| Autor: |
Götz, Christian, Rau, Alexander, Luft, Gerhard |
| Zdroj: |
Macromolecular Materials and Engineering; January 2002, Vol. 287 Issue: 1 p16-22, 7p |
| Abstrakt: |
The influence of aluminium alkyl compounds on metallocene-catalyzed high pressure polymerizations of ethylene has been investigated at 150 MPa and 180°C in a continuously operated autoclave. The catalysts were based on the metallocenes bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2) and diphenylmethylene (cyclopentadienylfluorenyl)zirconium dichloride (Ph2C-(CpFlu)ZrCl2), which were preactivated outside the reactor with triisobutylaluminium (TiBA) and N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate (DMAP, [PhNHMe2][B(C6F5)4]). The concentrations of triisobutylaluminium (TiBA) and triethylaluminium (TEA) in the reactor were varied over a wide range, using a separate dosing for these two aluminium alkyl compounds. Productivity and polymer properties strongly depended on the type and the concentration of the aluminium alkyl compound used. Highest productivities and molecular weights were obtained with low concentrations of TiBA in the reactor. Up to a concentration of 30 molppm Al in the reactor, unimodal polymers were formed with &Mmacr;w/&Mmacr;n between 2 and 3. With higher aluminium concentrations the products formed contained small amounts of waxes, due to oligomerization catalyzed by the aluminium alkyl compounds. The molecular weight distributions (MWDs) of these products could be described as a superimposition of two Schulz-Zimm distributions. All MWDs were analyzed with regard to the amount of waxes produced by ethylene oligomerization and with regard to the influence of chain transfer reactions to the aluminium. The rate constants of chain transfer to aluminium, in relation to the rate constants of insertion of ethylene, were estimated. |
| Databáze: |
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