On the Mechanism of Stereospecific Polymerization—Development of a Universal Model to Demonstrate the Relationship Between Metallocene Structure and Polymer Microstructure

With the discovery of stereorigid bridged metallocenes, soluble catalysts became available for the stereospecific polymerization of α‐olefins. A relatively simple mechanism was used to explain the stereospecificity, primarily in terms of the catalyst symmetry. In this paper we demonstrate that the simple rule of thumb that C 2 ‐symmetric catalysts produce isotactic and C s ‐symmetric catalysts syndiotactic polypropylene is too narrow. The introduction of one methyl group at the Cp ring in the [{ i Pr(CpFlu)}ZrCl 2 ]/ MAO system (Flu = fluorenyl, MAO = methylalumoxan) reduces the C s symmetry to C 1 , and the resulting catalyst produces hemiisotactic polypropylene. The analogous catalyst with a bulkier tert ‐butyl group at the Cp ring gives isotactic polypropylene. When the C 2 symmetry of [{Me 2 Si(Ind) 2 }ZrCl 2 ] (Ind = indenyl) is reduced to C 1 , a metallocene can be obtained that produces atactic polypropylene. We have broken away from the symmetry‐based model and developed a universal model, which accurately describes the experimental microstructures of the polymers by considering the four lowestenergy conformers of the metallocene species coordinating to prochiral propene ( R re , S re , S si , and R si ) and the positional changes that the polymer chain undergoes during insertion. The relative energy levels of the four diastereomers can be determined by molecular modeling calculations; these energy gradations, in particular the size of the energy gaps, are decisive in determining the stereospecificity. Also, the model permits the stereoerrors to be classified and explained. Through this model the stereosequence of a polymer chain can be calculated and predicted.