Pressure Modulates Stereoregularities in the Polymerization of Propylene Promoted by rac-Octahedral Heteroallylic Complexes Victoria Volkis, † Michal Shmulinson, † Claudia Averbuj, † Anatoli Lisovskii, † Frank T. Edelmann, ‡ and Moris S. Eisen* ,† Department of Chemistry, Technion, Israel Institute of Technology, Haifa 32000, Israel, and Chemisches Institut der Otto-von-Guericke, Universita ¨ t Magdeburg, Universita ¨ tsplatz 2, D-39106 Magdeburg, Germany Received April 30, 1998 Summary: This communication reports the synthesis and activity of four racemic mixtures of bis(benzamidi- nate) group-4 complexes, cis-[p-R′C 6 H 4 C(NR) 2 ] 2 MX 2 (R′ ) CH 3 ,R ) SiMe 3 ,M ) Ti, X ) Cl (1); R′ ) CH 3 ,R ) SiMe 3 ,M ) Zr, X ) Cl (2); R′ ) H, R ) i-pr, M ) Zr, X ) Cl (3); R′ ) CH 3 ,R ) SiMe 3 ,M ) Zr, X ) CH 3 (4)) as precatalysts for the stereoregular polymerization of pro- pylene. These complexes catalyze the stereoregular po- lymerization of propylene only under pressure in CH 2 Cl 2 , producing polypropylene with very large isotacticities (mmmm % ) ∼95-98), high melting points (140-154 °C). During the past decade an enormous advancement has been made in the design and synthesis of well- defined or “single-site” catalysts for the polymerization of olefins. 1-5 The majority of these catalysts belong to the metallocene ensemble, although some are known that contain one cyclopentadienyl ring and (for example) a pendant amido ligand. 6-8 Recently, however, com- plexes containing only chelating dialkoxide, 9-11 chelat- ing diamido, 12-15 or chelating benzamidine 16-20 ancillary ligands have received attention as potential Ziegler- Natta catalysts. Previously, we and others have inves- tigated the R-olefin polymerization chemistry catalyzed by several chelating bis(benzamidinate) complexes of group 4. The bis(benzamidinate) dichloride group 4 complexes (A) are obtained as a mixture of the racemic C 2 -symmetry cis-octahedral structures, and when acti- vated with methyl alumoxane (MAO), these complexes were found to be active catalytic precursors for the polymerization of ethylene, propylene, and the oligo- merization of 1,5-hexadiene. 16,21,22 Interestingly, for propylene, performing the polym- erization at atmospheric pressure produced polypropy- lene as an atactic oil, despite the expected isotactic polymer. 22,23 The resulting atactic polypropylene can be accounted for by two plausible mechanisms: (1) a “cationic” five-coordinated trigonal-bypyramid (B) or square-planar-pyramid intermediate (C) is formed, as the active complex, with no stereo-differentiation at the propene insertion; 24-26 (2) an intramolecular epimer- ization reaction of the growing polypropylene chain at the last-inserted monomeric unit is faster than the stereoregular insertion of propene (Scheme 1, vide infra). 27-29 In this paper, we report the synthesis and character- ization of non-Cp, racemic bis(benzamidinate) cationic † Israel Institute of Technology. ‡ Universita ¨ t Magdeburg. 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