Kinetic Study of Ethylene Polymerization over Homogeneous and Supported Catalysts Based on 2,6-bis(imino)pyridyl Complex of Iron and Cobalt
Method of polymerization inhibition by radioactive carbon monoxide (14CO) has been used to determine the number of active centers (CP) and propagation rate constant (kP) for ethylene polymerization with homogeneous and supported on the different supports (SiO2, Al2O3, MgCl2) catalysts based on the bis(imino)pyridine complexes of iron and cobalt. Methylalumoxane (MAO) or Al(i-Bu)3, were used as co-catalyst. In case of homogeneous system with both activators the rate profile of polymerization was unstable: high activity of the initial period sharply decreases. Rapid catalyst deactivation with polymerization time is shown to the decrease of the active sites number and average propagation rate constant (for instance, for catalysts based on Fe-complex in 1.5-2 and 3 times for CP and kP values, respectively). In contrast to homogeneous systems based on bis(imino)pyridine complexes of iron and cobalt, the supported catalysts are highly active and stable in ethylene polymerization at 70-80 C. Data on the effect of hydrogen over the calculated CP and kP values were obtained. Molecular-mass distribution (MMD) of polyethylenes produced was determined by gel-permeated chromatography method. Polyethylene with broad MMD was divided by using Temperature Rising Elution Fractionation (TREF) on separate fractions with narrow MMD and the distribution of CP and kP values on polymer fractions were determined. The obtained CP and kP values and data on the polymer molecular mass and MMD lead to conclusion that the nature of the support has almost no effect on the active centers structure and distribution of their reactivity.