Supported SiO₂-ⁿBuSnCl₃/MAO/( ⁿBuCp)₂ZrCl₂ catalyzing MAO cocatalyst-free ethylene polymerization: Study of hydrogen responsiveness

A supported metallocene catalyst was synthesized by sequentially loading methylaluminoxane (MAO) (30 wt % in toluene) and (ⁿBuCp) ₂ZrCl₂ on partially dehydroxylated silica ES 70 modified by ⁿBuSnCl₃. Its shock load hydrogen responsiveness was evaluated by polymerizing ethylene for 1 h at 8.5 bar (g) and 75°C without separately feeding the MAP cocatalyst. The shock load hydrogen feeding increased the ethylene consumption (at a fairly constant rate), catalyst productivity, as well as the resin bulk density and average particle size at ΔP (of hydrogen) ≥∼3.0 psi. The bulk density increased from 0.25 to 0.31 g/cm³. This shows a procedure for overcoming the inherent drop in catalyst productivity caused by heterogenization of metallocenes (that is a method for catalyst activation) and improving the resulting resin bulk density. The volume-weighted mean particle diameter of the resulting polyethylenes was found to be 5.80-11.12-fold that of the catalyst corresponding to ΔP = 0.00-7.11 psi, respectively. The resulting kinetic profiles showed to be fairly stable. However, M_w and polydispersity index were not affected. The particle size distribution, average particle size, and the scanning electron microscope photographs of the resulting resin particles confirmed the occurrence of the replication phenomenon. On the basis of the above findings, the mechanism of ethylene polymerization under the present experimental conditions has been revisited.