Metallocenes and Methyl Aluminoxane

Typical examples of single-site metallocene catalysts are shown by structures 6. -6.12. Structures 6.9 and 6.10 are general structures that represent the basic framework of metallocene and half-metaUocene catalysts, respectively. In both the structures, the points to note are as follows. 

the five-membered ring instead of being a cyclopentadineyl (C H ) may also be an indenyl (C H. ) or a fluorinenyl (CjjH,) ring. Second, in 6.9 and 6.10, M is titanium or zirconium in the oxidation state of four. Third, A is an optional bridging atom, generally a carbon or a silicon atom with Rj usually methyl, and R = H, CHj, etc. Finally, in 6.9, X is usually Cl or CH3.  

As we saw in Section 5.7, catalysts with the general structure of 6.10 are also called constrained geometry catalysts (CGCs). They were the 

Complex 6.11 has the novel ligand C3HjB(OEt), and behaves quite differently from other metallocene catalysts. Higher alkenes such as decene, dodecene rather than PE are obtained when 6.11 is used as the catalyst. It should be noted that C H BCOR) is a pseudo-pentadienyl ligand. 

The other two structures, 6.12 and 6.13, have been studied much, and their structural difference in terms of symmetry has an important bearing on the tacticity of PP produced by them. As we will see, such observations have provided strong evidences for the enantioselection mechanisms. 

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