New phenyl–nickel complexes of bulky 2-iminopyrrolyl chelates: synthesis, characterisation and application as aluminium-free catalysts for the production of hyperbranched polyethylene

Abstract

A family of mono(2-iminopyrrolyl) complexes with the general formula [Ni{κ²N,N′-5-(aryl)-NC₄H₂-2-C(H)N-2,6-(aryl)}(C₆H₅)(PPh₃)] were obtained from the reaction of the sodium salts of the newly synthesised 5-aryl-2-(N-arylformimino)pyrroles with the square planar complex trans-[Ni(C₆H₅)(PPh₃)₂Cl]. These new iminopyrrole ligand precursors, designed with increasing bulkiness and different electronic properties, and their corresponding nickel(II) complexes were characterised by NMR spectroscopy and elemental analysis, and their structural features were analysed by single crystal X-ray diffraction. The nickel complexes were tested as aluminium-free catalysts for the polymerisation of ethylene, at low to moderate pressures and different temperatures and in the absence or presence of the phosphine scavenger [Ni(COD)₂], giving rise to catalytic activities in the range of 3.61–73.12 kg_PE mol_Ni⁻¹ h⁻¹ bar⁻¹. The polyethylene products formed in these catalytic reactions were characterised by GPC/SEC and NMR spectroscopy. Generally, low molecular weight (M_n 510–1300 g mol⁻¹) low viscosity oils were obtained, presenting high branching degrees (80–125 branches per 1000 C atoms), which are characteristic of hyperbranched polyethylene products. In particular, polymerisation reactions using catalyst 7 led to higher viscosity oils with molecular weights between 11 000 and 20 000 g mol⁻¹, and branching degrees of 100–120 branches per 1000 C atoms.