a low catalytic activity. 

Cationic zirconocene complexes are well-known very reactive species with interesting applications owing to their Lewis acidic character see Lewis Acids Bases). In particular, they are often involved into various catalytic processes including Ziegler-Natta type polymerizations see Ziegler-Natta Catalysts). 

Bis 8-quinolmato metal complexes are representative examples of distorted octahedral structures of the type (87). It was noted that these systems bear a topological resemblance to the chiral, octahedral, active metal sites that have been proposed for supported Ziegler-Natta catalysts, and are interesting for modeling catalytic properties. Similar comportment has been observed with diamine bis (phenolate) ligands. ... 

Acidic chloroaluminates (hard anions) have been utilized in catalytic reactions which need the presence of Lewis acidity (Ziegler-Natta type catalysts) and for which there is no Lewis acid-base interaction between the reaction products and aluminum chloride. Polymerization of ethylene catalyzed by (CjHjJjTiClj/Alj. Me3Cl3 complexes has been carried out in acidic chloroaluminates [20],... 

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