Olefin polymerization behavior of titanium(IV) pyridine-2-phenolate-6- (σ-aryl) catalysts: Impact of "py-adjacent" and phenolate substituents

A series of Ti(IV) post-metallocene bis(benzyl) precatalysts supported by tridentate pyridine-2-phenolate-6-(σ-aryl) [O,N,C] ligands, featuring various substituents on the σ-aryl (directly adjacent to the pyridine ring: fluoro, trifluoromethyl, benzo [C₄H₄]) and phenolate groups (tert-butyl, trifluoromethyl, cumyl, 1,1-diphenylethyl), have been prepared. Multinuclear (including ¹H, ¹³C, and ¹⁹F) NMR characterizations of the complexes have been performed. The principal purpose of this study was to investigate the impact of these substituents upon ethylene polymerization reactivity and polymer properties. The cumyl-phenolate σ-naphthyl Ti precatalyst, in conjunction with [Ph ₃C][B(C₆F₅)₄], displays good activity and produces polyethylene with exceptionally high MW (M_n = 4 × 10⁶) and an M_w/M_n value (2.5) approaching single-site character at 50 C, but multisite behavior is apparent for other catalysts. DFT calculations have been performed to probe the polymerization behavior and the role of the py-adjacent substituent. These studies revealed the possibility of two distinct polymerization reactions, namely conventional and ethylene-assimilated (comprising initial ethylene insertion into the Ti-C(σ-aryl) bond) chain propagation, and found that the latter is kinetically preferred. Furthermore, the viability of another kinetically competitive pathway, namely the isomerization of the ethylene-assimilated [Ti-CH₂CH₂-aryl] species via β-H elimination and 2,1-reinsertion, was also indicated. © 2013 American Chemical Society.