Combining hydrogen bonding interactions with steric and electronic modifications for thermally robust α-diimine palladium catalysts toward ethylene (co)polymerization†
Development of thermally robust palladium-based catalysts for (co)polymerization of ethylene and polar monomers with high activities is a continuing challenge. Combining hydrogen bonding interactions with steric and electronic modifications, dibenzobarrelene-based α-diimine palladium complexes with different substituents (X = OMe, H, Cl, Br, and I) have been synthesized and characterized. The steric effect of the palladium complexes was elucidated by their buried volumes, and the electronic effect of the substituents was clarified by the Hammett constants (σ) of the substituents and 1H NMR analysis of the Pd-bound methyl. The hydrogen bonding interactions (H⋯Cl and H⋯OMe) were confirmed by single crystal structures of chloro- and methoxy-substituted neutral and cationic palladium complexes. Contributed by the steric and electronic effects as well as the hydrogen bonding, the chloro-substituted palladium catalyst was thermally robust at temperatures as high as 100 °C for ethylene polymerization, while the methoxy-substituted palladium catalyst showed excellent tolerance toward high temperature and polar groups and was able to copolymerize ethylene and methyl acrylate (MA) at 80 °C to produce the copolymer with high MA incorporation up to 9.5 mol%.