Recent Developments in Ziegler-Natta Catalysis

As discussed above, traditional Ziegler-Natta catalysts are heterogeneous in nature. There have been several attempts to prepare homogenous versions of these catalysts [21-28]. Many of these attempts have revolved around the use of metallocenes [18-20, 27]. In order to understand this topic a brief background on metallocenes is necessary. Metalloeenes are a large family of organometallic compounds where a metal atom/ion is sandwiched between at least two aromatic rings. Well-known examples of metallocenes are ferrocene and chromocene (Fig. 2.23). 

Ligand or metal Electrons contributed Total electrons 

The titanium (or zirconium) present in these complexes is in an oxidation state of +4 and therefore has 0 valence electrons. Each Cp contributes 6e each, making a total of 12e. Each Cf contributes 2e each. Thus, Cp2ZrCl2 and Cp2TiCl2 are 16e compounds and therefore are more reactive than ferrocene. Further the bent structure of metallocenes allows additional ligands to approach the metal ion. 

The reactive three-coordinate zirconium compound is involved in a reaction with ethylene (ethylene coordination). This leads to a metal-olefin complex. 

The next step is the migratory insertion reaction. This leads to the formation of a metal-alkyl complex. Note that we have ended up again with a three-coordinate zirconium cation, which has a metal-alkyl bond. In effect we have lengthened the alkyl chain by the process of olefin coordination followed by migratory insertion. 

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