Newer Metallocene Catalysts for Olefin Polymerization

Metallocene are group IV metals (Tj, Zr, Hf, and Rf, but commonly Zr is used) complexed with cyclopentadiene and can be activated by methyl aluminoxane (MAO) as follows  

The above MAO is a reaction product of partially hydrolyzed triethyl aluminium and is mostly used in homogeneous solution. The MAO provides a cage for the cation and the pair as a whole serves as the catalyst for polymerization. Some of 

The first step in the catalyst polymerization, as discussed in Section 5.12, of the olefin to the Lewis acid metal center. The chain propagation occurs by insertion of the olefin between the metal carbon bond as follows  

The insertion step consists of an alkyl migration to the olefin hgand. At the same time, a new tree coordination site is generated at the vacant piston of the former alkyl ligand. Depending on the orientation of the monomer diuing insertion, the following (1,2) or (2,1) possibilities exist  

In view of this, in the polymerization of propylene, the following racemic and meso diads are formed  

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