Metallocenes, ansa Propagation

One major drawback of rflc-C2-symmetric ansa-metallocene catalysts is that either during the synthesis of the precursors or by subsequent epimerization the meso-form with Cs-symmetry can also be obtained. The active sites of catalytic species formed from meso-Cs-symmetric precursors are obviously nonchiro-topic, and cannot exert any stereocontrol on the chain propagation. As a result, some atactic PP is invariably obtained. ... 

The mechanism of monomer insertion and steric control in polymerizations of a-olefins by the metallocene catalysts received considerable attention [293-297]. There is no consensus on the mechanism of polymerization. Many studies of chain propagation tend to support the Cossee-Arleman mechanism [293-297]. An example is work by Miyake et al. [294] who synthesized unsymmetrical ansa-metallocenes and separated them into threo and erythro isomers. Both isomers coupled with methylaluminoxane polymerize propylene in toluene to highly isotactic polymers of = 105,000. The isotactic placement is greater that 99.6% and the polymer melting point is 161°C. 

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