Polymerization of olefins with titanium and zirconium complexes containing hydrotris(pyrazolyl)borate or hydrotris(3,5-dimethylpyrazolyl)borate

Zirconium and titanium complexes containing hydrotris(pyrazolyl)borate (Tp) or hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) as a ligand have been examined as polymerization precatalysts for ethylene, propylene and styrene. Activation of the group 4 transition-metal complexes by methylaluminoxane exhibits a greater catalytic activity in ethylene polymerization for TpMCl3 (M=Zr, Ti) than for the corresponding mono-cyclopentadienyl analogue, CpMCl3. Dimethyl substitution on the pyrazolyl groups enhances the activity for the Zr complex (Tp*ZrCl3), but not for the Ti complex (Tp*TiCl3). Complete substitution of OtBu or OPh groups for Cl ligands in Tp*ZrCl3 greatly decreases the activity. Mono substitution of an OPh group for one Cl ligand in Tp*TiCl3 does not affect the activity. The addition of propylene comonomer reduces the catalytic activity of Zr complexes containing Tp or Tp* to a considerable degree. The Tp and Tp* complexes of Zr and Ti produce atactic polymers from styrene.