Ethylene polymerization catalyzed by salicylaldimine Fe or Cr complexes: A DFT study of the metal effects

[Display omitted] •Metal centers influence chain propagation through dissociative or associative insertion mechanisms.•Metal centers Fe/Cr center’s different hydride affinity result in linear and branched products. Salicylaldimine transition metal catalysts are of great interest in the field of ethylene polymerization. Especially, the late transition metal Fe and the middle transition metal Cr complexes exhibited completely different catalytic behaviors. We carried out systematic DFT studies to understand the catalytic mechanism in detail and revealed the intriguing metal effect on ethylene polymerization in propagation, β-hydride elimination, and termination stages. The metal centers would lead to dissociative or associative insertion mechanisms in the chain propagation stage due to their preference for different coordination features. The metal centers also significantly influence the β-hydride elimination, i.e., the late transition metal Fe center much prefers the β-hydride elimination over the middle transition metal Cr, due to the stronger hydride affinity of the Fe center than that of the Cr center. Therefore, salicylaldimine Fe produces a highly branched product, however, salicylaldimine Cr prevents β-hydride elimination producing linear high-density polyethylene. On the other hand, the higher hydride affinity of the Fe center leads to an easy chain transfer, resulting in only oligomer for salicylaldimine Fe systems. In contrast, the chain transfer step is difficult for the Cr center, leading to a successful ethylene polymerization for the salicylaldimine Cr system. These mechanistic insights into the metal center effects well explain the distinct ethylene polymerization behaviors for salicylaldimine Fe and Cr systems, providing helpful guidelines for the design of structure-controllable polymerization catalysts with different metal centers.