Polymerization of ethylene and propylene promoted by group 4 metal complexes bearing thioetherphenolate ligands

Abstract

The synthesis of four new group 4 metal complexes 1–4 (1 = (^t-BuOS)₂TiCl₂; 2 = (^CumOS)₂TiCl₂; 3 = (^t-BuOS)₂Zr(CH₂Ph)₂; 4 = (^CumOS)₂Zr(CH₂Ph)₂) bearing two bidentate thioetherphenolate ligands (^t-BuOS-H = 4,6-di-tert-butyl-2-phenylsulfanylphenol; ^CumOS-H = 4,6-bis-(α,α-dimethylbenzyl)-2-phenylsulfanylphenol) has been accomplished. These complexes exhibit fluxional behaviour in solution and this was revealed by VT ¹H NMR and supported by density functional theory (DFT) calculations. All these complexes are active catalysts in ethylene polymerization, producing linear polyethylene. Notably, the zirconium complex 3 displays, under proper reaction conditions, very high activity (1422 kg_PE mol_cat⁻¹ bar⁻¹ h⁻¹), which compares well with that of the most active post-metallocene catalysts. Furthermore, propylene polymerization catalyzed by the titanium complex 1 yields atactic polypropylene, whereas the zirconium complexes 3 and 4 selectively produce oligopropylene with Schultz–Flory distribution. NMR analysis of the unsaturated chain ends in the latter samples provides evidence of a regioselective propagation reaction with a large preference for 1,2-monomer insertion. DFT calculations allowed the modelling of the elementary reaction steps, namely, the chain propagation reaction, β-hydrogen elimination and transfer, highlighting the importance of the flexibility and steric hindrance of the ancillary ligands in determining the high activity of the title catalysts.