Axial-phenyl-constrained bis(imino)acenaphthene-nickel precatalysts enhance ethylene polymerization†

To enhance the performance of the bis(imino)acenaphthene-nickel precatalyst, an axial phenyl group was introduced on the acenaphthenyl backbone, and five analogues were formed: bis(aryl)imino-5-phenylacenaphthene nickel bromides (aryl: 2,6-dimethylphenyl, Ni1; 2,6-diethylphenyl, Ni2; 2,6-diisopropylphenyl, Ni3; 2,4,6-trimethylphenyl, Ni4; and 2,6-diethyl-4-methylphenyl, Ni5). The molecular structures of mononuclear Ni2 and dinuclear Ni3 were confirmed as distorted tetrahedron and square-pyramidal geometries around the nickel core, respectively. All five precatalysts achieved better activity as well as thermostability for ethylene polymerization and delivered polyethylene elastomer with higher molecular weight and more branches than the benchmark nickel precatalyst proposed by Brookhart. Specifically, Ni2/EASC maintained a catalytic activity of 4.52 × 10⁶ g PE per mol (Ni) per h even at 100 °C. More importantly, the resultant polyethylenes exhibited excellent mechanical properties with high strain (>1000%) and stress (>10 MPa) values observed during tensile stress–strain tests. Topographic steric maps showed that there is sufficient space around the nickel center for ethylene to conduct coordination and facilitate polymeric propagation. The introduction of an axial phenyl group into the backbone of nickel complexes not only elevates the catalytic performance, but also provides these polyethylene elastomers with attractive properties.