Kinetic study of ethylene polymerization over 2,6-bis(imino)pyridine cobalt complexes with bulky substituents in ligand

The number of active centers (СP) and propagation rate constants (kP) for ethylene polymerization with single-site homogeneous catalysts composed of cobalt (II) bis(imino)pyridine complexes: 2-tBuLCoCl2 and 2,5-tBu2LCoCl2 (L: 2,6-(C6H3NCMe)2C5H3N) and methylaluminoxane (MAO) as an activator have been determined by polymerization quenching with radioactive carbon monoxide. The above cobalt complexes provided formation of catalysts, characterized with an increased activity due to the increase in the СP value. The values of propagation rate constant, determined for the studied catalysts, were close (3.6–2.4 × 103 L mol−1 s−1). The values of the constant of the chain transfer reaction with ethylene decreases sharply in the order 2,6-Me2LCoCl2 > 2-tBuLCoCl2 > 2,5-tBu2LCoCl2, being the reason for the increase in the molecular weight of PE, produced with corresponding complexes. For the 2,5-tBu2LCoCl2 + MAO catalyst, the effect of polymerization temperature on the CP and kP values have been studied and the activation energy of propagation reaction was calculated (4.5 kcal mol−1), that was noticeably lower as compared with the effective activation energy (12.5 kcal mol−1), owing to the increase in the CP value with polymerization temperature growth.

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► The active centers number and reactivity for single-sites catalysts were determined.
► There is no effect of ligand bulk on propagation rate constant.
► The chain transfer to monomer was found to be the main reaction of chain termination.
► We have obtained the value of the activation energy of the propagation reaction.