Investigations on ternary metallocene-based catalyst systems
The influence of the cocatalyst triisobutyl aluminium (TiBA) on the two step reaction of the ternary activation of the metallocene precursor Ph2C(CpFlu)ZrCl2 with TiBA and the activator N,N-dimethylanilinium-tetrakis(pentafluorophenyl)borat (DMAB) was investigated by means of NMR spectroscopy. More than 5 equivalents of TiBA are required for a total conversion of Ph2C(CpFlu)ZrCl2. The reaction exclusiveley leads to the monoalkyl complex Ph2C(CpFlu)ZrClBui. independent of the Al / Zr ratio. The reaction of the Ph2C(CpFlu)ZrCl2 / TiBA mixtures, using 10, 20 and 50 equivalents of TiBA with DMAB, generates two cationic metallocene species. Using 100 equivalents of TiBA exclusiveley leads to the formation of the cationic heterodinuclear metallocene complex [Ph2C(CpFlu)Zr-(μ-H)- (μ-C4H7)-AlBui2]+. In no case was the degradation or complexation of B(C6F5)4]- detected, whereas the direct reaction between TiBA and DMAB gives AlBui3-x(C6F5)x compounds. A mechanism was postulated to explain the formation of [Ph2C(CpFlu)Zr-(μ-H)-(μ-C4H7) -AlBui2]+. The catalyst system Ph2C(CpFlu)ZrCl2 / TiBA / DMAB was used for continuous high pressure polymerizations of ethylene at 150 MPa and 180 °C. To investigate the influence of additional TiBA and triethyl aluminium (TEA) in the reactor, their concentrations were varied over a wide range. 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 Mw / Mn between 2 and 3. With higher aluminium concentrations, the products formed contained small amounts of waxes. This was due to oligomerization catalyzed by the aluminium alkyl compounds. Using the Schulz-Zimm distribution 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.