Kinetic evidences for reversible transformations of active centers in ethylene polymerization by titanium–magnesium catalyst: Effect of the polymerization temperature

•Polymerization temperature affects on the number of active centers.•The number of active centers reversible changes with polymerization temperature.•Activation energy of propagation reaction is lower than effective activation energy.•Kinetic scheme of reversible transformations of active centers at polymerization is proposed.

The kinetics of ethylene polymerization at varied polymerization temperature over two modifications of titanium–magnesium catalysts that differ in their compositions and the molecular weight of the resulting polymers were studied by polymerization quenching by radioactive carbon monoxide. An increase in polymerization temperature in the range of 40–80 °C was found to significantly increase the number of active centers (CP), which contributes to the polymerization rate more significantly than an increase in the propagation rate constant (kP) does. The rise in the number of active centers with increasing polymerization temperature is shown to result in effective activation energy (Eeff) determined from the temperature dependence of polymerization rate being significantly higher than the activation energy of propagation reaction (EP) determined from the temperature dependence of the kP value. It was shown too the changes of the number of active centers and polymerization rate at variation of temperature in the single experiment have the reversible nature. The data on the number of active centers and the propagation rate constants at ethylene polymerization in the presence of hydrogen as a chain transfer agent are obtained. Introduction of hydrogen reduces the number of active centers at elevated polymerization temperatures (60–80 °C) and has virtually no effect on the CP value at lower temperatures; it also reduces the kP value at 40–80 °C. The findings on the effect of polymerization temperature on the number of active centers were used to propose a kinetic scheme including the reaction of reversible transformation of the precursors of active centers (alkyl derivatives of titanium) into an inactive state due to adsorption of a co-catalyst (triethylaluminum). The transformation of these complexes into active centers depends on polymerization temperature, presence of hydrogen, concentrations of monomer and organoaluminum activator.

Graphical abstractBased on the data on the effect of polymerization temperature on the number of active centers a kinetic scheme comprising the reactions of the reversible transformation of the precursors of active centers Cpr to inactive state due to adsorption of the co-catalyst was proposed. The transformation of these complexes into active centers CP depends on polymerization temperature, presence of hydrogen, concentrations of monomer and organoaluminum activator.Figure optionsDownload full-size imageDownload as PowerPoint slide