Copolymerization of ethylene with propylene and higher α-olefins catalyzed by (imido)vanadium(iv) dichloride complexes

Abstract

We have synthesized and characterized a series of dimethylamine–imido V(NR)Cl₂(NHMe₂)₂ [R = ^tBu (1a), CPh₃ (1b), 2,6-CHPh₂-4-Cl-C₆H₂ (1c)], and pyridine–imido V(NR)Cl₂(Py)₃ [R = ^tBu (2a), CPh₃ (2b), 2,6-CHPh₂-4-Cl-C₆H₂ (2c)] complexes. The solid-state structures of 1a–1c, and 2c were determined by X-ray crystallography. Complexes 1a and 2a–2c, in combination with Et₂AlCl and Cl₃CCO₂Et, have been screened as catalysts for the copolymerization of ethylene with various α-olefins (i.e., propylene, 1-hexene, 1-octene, and 4-methyl-1-pentene). The results are compared with the known PMe₂Ph–imido V(NR)Cl₂(PMe₂Ph)₂ [R = ^tBu (3a), 2,6-ⁱPr₂-C₆H₃ (3d)] complexes. Differences in the (co)polymerization regarding the activity and reactivity toward the target comonomers are investigated to probe the effect of imido ligand substitution, and of the coligand. With the exception of dimethylamine 1a, 2 and 3 are instantaneously activated and exhibit good activity, affording copolymers with a moderate comonomer content (4.2 < mol% < 13.7), from low to high molecular weight (36 < M_w × 10³ g mol⁻¹ < 270), and unimodal molecular weight distribution (2.1 < M_w/M_n < 2.7), strongly depending on the type of comonomer, copolymerization temperature, and, to a lesser extent, the type of ligand set employed. ¹³C NMR spectra of poly(ethylene-co-propylene)s have been fully interpreted as a result of uninterrupted methylene sequence distribution, the ethylene–propylene sequence, and inverted propylene units. In addition, the copolymers were characterized by DSC, TGA, and successive self-nucleation and annealing (SSA). A preliminary investigation of the tensile behavior of the copolymers was performed by uniaxial stretching until failure.