Influence of the N–H functionality on activating O,N,N-titanium complexes and controlling polyethylene molecular weights and distributions

Abstract

The titanium trichloride complex 1-TiCl₃ ((C₆H₅–NH–C₆H₄–NCH–C₆H₂-3,5-^tBu-2-O)TiCl₃) with the phenoxy-imine–amine (O,N,N) ligand containing an N–H group was synthesized and used as a pre-catalyst for olefin polymerization. Its structure, activation mechanism, and catalytic properties were compared with those of the titanium dichloride analogue 1-TiCl₂ ((C₆H₅–N–C₆H₄–NCH–C₆H₂–3,5-^tBu-2-O)TiCl₂) bearing the same ligand but devoid of N–H groups. Under optimum conditions, 1-TiCl₃ showed unprecedented activity of up to 5.10 × 10⁸ g(PE) mol⁻¹(Ti) h⁻¹ leading to polyethylenes with unimodal or bimodal distributions (2.9 ≤ Đ (dispersity) ≤ 6.1), while 1-TiCl₂ afforded polyethylene with narrow and unimodal distribution (Đ = 1.8). In situ NMR and single-crystal X-ray diffraction experiments revealed that the activation of 1-TiCl₃ with 5 equiv. of d-MAO (dried methylaluminoxane) was slow, probably resulting in the production of polyethylene with a bimodal/broad distribution. When pre-activated with 200 equiv. of d-MAO, 1-TiCl₃ and 1-TiCl₂ showed very similar catalytic properties, including their activity and the molecular weights and mass distributions of the polymers formed, which is fully consistent with the results of in situ NMR studies. For comparison, a methyl group was introduced on the amine-N and the corresponding ligand 2-L (C₆H₅–NMe–C₆H₄–NCH–C₆H₂-3,5-^tBu-2-OH) afforded 2-TiCl₃ ((C₆H₅–NMe–C₆H₄–NCH–C₆H₂-3,5-^tBu-2-O)TiCl₃). Its catalytic properties were similar whether pre-activation with excess d-MAO was performed or not, further establishing the strong influence of the amine-N substituent on catalysis. Remarkably, 2-TiCl₃ exhibited high activity (8.10 × 10⁷ g(polymer) mol⁻¹(Ti) h⁻¹) in ethylene/1-octene copolymerization and produced a poly(ethylene-co-1-octene) copolymer with an ultra-high-molecular-weight and narrow distribution (M_w = 362 × 10⁴ g mol⁻¹, Đ = 1.4).