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)(2)TiCl2; 2 = ((OS)-O-Cum)(2)TiCl2; 3 = (t-BuOS)(2)Zr(CH2Ph)(2); 4 = ((OS)-O-Cum)(2)Zr(CH2Ph)(2)) bearing two bidentate thioetherphenolate ligands (t-BuOS-H = 4,6-di-tert-butyl-2-phenylsulfanylphenol; (OS)-O-Cum-H = 4,6-bis-(alpha, alpha-dimethylbenzyl)-2-phenylsulfanylphenol) has been accomplished. These complexes exhibit fluxional behaviour in solution and this was revealed by VT H-1 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, beta-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.