Titanium and zirconium complexes containing sterically hindered hydrotris(pyrazolyl)borate ligands: synthesis, structural characterization, and ethylene polymerization studies

The synthesis, characterization and ethylene polymerization behavior of a set of Tp′MCl3 complexes (4, M=Ti, Tp′=HB(3-neopentyl-pyrazolyl)3−(TpNp); 5, M=Ti, Tp′=HB(3-tert-butyl-pyrazolyl)3−(TptBu); 6, M = Ti, Tp′=HB(3-phenyl-pyrazolyl)3−(TpPh); 7, M=Zr, Tp′=HB(3-phenyl-pyrazolyl)3−(TpPh); 8, M=Zr, Tp′ = HB(3-tert-butyl-pyrazolyl)3−(TptBu)) is described. Treatment of these tris(pyrazolyl)borate Group IV compounds with methylalumoxane (MAO) generates active catalysts for ethylene polymerization. For the polymerization reactions performed in toluene at 60 °C and 3 atm of ethylene pressure, the activities varied between 1.3 and 5.1 × 103 g of PE/mol[M] · h. The highest activity is reached using more sterically open catalyst precursor 4. The viscosity-average molecular weights () of the PE’s produced with these catalyst precursors varying from 3.57 to 20.23 × 105 g mol−1 with melting temperatures in the range of 127–134 °C. Further polymerization studies employing 7 varying Al/Zr molar ratio and temperature of polymerization showed that the activity as well as the polymer properties are dependent on these parameters. In that case, higher activity was attained at 60 °C. The viscosity-average molecular weights of the polyethylene’s decreases with increasing Al/Zr molar ratio.