The synthesis of branched polyethylenes by ethylene polymerization with new tandem catalyst systems consisting of methylaluminoxane-preactivated linked cyclopentadienyl-amido titanium catalysts [Ti( 5 : 1 -C 5 Me 4 SiMe 2 NR)Cl 2 (R ϭ Me or t Bu)] supported on pyridylethylsilane-modified silica (PySTiNMe and PySTiN t Bu) and homogeneous dibromo nickel catalyst having a pyridyl-2,6-diisopropylphenylimine ligand (PyminNiBr 2 ) in the presence of modified methylaluminoxane was investigated. Ethylene polymerization with only PyminNiBr 2 yielded a mixture of 1-and 2-olefin oligomers with methyl branches [weight-average molecular weight (M w ) ϳ 460)] with a ratio of about 1:7. By the combination of this nickel catalyst with PySTiN t Bu, polyethylenes with long-chain branches (M w ϭ 15,000 -50,000) were produced. No incorporation of 2-olefin oligomers was observed in the 13 C NMR spectra. Unexpectedly, the combination of the nickel catalyst with PySTiNMe produced lower molecular weight polyethylenes with only methyl branches. The molecular weight distributions of branched polyethylenes obtained with both PySTiNMe and PySTiN t Bu combined with the nickel catalyst were broad (weight-average molecular weight/number-average molecular weight Ͻ 9). Bimodal gel permeation chromatography (GPC) curves were clearly observed in the PySTiNMe system, whereas GPC curves with small shoulders in low molecular weight areas were observed for PySTiN t Bu. The synthesis of branched polyethylenes with tandem catalyst systems of corresponding homogeneous titanium catalysts and the nickel catalyst was also investigated for comparison.
Bis(cyclopentadienyl)zirconocene dimethyl (Cp2ZrMe2) combined with triphenylcarbenium tetrakis(pentafluorophenyl)borate ([Ph3C][B(C6F5)4]) was brought into contact with a suspension of 2% cross‐linked poly(4‐vinylpyridine) to give a new type of polymer‐supported cationic zirconocene catalyst. The resulting polymer‐supported catalyst system combined with Al(i‐Bu3) showed markedly high activity for ethylene polymerization in even a non‐polar solvent like hexane at 25–60°C and [Al]/[Zr] molar ratio 40–200. By the analysis of Zr content of the hexane solution, it was found that no Zr was detected in the solution, i. e. no leaching of the cationic catalyst into the hexane medium. The catalytic activity was found to increase with an increase of polymerization temperature and showed the highest at [Al]/[Zr] = 100. The molecular weight, crystalline melting temperature, crystallinity, and bulk density of polyethylene formed were higher than those of the polymer obtained from the homogeneous system.
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