Effect of temperature on the number of active sites and propagation rate constant at ethylene polymerization over supported bis(imino)pyridine iron catalysts

The inhibition of ethylene polymerization with radioactive carbon monoxide (¹⁴CO) was used to obtain data on the number of active sites (C_P) and propagation rate constant (k_P) at ethylene polymerization in the temperature range of 35–70 °C over supported catalysts LFeCl₂/Al₂O₃, LFeCl₂/SiO₂, and LFeCl₂/MgCl₂ (L: 2,6‐(2,6‐(Me)₂C₆H₃N = CMe)₂C₅H₃N) with activator Al(i‐Bu)₃. The values of effective activation energy (E_eff), activation energy of propagation reaction (E_P), and temperature coefficients of variation of the number of active sites (E_Cp = E_eff ? E_P) were determined. The activation energies of propagation reaction for catalysts LFeCl₂/Al₂O₃, LFeCl₂/SiO₂, and LFeCl₂/MgCl₂ were found to be quite similar (5.2–5.7 kcal/mol). The number of active sites diminished considerably as the polymerization temperature decreased, the E_Cp value being 5.2–6.2 kcal/mol for these catalysts at polymerization in the presence of hydrogen. The reactions of reversible transformations of active centers to the surface hydride species at polymerization in the presence and absence of hydrogen are proposed as the derivation of E_Cp. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6621–6629, 2008