Bridged fluorenyl/indenyl metallocenes

Patents have been reported for a series of bimetallic metallocene catalysts based on lanthanide and group IV metals. The bridging ligand is a substituted ethyl-linked fluorenyl indenyl bearing a substituent of varying length [26]. The complexes are reported to act as good olefin polymerization catalysts in the presence of MAO.52,53... 

Activation of these catalysts is done using either MAO or triisobutylalu-minium. In all cases the polymers obtained are isotactic despite the fact that synthesis of fixed bis(indenyl) metallocenes may result in inseparable meso and racemic diastereomers and bridged bis(fluorenyl) metallocenes are not chiral. 

Stereoblock polypropylene is synthesized by unbridged metallocenes such as bis(neomenthyl)zirconium dichloride (7) with cyclopentadienyl or phenyl substituted indenyl ligands [55, 56]. The stereoblock length increases at lower polymerization temperatures. Products containing isotactic and atactic blocks are elastomeric if the isotactic block length is short. Rieger [57] obtained similar polypropylenes by bridged fluorenyl-substituted indenyl zirconium complexes. 

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. 

Extensive efforts have also been made to develop olefin polymerisation catalysts based on metallocenes with only one ligand of the cyclopentadienyl type. Ethylene-,dimethylsilylene- or tetramethyldisilylene-bridged mono(l-tetra -methylcyclopentadienyl), mono(l-indenyl) or mono(9-fluorenyl)-amidotita-nium complexes, such as dimethylsilylene(l-tetramethylcyclopentadienyl)(t-butyl)amidotitanium dichloride [Me2Si(Me4Cp)N(/-Bu)TiCl2] (Figure 3.10), have recently attracted both industrial and scientific interest as precursors for methylaluminoxane-activated catalysts, which polymerise ethylene and copolymerise ethylene with 1-butene, 1-hexene and 1-octene [30,105,148-152]. 

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