Effect of hydrogen on the number of active centers and the propagation rate constant at ethylene polymerization over titanium-magnesium Ziegler-Natta catalysts

The effect of hydrogen on the number of active centers (С_Р) and the propagation rate constant (k_P) at ethylene polymerization over titanium–magnesium catalysts is studied using quenching by radioactive carbon monoxide (¹⁴СО). The polymerization rate drops after hydrogen was added to the polymerization medium mainly due to the decrease in the calculated k_P value. The changes in the polymerization rate and the experimental k_P value observed after hydrogen was added or removed are found to be reversible. The data on the effect of the [H₂]/[C₂H₄] molar ratio in the gas phase within the range of 0.125–1 on polymerization rate and the С_Р and k_P values are obtained. Reduction of the k_P value and a small decrease in the number of active centers are observed at the H₂/C₂H₄ molar ratio ≥ 0.5. These findings were used to propose a scheme interpreting the effect of hydrogen on the С_Р and k_P values. The scheme suggests that the decrease in the calculated k_P value observed in the presence of hydrogen is related to the reversible formation of temporarily inactive dormant sites containing the titanium–polymer bond.