Ethylene Polymerization over Titanium-Magnesium Catalysts: Effect of Hydrogen on the Numbers of Active Centers and Propagation Rate Constant
Hydrogen is widely used in industrial processes of polyethylene production with titanium-magnesium catalysts (TMC) as the molecular weight control agent for preparation of different polymer grades. It is known that hydrogen reduce the activity of titanium-magnesium catalysts at ethylene polymerization. There are a lot of speculations in literature about the reasons of this effect, but still there isn’t generally accepted theory which could explain the deactivating effect of hydrogen at ethylene polymerization. In this work the data on the hydrogen effect on the number of active centers (Cp) and propagation rate constant (kp) at ethylene polymerization over titanium-magnesium catalysts with different compound and at the variation of polymerization conditions were obtained by the method of polymerization inhibition with radioactive monoxide (14CO).
It was found that the decrease of the activity at hydrogen introduction to the polymerization media is mostly caused by reduction of the determined average kp value. The number of active centers is weakly depends on hydrogen. Effect of the hydrogen concentration on Cp and kp values is studied. It was shown the decrease of the polymerization rate with the increase of hydrogen concentration is caused mainly by the decrease of determined kp value. Only small decrease of the number of active centers is observed at large molar ratios of H2/C2H4 in gas phase. It was found that the hydrogen effect on the activity and Cp and kp values do not depend on the monomer pressure in the range of 1-4 bar.
Rest upon the data obtained in this work and using literature data about the hydrogen effect at ethylene polymerization on activity, CP and kP values for vanadium-magnesium catalysts the scheme of active centers transformation in the hydrogen presence at ethylene polymerization on TMC is proposed. The scheme involves the diethylaluminumhydride reversible adsorption on active centers resulting in temporary stop of the propagation reaction and explains the decrease of polymerization rate and determined value kp in the presents of hydrogen.