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 alpha-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 (sigma) of the substituents and H-1 NMR analysis of the Pd-bound methyl. The hydrogen bonding interactions (H center dot center dot center dot Cl and H center dot center dot center dot 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 degrees 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 degrees C to produce the copolymer with high MA incorporation up to 9.5 mol%.

Automated summary

The development of thermally robust palladium-based catalysts for (co)polymerization of ethylene and polar monomers with high activities remains a challenging task. By combining hydrogen bonding interactions with steric and electronic modifications, new dibenzobarrelene-based alpha-diimine palladium complexes with different substituents have been synthesized and characterized. Through the steric and electronic effects as well as hydrogen bonding contributions, chloro- and methoxy-substituted palladium catalysts showed excellent performance in ethylene polymerization and copolymerization with high MA incorporation, respectively.