Ethene polymerization activity and coordination gap aperture in non-ansa alkyl-substituted cyclopentadienyl- and phospholyl-zirconium/MAO catalysts

The ethene polymerization activities of a series of Cp'(C5H5)ZrCl2 and Cp(2)'ZrCl2 pre-catalysts (Cp' = C(5)HMe(4), C(4)Me(4)P, C(5)Me(5), C(5)H(4)tBu, C5H3-1,3-tBu(2), C5H2-1,2,4-tBu(3)) together with (C5H5)(2)ZrCl2 have been correlated with the coordination gap aperture. In the case of the mixed substituted C(5)R(5-n)R(n)' ligands, the coordination gap aperture has been determined with the help of the cyclopentadienyl cone angle and the bending angle at zirconium. A hindered rotatability of the tert-butyl substituted systems has been evaluated by molecular modeling calculations and was included in the gap aperture estimation. Deviations from the activity-gap aperture correlation in the case of the phospholyl (C(4)Me(4)P) systems could be accounted for in terms of adduct formation between aluminum species and the phosphorus donors. These Lewis acid/base adducts form in an equilibrium reaction at high Al/Zr ratios, as shown by P-31 NMR. Ab initio calculations on model L(2)TiMe(+) complexes (L = C5H5, C5H4N and C5H4P) for the insertion of ethene in the Ti-Me bond suggest a high electronic similarity for phospholyl and cyclopentadienyl. The Zr-91-NMR data (chemical shift and Line width) for the above zirconocene series are reported. The Zr-91 chemical shift values increase with a good linear correlation to the number of methyl or tert-butyl groups, which is traced to the additive electron donating effect of the alkyl groups. The hyperconjugative donor effect of a tert-butyl group is found here to be 1.25 times that of a methyl group. The X-ray structure of (C5H3-1,3-tBu(2))(C5H5)ZrCl2 has been determined (monoclinic, P2(1)/n, a = 6.631(3), b = 18.843(9), c = 15.265(5) Angstrom, beta = 91.3 degrees, Z = 4).