Mechanism of dimethylzirconocene activation with methylaluminoxane: NMR monitoring of intermediates at high Al/Zr ratios

Using 1H and 13C NMR spectroscopy, the interaction between Cp2ZrMe2 and MAO in toluene solution was investigated in a wide range of Al/Zr ratios (10–4 000). Several intermediates (I–IV) were detected in the reaction. The structures of these intermediates were elucidated from detailed analysis of 13C-1H NMR data. Intermediate I is a weak complex of Cp2ZrMe2 with MAO. Intermediate II is the complex contact ion-pair [Cp2ZrMe(μ-Me)Cp2ZrMe]+[Me-MAO]– with symmetric binuclear cationic part. Species III a and III b are the heterodinuclear complex [Cp2Zr(μ-Me)2AlMe2]+[Me-MAO]– contact ion-pairs and the similar separated ion-pairs. Species IV are represented by the unsymmetrically Me-bridged complex Cp2(Me)Zr+ ← Me–-Al ≡ (MAO). Reaction equilibria involving intermediates I–IV were studied with varying Al/Zr ratio, MAO concentration and Al2Me6 content. With increasing Al/Zr ratio, complexes I, II, and free Cp2ZrMe2 gradually disappear and at high Al/Zr ratios at conditions, close to the usual polymerization conditions, only species III and IV were detected. At high Al/Zr ratios the molar ratio IV/III appears to be 1–4 depending on the Al2Me6 content, whereas at low Al/Zr (10–40) it is much greater and III is present only in very small amounts. These observations are rationalized assuming a gradual decrease of the average dissociation tendency of IV as the diverse Lewis acidic sites of MAO become occupied. A possible scheme of Cp2ZrMe2 interaction with MAO is suggested. Data on the interaction between Cp2ZrCl2 and MAO are given for comparison.