Olefin Polymerization Catalyzed by Amide Vanadium (IV) complexes: Stereo and regiochemistry of Propene Insertion

The reaction of VCl3(THF)3 with 1 equiv of the lithium salt of ligand ArNH(Me2SiCH2CH2SiMe2)NHAr or ArNH(SiMe3) (Ar = 2,6-Me2C6H3) afforded the corresponding V(IV) amide complexes, [1,2-CH2CH2(Me2SiNAr)2]VCl2 (3) and (Me3Si-NAr)2VCl2 (4). The activation of 3 and 4 with the alkyl aluminum compound Al2Et3Cl3 or AlEt2Cl produced active ethylene polymerization catalysts exhibiting productivity values among the highest reported for vanadium amide based catalysts. Moreover, syndiotactic specific propylene polymerization was successfully conducted at 40 °C in the presence of 3/Al2Et3Cl3 and 4/Al2Et3Cl3. Syndiotactic polypropylenes with moderate stereoregularity ([rr] = 0.66) and a concentration of regioirregular propylene of 6.9 mol % were obtained. Monomodal molecular weight distributions and polydispersity indices lower than 2 were observed in the polymerization runs carried out in heptane solutions. Thus, ethylene–propylene copolymers with propylene concentrations up to 45 mol % were synthesized and characterized by 13C NMR and thermal analysis. Good alternation and random distribution of the two monomers were actually obtained. Samples with elevated concentrations of propylene were completely amorphous, with a glass-transition temperature of 50 °C. The properties and structure of the copolymers produced with amide vanadium catalysts 3 and 4 were similar to those reported for ethylene–propylenes produced with industrial vanadium-based catalysts, suggesting the presence of the same active catalyst species.