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'2ZrCl2 pre‐catalysts (Cp' = C5HMe4, C4Me4P, C5Me5, C5H4tBu, C5H3‐1,3‐tBu2, C5H2‐1,2,4‐tBu3) together with (C5H5)2ZrCl2 have been correlated with the coordination gap aperture. In the case of the mixed substituted C5R5‐nR'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 (C4Me4P) 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 31P NMR. Ab initio calculations on model L2TiMe+ 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 91Zr‐NMR data (chemical shift and line width) for the above zirconocene series are reported. The 91Zr 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‐tBu2)(C5H5)ZrCl2 has been determined (monoclinic, P21/n, a = 6.631(3), b = 18.843(9), c = 15.265(5) Å, β = 91.3°, Z = 4).