Bridged Cyclopentadienyl-Fluorenyl Metallocene Molecules

The fact that the metallocene complexes 1 and 2 are active for olefin polymerization was, of corrrse, of no surprise Kaminsky and Sirm had discovered in the mid-1970s that MAO obtained from partial hydrolysis of tri-methylaluminum (TMA) could activate metallocene dichloride complexes to very efficient olefin 

What was unprecedented in the polymerization behavior of the newly discovered catalyst systems, however, was the fact that an initially nonchiral metallocene 

To imderstand the functioning mode of this catalyst system and the mechanistic aspects of the stereochemical events involved in the formation of s-PP chains, it is very important to first examine more closely the molecular structure and structural characteristics of the complex 1 (and/or 2). The structural data can be then correlated to the information gleaned from different polymer analyses in order to determine the polymerization mechanism and the elementary steps involved in the formation of s-PP chains on the basis of the well-established principle of catalyst structure-polypropylene chain microstructure interrelationship. 

1 Molecular Structure ofIsopropyttdene(cyclopentadienyl-fluorenyl)MCl2 (M = Zr, Hf) Bonding and Symmetry 

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In most cases the catalyst precursor is a metallocene dichloride complex consisting of two aromatic five-membered ring systems that can be tethered by a bridging unit ansa metallocene complexes) or not. The two aromatic ligands at the metal can be of the same type. i.e.. cyclopentadienyl. indenyl. or fluorenyl. The introduction of substituents at certain positions of the two aromatic ligands and/or the bridge modifies not only the steric and electronic conditions in the molecule but also the symmetry of such a metallocene dichloride complex. Another variable parameter is the metal M = Ti. Zr. Hf. 

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