Regulation of the Components in Broad Molecular Weight Distribution Polyethylene by C2-Symmetric α-Diimine Nickel Catalysts With Different N-Aryl Substituents

Abstract

Among the catalyst design strategies for α-diimine nickel systems, the C₂-symmetric strategy is an effective and widely used approach. To investigate the steric effect of the N-aryl substitutes on the isomerization behavior of C₂-symmetric catalysts and the properties of ethylene polymerization, a series of α-diimine nickel complexes were constructed by introducing phenyl and alkyl groups on the N-aryl ortho- and para-positions. From 40°C to 80°C, Ni1 ~ Ni5 exhibited highly ethylene polymerization activity (10⁷ g mol⁻¹ h⁻¹), yielding broadly distributed or bimodal polyethylene dominated by high molecular weight component. This is attributed to the rotation of the N-aryl bond resulting in the presence of both anti and syn isomers. Increasing the temperature or decreasing the size of the ortho-alkyl substitutes results in a faster conversion rate between the syn and anti isomers. It is believed that at higher temperatures or with larger N-aryl ortho-alkyl groups, the catalyst tends to exist in a more stable anti conformation. As a result, the bimodal polyethylene products are dominated by high molecular weight component in adjustable proportions and exhibit good mechanical and processing properties, showing great prospects for the application of solar cell package films.