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

Using H-1 and C-13 NMR spectroscopy, the interaction between Cp2ZrMe2 and M AO in toluene solution was investigated in a wide range of Al/Zr ratios (10 -4000). Several intermediates (I-IV) were detected in the Fraction. The str uctures of these intermediates were elucidated from detailed analysis of C- 13-H-1 NMR data. Intermediate I is a weak complex of Cp2ZrMe2 with MAO. Int ermediate II is the complex contact ion-pair [Cp2ZrMe(mu-Me)Cp2ZrMe](+)[Me- MAO](-) with symmetric binuclear cationic part. Species IIIa and IIIb are t he heterodinuclear complex [Cp2Zr(mu-Me)(2)AlMe2](+)[Me-MAO](-) contact ion -pairs and the similar separated ion-pairs. Species IV an represented by th e unsymmetrically Me-bridged complex Cp-2(Me)Zr+ <-- Me--Al = (MAO). Reacti on equilibria involving intermediates I-IV were studied with varying Al/Zr ratio, MAO concentration and Al2Me6 content. With increasing Al/Zr ratio, c omplexes I, II, and free Cp2ZrMe2 gradually disappear and at high Al/Zr rat ios at conditions, close to the usual polymerization conditions, only speci es III and IV were detected. At high Al/Zr ratios the molar ratio IV/III ap pears to be 1-4 depending on the Al2Me6 content, whereas at low Al/Zr (10-4 0) it is much greater and III is present only in very small amounts. These observations are rationalized assuming a gradual decrease of the average di ssociation tendency of IV as the diverse Lewis acidic sites of MAO become o ccupied. A possible scheme of Cp2ZrMe2 interaction with MAO is suggested. D ata on the interaction between Cp2ZrCl2 and MAO are given for comparison.