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

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 kgPE molcat−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.