1H-, 13C-NMR and ethylene polymerization studies of zirconocene/MAO catalysts:: effect of the ligand structure on the formation of active intermediates and polymerization kinetics

Using H-1- and C-13-NMR spectroscopies, cationic intermediates formed by activation of L2ZrCl2 with methylaluminoxane (MAO) in toluene were monitored at Al/Zr ratios from 50 to 1000 (L-2 are various cyclopentadienyl (Cp), indenyl (Ind) and fluorenyl (Flu) ligands). The following catalysts were studied: (Cp-R)(2)ZrCl2 (R = Me, 1,2-Me2, 1,2,3-Me-3, 1,2,4-Me-3, Me-4, Me-5, n-Bu, t-Bu), rac-ethanediyl(Ind)(2)ZrCl2, rac-Me2Si(Ind)(2)ZrCl2, rac-Me2Si(1-Ind-2-Me)(2)ZrCl2, rac-ethanediyl(1-Ind-4,5,6,7-H-4)(2)ZrCl2, (Ind-2Me)(2)ZrCl2, Me2C(CP)(Flu)ZrCl2, Me2C(Cp-3-Me)(Flu)ZrCl2 and Me2Si(Flu)(2)ZrCl2- Correlations between spectroscopic and ethene polymerization data for catalysts (Cp-R)(2)ZrCl2/MAO (R = H, Me, 1,2-Me-2, 1,2,3-Me-3, 1,2,4-Me-3, Me-4, Me-5, n -Bu, t-Bu) and rac-Me2Si(Ind)(2)ZrCl2 were established. The catalysts (Cp-R)(2)ZrCl2/AlMe3/CPh3+B(C6F5)(4)(-) (R= Me, 1,2-Me-2, 1,2,3-Me-3, 1,2,4-Me-3, Me-4, n -Bu, t -Bu) were also studied for comparison of spectroscopic and polymerization data with MAO-based systems. Complexes of type (Cp-R)(2)ZrMe+ <-- Me- -Al equivalent to MAO (IV) with different [Me-MAO](-) counteranions have been identified in the (Cp-R)(2)ZrCl2/MAO (R = H-Bu, t-Bu) systems at low Al/Zr ratios (50-200). At Al/Zr ratios of 500-1000, the complex [L2Zr(mu-Me)(2)AlMe2](+)[Me-MAO](-) (HI) dominates in all MAO-based reaction systems studied. Ethene polymerization activity strongly depends on the Al/Zr ratio (Al/Zr = 200 - 1000) for the systems (Cp-R)(2)ZrCl2/MAO (R = H, Me, n -Bu, t -Bu), while it is virtually constant in the same range of Al/Zr ratios for the catalytic systems (Cp-R)(2)ZrCl2/MAO (R = 1,2-Me-2, 1,2,3-Me-3, 1,2,4-Me-3, Me-4) and rac-Me2Si(Ind)(2)ZrCl2/MAO. The data obtained are interpreted on assumption that complex III is the main precursor of the active centers of polymerization in MAO-based systems. (C) 2003 Elsevier Science B.V. All rights reserved.