AbstractUsing 1H and 13C NMR spectroscopy, cationic intermediates formed by activation of (Cp‐R)2ZrCl2 (R = nBu, tBu and 1,2,3‐Me3) with MAO in toluene were monitored at Al/Zr ratios from 50 to 1 000. The catalysts (Cp‐R)2ZrCl2/AlMe3/CPh3+B(C6F5)4− (nBu, tBu and 1,2,3‐Me3) were also studied for comparison of spectroscopic and polymerization data with MAO based systems. Complexes of type (Cp‐R)2ZrMe+ ← Me−‐AlMAO (IV) with different Me‐MAO− counter anions have been identified in the (Cp‐R)2ZrCl2/MAO systems at low Al/Zr ratios. At Al/Zr ratios of 200–1 000, the complex [(Cp‐R)2Zr(μ‐Me)2AlMe2]+ Me‐MAO− (III) dominates in all MAO‐based reaction systems. Ethene polymerization activity strongly depends on the Al/Zr ratio (Al/Zr = 200–1 000) for the systems (Cp‐nBu)2ZrCl2/MAO and (Cp‐tBu)2ZrCl2/MAO, while it is virtually constant in the same range of Al/Zr ratios for the catalytic system (Cp‐1,2,3‐Me3)2ZrCl2/MAO. The data obtained are interpreted on the assumption that complex III is the actual precursor of active centers of polymerization in MAO based systems.Formation of cationic intermediates by activation with MAO.magnified imageFormation of cationic intermediates by activation with MAO.