Kinetics of ethylene polymerization in the presence of a homogeneous catalyst based on a bis(phenoxyimine) complex of zirconium(IV)

Changes in the molecular-weight characteristics of the product of ethylene polymerization in the course of reaction in the presence of a homogeneous catalytic system and in the number and reactivity of catalyst active sites were studied. The catalytic system consisted of bisN-(3-tert-butylsalicylidene)anilinato]zirconium dichloride and methylalumoxane as an activator. This catalytic system exhibited the signs of unsteady-state conditions: the rate of polymerization dramatically decreased as the reaction time increased. At the onset of polymerization (to 5 min), the catalyst was single-site, and it produced low-molecular-weight polyethylene with M _w = (4–10) × 10³ g/mol. The fraction of active sites at the initial point in time was as high as 11% based on the initial amount of the zirconium complex. The reactivity of these centers was very high (the rate constant of polymer chain growth was 5.4 × 10⁴ l mol⁻¹ s⁻¹ at 35°C). As the polymerization time increased, the number of active sites decreased and the molecular-weight distribution of polyethylene broadened because of the decay of a portion of initial centers and the formation of new centers that produced high-molecular-weight polyethylene with M _w to 130 × 10⁴ g/mol. The propagation rate constant measured at a sufficiently long polymerization time (20 min) was lower than that at the initial point in time; this fact suggests the much lower reactivity of the new active sites.