Substituent Effects of the Backbone in α‑Diimine Palladium Catalysts on Homo- and Copolymerization of Ethylene with Methyl Acrylate

On the basis of different approaches for modifying α-diimine
palladium catalysts, a series of methyl chloride palladium complexes
with various α-diimine ligand backbones were synthesized and
characterized. The corresponding cationic palladium complex chelating
esters were further obtained by treatment of methyl chloride palladium
complexes with methyl acrylate (MA). It was observed that decomposition
of a cationic palladium complex chelating ester can occur to produce
a new cationic palladium complex chelating two ligands and two counteranions,
which provides a new pathway for deactivation of palladium catalysts
and formation of palladium black by a fragmentation pattern with ester
loss. These α-diimine palladium catalysts were employed in the
homopolymerization of ethylene and copolymerization of ethylene and
MA to evaluate substituent effects of the ligand backbone. A bulky
camphyl α-diimine palladium catalyst was found to show better
thermal stability and afford high-molecular-weight copolymer with
higher incorporation of polar monomer. Longstanding living polymerization
of ethylene was also achieved within 12 h using a bulky camphyl α-diimine
palladium catalyst.