Copolymerization of ethylene with methyl methacrylate with neutral nickel(II) complexes bearing β-ketoiminato chelate ligands

New neutral nickel(II) complexes bearing asymmetric β-ketoiminato (N,O) ligands (5a-c), [(2,6-ⁱPr₂C₆H ₃)NC(R₂)C(H)C(R₁)O]Ni(Ph)(PPh₃), have been synthesized. X-ray crystal structures reveal that each of complexes 5a-c adopts a near square-planar coordination geometry and the triphenylphosphine group is always trans to the arylamine group. With modified methylaluminoxane (MMAO) as a cocatalyst, complexes 5a-c can promote the polymerization of methyl methacrylate (MMA) in excellent yields (>98%), producing syndiotactic-rich PMMAs (rr around 70%). It is very exciting that the 5a-c/MMAO catalyst systems exhibit highly catalytic activities for the copolymerization of ethylene with MMA, producing high molecular weight functionalized polyethylenes with up to 16.7 mol % of methacrylate units built into the moderately branched polyethylene backbone. Further mechanistic studies have shown that PMMAs and copolymers obtained using these complexes possibly arise from a coordination-insertion mechanism rather than a radical mechanism.