ISSN:
0947-6539
Schlagwort(e):
asymmetric catalysis
;
metallocenes
;
polymerizations
;
molecular modeling
;
reaction mechanisms
;
Chemistry
;
General Chemistry
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
Notizen:
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 C2-symmetric catalysts produce isotactic and Cs-symmetric catalysts syndiotactic polypropylene is too narrow. The introduction of one methyl group at the Cp ring in the [{iPr(CpFlu)}ZrCl2]/ MAO system (Flu = fluorenyl, MAO = methylalumoxan) reduces the Cs symmetry to C1, 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 C2 symmetry of [{Me2Si(Ind)2}ZrCl2] (Ind = indenyl) is reduced to C1, 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 (Rre, Sre, Ssi, and Rsi) 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.
Zusätzliches Material:
17 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/chem.19970030414
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