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

Abstract

Method of polymerization quenching with radioactive carbon monoxide (14СО) has been used to determine the number of active centers (СP) and propagation rate constants (kP) 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 molV−1 and the kP 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 СP and kP 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 kP values of these centers. For the 2,6-Et2LVCl3+МАО catalyst, the effect of polymerization temperature on the СP and kP values has been studied. It has been found that with polymerization temperature growth from 25 to 60 °С, the СP value substantially increases. As a result, the value of the effective activation energy (Eeff = 22.6 kcal mol−1) noticeably exceeded that of the activation energy of propagation reaction (EP = 14.2 kcal mol−1). It is proposed that the reason for the unusually high EP value is formation of “dormant” sites with vanadium-polymer bond at lower polymerization temperature.