Dearomatized diimines for nickel(ii) and palladium(ii) catalyzed ethylene polymerization and copolymerization

Abstract

Steric tuning of the aryl ortho-positions and backbone of an α-diimine pro-ligand significantly impacts Ni(II) and Pd(II)-catalyzed olefin (co)polymerization. Herein, dual steric enhancement in the ortho-positions and backbone was achieved through the combination of a bulky vicinal t-butylated dearomatized acenaphthyl backbone and bulky ortho substituents (i-Pr or CHAr₂). Single-crystal X-ray analysis showed that this combination shielded the back and axial spaces of the metal center. In the ortho-diisopropyl (i-Pr) substituted system, compared to Ni1 and Pd1 with the acenaphthyl backbone, Ni2 and Pd2 with the bulky dearomatized acenaphthyl backbone showed improved catalytic activity and generated a polymer with a higher molecular weight and slightly changed branching density. In the ortho-diarylmethyl (CHAr₂) substituted system, the opposite trend that the bulky dearomatized acenaphthyl backbone led to lower catalytic activity and polymer molecular weight was observed, which may be attributed to the slow propagation rate caused by the excessively high and rigid steric effects in these catalysts. In contrast to ortho-diarylmethyl substituted Ni(II) catalysts, the branching density of the polymer generated by the Pd(II) analogues decreased when the more bulky dearomatized acenaphthyl backbone was employed. The non-dependence of the branching density on polymerization temperature in catalysts with a dearomatized acenaphthyl backbone (Ni4 and Pd4) was also observed in comparison with catalysts with the acenaphthyl backbone (Ni3 and Pd3), suggesting the disruption of the π–π interaction between the diarylmethyl moiety and the acenaphthene aromatic plane in Ni4 and Pd4. Particularly, the dearomatized acenaphthyl backbone promoted the incorporation of polar monomers (methyl acrylate (MA) and ethylene-acrylic acid (AA)) in ethylene–polar monomer copolymerization.