Abstract:With the discovery of stereorigid bridged metallocencs, soluble catalysts became available for the stereospecific polymerization of cc-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,-symmetric catalysts produce isotactic and C',-symmetric catalysts syndiotactic polypropylene is too narrow. The introduction of one methyl group at the Cp ring in the [jiPr(CpFlu))ZrCI,]/ M A 0 system (Flu = fluorenyl, MA0 = methylalumoxan) rcduces the C, syninietry to C , , and the resulting catalyst products hemiisotactic polypropylene. The analogous catalyst with a bulkier tcrtbutyl group at the Cp ring gives isotactic polypropylene. When the C, symmetry of [{Me,Si(lnd),)ZrCl,] (Ind = indenyl) is reduced to C , , 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

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