Living regioselective copolymerization of ethylene with nonconjugated bicyclic dienes using (salicylaldiminato)(β-enaminoketonato) titanium catalysts

A series of heteroligated (salicylaldiminato)(β-enaminoketonato) titanium complexes [3-^tBu-2-OC₆H₃CH=N(C ₆F₅)] [PhN=C(CF₃)CHCRO]TiCl₂ [3a: R = Ph, 3b: R = C₆H₄Cl(p), 3c: R = C₆H ₄OMe(p), 3d: R = C₆H₄Me(p), 3e: R = C ₆H₄Me(o)] were synthesized and characterized. Molecular structures of 3b and 3c were further confirmed by X-ray crystallographic analyses. In the presence of modified methylaluminoxane as a cocatalyst, these unsymmetric catalysts displayed favorable ability to incorporate 5-vinyl-2-norbornene (VNB) and 5-ethylidene-2-norbornene (ENB) into the polymer chains, affording high-molecular weight copolymers with high-comonomer incorporations and alternating sequence under the mild conditions. The comonomer concentration in the polymerization medium had a profound influence on the molecular weight distribution of the resultant copolymer. At initial comonomer concentration of higher than 0.4 mol/L, the titanium complexes with electron-donating groups in the β-enaminoketonato moiety mediated room-temperature living ethylene/VNB or ENB copolymerizations. Polymerization results coupled with density functional theory calculations suggested that the highly controlled living copolymerization is probably a consequence of the difficulty in chain transfer of VNB (or ENB)-last-inserted species and some characteristics of living ethylene polymerization under limited conditions.