Ethylene polymerization over homogeneous Bis(imino)pyridine vanadium catalysts: data on the number and reactivity of active sites

Method of polymerization quenching with radioactive carbon monoxide (¹⁴СО) has been used to determine the number of active centers (С _P ) and propagation rate constants (k _P ) for ethylene polymerization over the homogeneous catalysts 2,6-R₂LVCl₃+MAO (L: 2,6-(C₆H₃N=CMe)₂C₅H₃N, R=Me, Et). For both catalysts the maximum number of active centers at 60 °C was found to be 0.42–0.50 mol mol_V ⁻¹ and the k _P values—(16.4–20.0) × 10³ L mol⁻¹ s⁻¹. It has been shown that noticeable decrease in the catalysts activity with polymerization time results from the decrease in the active centers number and simultaneous transformation of some part of the initial centers into a new ones with lower reactivity. Analysis of the С _P and k _P values, measured at different polymerization times, together with the values of the polyethylene molecular-weight distribution, allowed to calculate the content of the active centers of both types and the k _P values of these centers. For the 2,6-Et₂LVCl₃+МАО catalyst, the effect of polymerization temperature on the С _P and k _P values has been studied. It has been found that with polymerization temperature growth from 25 to 60 °С, the С _P value substantially increases. As a result, the value of the effective activation energy (E _eff  = 22.6 kcal mol⁻¹) noticeably exceeded that of the activation energy of propagation reaction (E _P  = 14.2 kcal mol⁻¹). It is proposed that the reason for the unusually high E _P value is formation of “dormant” sites with vanadium-polymer bond at lower polymerization temperature.