Multimodal Polyethylenes Produced by Hybrid and Unsymmetric α‑Diimine Nickel Catalysts: The Roles of Syn/Anti Diastereomers and Imine Metathesis

The broad applicability requires polyethylene (PE) to
possess good
processability and mechanical properties; direct synthesis of bimodal
or multimodal PEs using a well-defined catalyst is therefore of considerable
academic and industrial interest. In this paper, a series of dibenzobarrelene-derived
α-diimine nickel complexes with a substituted 8-(p-R-phenyl)naphthylamine (R = OMe, Me, CF3) and a naphthylamine
were designed and synthesized for ethylene polymerization by unsymmetric
and hybrid aniline strategies. At low temperature (<50 °C),
bimodal PEs are obtained because of the presence of syn and anti diastereomers
of α-diimine nickel catalysts. At high temperatures (>65
°C),
multimodal PEs with broad molecular weight distributions are produced,
which arises from the cooperative contributions of syn/anti diastereomers
of α-diimine nickel catalysts and imine metathesis. The imine
metathesis observed for these hybrid α-diimine nickel complexes
provides a new mechanistic perspective on their unique ethylene polymerization
behavior and a strategy for tailoring PE molecular weight distributions
by using well-defined catalysts.