Highly active three-component catalytic systems based on dialkylzirconocenes, triisobutylaluminum, and perfluorophenyl borates for synthesis of low-molecular-weight polyethylene
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The catalytic properties of the zirconium complex with “constrained geometry” Me2SiCp*NBu1ZrX2 (Cp*=C5Me4, X=Cl (1a), Me (1b)) and bridged bis(cyclopentadienyl)zirconocene Me2SiCp2ZrX2 (X=Cl (2a), Me (2b)) during their activation with triisobutylaluminum/perfluorophenyl borates (TIBA/LB(C6F5)4, L=CPh3 (3), Me2HNPh (4)) in ethylene polymerization under a monomer pressure of 2–20 atm were studied by comparison. Both dichloride complexes exhibit moderate activity under the action of the combined TIBA/3 activating agent and give linear high-molecular-weight polyethylene (PE). The interaction of the dimethyl complexes with TIBA/3(4) afford active sites in which the growing polymeric chain is intensely transferred to the monomer, due to which low-molecular-weight PE is formed. The dichloride complexes affected by TIBA/4 also afford low-molecular-weight PE. Analysis of the structure of the polymeric products (1H NMR spectrometry, IR spectroscopy), molecular-weight parameters of the PE samples (gel permeation chromatography (GPC)), and kinetics of polymerization suggested that the active site contains AlBui 3 as a heteronuclear bridged cationic complex. The influence of various basic substrates (the products of chain transfer with the terminal vinyl groups, the dimethylaniline fragment of borate4 or other amine specially introduced into the reaction mixture) on the catalytic properties of the Zr−Al site was revealed. The polymerization rate and molecular-weight parameters of PE as functions of the reaction temperature, ethylene pressure, and modifying additives were studied.
Key wordsmetallocene catalysts ethylene polymerization ethylene oligomerization polymerization kinetics active site perfluorophenyl borates triisobutylaluminum polyethylene
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