Jan L. Eilertsen

Methylaluminoxane as a Cocatalyst for Olefin Polymerization. Structure, Reactivity and Cocatalytic Effect

The structure and reactivity of methylaluminoxane (MAO), used as a cocatalyst for olefin polymerization, has been investigated by in situ IR spectroscopy, polymerization experiments and density functional calculations. We have suggested a few Me18Al12O9 cage structures, including a highly regular one with C3h symmetry, which may serve as models for methylaluminoxane solutions. Three reactive methyl bridges, presumably the key elements in metallocene activation, are situated at the cage surfaces. Further, exchange reactions show that the methyl groups are readily exchanged with chlorine, while non-bridging methyl groups are inert. The chlorinated MAO thus formed (MAO-Cl) is unable to activate bis(pentamethylcyclopendadienyl)zirconium dichloride (Cp*2ZrCl2), even with a surplus of added trimethylaluminium (TMA). MAO and TMA are present as separate FTIR-spectroscopic entities, with TMA acting independently as chain transfer agent for this catalyst.

In situ FTIR spectroscopy shows no evidence of reaction between MAO and TMA

In situ infrared spectra of TMA depleted commercial MAO with increasing additions of TMA have been recorded at 25 °C in toluene solution. The spectra of the mixtures are the sums of the spectra of TMA depleted MAO (CH3/A1 ratio 1.5) and of TMA. There is no evidence of any reaction between these compounds; the basic MAO entity seems to be completely uninfluenced by additions of TMA at this temperature.

Activation Reactions of Cp2 ZrCl2 and Cp2 ZrMe2 with Aluminium Alkyl Type Cocatalysts Studied by in situ FTIR Spectroscopy

The reactions of Cp2ZrCl2 and Cp2ZrMe2 with methylaluminoxane (MAO), trimethylaluminium (TMA) and dimethylaluminium chloride (DMAC) have been investigated by in situ FTIR spectroscopy. The studies have been performed in a cell that allows continuous monitoring of the reactions and stepwise additions of reactants. Most bands of the zirconocenes are unaffected by the reactions, but a strong Cp band at 803-822 cm1 was found to give distinct information on structural changes in the zirconocenes. A slow formation of the monochloro-monomethyl compound Cp2ZrClMe from a mixture of Cp2ZrCl2 and Cp2ZrMe2 has been verified. Only weak complexes are formed in mixtures of zirconocenes and TMA or DMAC. The chemical potential for methylation of zirconocenes is primarily due to MAO Clusters, but TMA may be important in the mechanism. Our IR data is consistent with the formation of stable ompounds during activation, which we assume include methyl or chlorine bridges between zirconium and aluminium, but do not differentiate between ionic or neutral complexes. Observed disappearance of C-H stretching bands may indicate double bridges between zirconium and aluminium.