Control of Chain Transfer by Tailored Metal-Phenyl π-Interactions in Bis(Imino)Pyridyl Cobalt-Catalyzed Ethylene Polymerization

In bis(imino)pyridyl iron/cobalt-catalyzed ethylene polymerization, broad and bimodal molecular weight distributions of polyethylenes (PEs) are often produced because of two different chain transfer pathways. In this paper, a combined theoretical and experimental study demonstrates how weak attractive metal-phenyl π-interactions can favor β-H transfer to ethylene over chain transfer to Al to achieve a unimodal distribution. A series of bis(imino)pyridyl cobalt complexes with a substituted 8-(p-R-phenyl)naphthylamine (R = OMe, Me, CF₃) were designed and synthesized for ethylene polymerization. The intramolecular Co-phenyl π-interactions between the capping aryl groups and the cobalt metal are clearly identified in "half-sandwich" bis(imino)pyridyl cobalt complexes. Ethylene polymerization results show that bis(imino)pyridyl cobalt catalyst with an electron-donating methoxy group can produce low-molecular-weight polyethylene with a unimodal distribution. This control of β-H transfer to ethylene monomer is promoted by weak Co-phenyl interactions, operative only for the case of sufficiently electron-rich aryls (R = OMe) and low temperatures (-10°C-30°C).