Nature of Nickel(II) Compounds Formed upon Interaction of an Amine–Imine Ni(II) Complex with MAO and AlMe3 According to In Situ NMR Data

The mechanism of catalytic action of α-diimine nickel complexes, one of the most widely used post-metallocene catalysts for ethylene polymerization, remains the subject of extensive discussion. It is generally recognized that so-called “cationic” Ni(II) complexes play a key role in the catalytic cycle, serving as the sites where chain initiation, branching, and chain growth occur. However, most of these highly unstable complexes have been detected in model systems, whereas very limited information is available on nickel(II) complexes present in real catalytic systems. In this work, the nature of intermediates formed during the activation of nickel(II) complexes bearing neutral N,N'-donor amine–imine ligands, related to classical α-diimine complexes, by commonly used cocatalysts was studied using in situ NMR spectroscopy. It was established that unstable “cationic” Ni(II) complexes are also formed in these systems and act as direct precursors of the active catalytic sites.