Symmetric metallocenes, polymer structures

The copolymerization parameter rt which indicates how much faster an ethene is incorporated in the growing polymer chain than an a-olefin, when the last inserted monomer was an ethene unit, lies between 1 and 60 depending on the kind of comonomer and catalyst. The copolymerization parameter r2 is the analogous ratio for the a-olefin. The product r r2 is important for the distribution of the comonomer and is close to unity when using C2 symmetric metallocenes, indicating a randomly distributed comonomer. It is less than unity with a more alternating structure for Cs-symmetric catalysts [62-65] (Table 5). 

Variation of Cs-symmetric metallocenes leads to Ci-symmetric ones (structure (60)). If a methyl group is introduced at position 3 of the cyclopentadienyl ring, stereospecificity is disturbed at one of the reaction sites so every second insertion is random. A hemiisotactic polymer is produced [440-442]. If steric hindrance is bigger (for example a tert-hvXy group is introduced instead of the methyl group), stereo selectivity is inverted and the metallocene catalyses the production of isotactic polymers [432,443-450]. 

Looking at C2-symmetric catalysts, there exist in principle two different metallocene-olefin complexes (Fig. 4). Structure a is energetically favored because the methyl group of the propene is in trans-position relative to the (3-methyl structure of the polymer chain. Both methyl groups in structure b are in cis position to each... 

When propylene and higher olefins are polymerized the configuration of the polymer is controlled by the catalyst structure. Catalyst 9-25 contains two equivalent Cl atoms, because the molecule is symmetrical about the Zr-indenyl bonds. Recall that in the active catalyst one Cl will be replaced by the growing polymer and the other by the incoming monomer. The polymer chain could occupy either of the two Cl positions, while the monomer could coordinate at the other position. As a result, both positions are equivalent and this metallocene produces isotactic polypropylene. In structure 9-26, the two Cl sites are not equivalent. At one position, the monomer would find the same environment as in 9-25, but the second... 

Two cyclopentadienyl-sandwiched Cp2ZrX2/MAO complexes (Cp = cyclopentadienyl X = halogen or alkyl) with C2v-symmetry form the earliest metallocene system that has been used for ethylene homopolymerization with remarkably high activity. The application in propylene polymerization is less useful. They produce PP with low activity and low molecular weight.f Unexpectedly, the Cp2TiPh2/MAO system can be used for the synthesis of predominately i-PP below room temperature. The stereochemical structure of the resulting polymer indicates a chain-end-controlled model in the polymerization. Bridged C2v-symmetric... 

The model for syndiotactic polymerization is a bit more complex than the model needed for isotactic polymerization. In 1988 Ewen and co-workers reported the discovery of a metallocene catalyst, iso-propyl(cyclopentadienyl-1 -fluorenyl)zirconium dichloride, 7, that would produce syndiotactic polypropylene, that is, a polymer formed from the sequential reaction of alternate olefin r-faces (see Figure 1 for the molecular structure). In contrast to the family of catalysts that have C2-symmetric catalyst precursors, 7 is Cs-symmetric. Ewen proposed that, for this catalyst to produce syndiotactic polymer, the active site must isomerize after each insertion consistent with the polymer chain flipping from one side to the other during insertion. Stereoerrors were thought to be due to chain back-skipping or reaction with the wrong olefin face. Eor Cz symmetric catalyst precursors, the active site does not isomerize if the polymer chain flips from side to side. 

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