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.

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On the basis of the steric effects of ligand, a series of imine–N-heterocyclic carbene (NHC) ligands and their corresponding five-membered palladium complexes with bulky substituents on both the imine and the NHC moieties were synthesized and characterized. Transpalladation of silver carbene complexes with (COD)­PdCl2 and (COD)­PdMeCl afforded the palladium dichloride and methylpalladium complexes, respectively. Bulky cationic palladium complexes were further obtained by treatment of the methylpalladium complexes with sodium tetrakis­(3,5-bis­(trifluoromethyl)­phenyl)­borate (NaBAF) in CH3CN. Well-defined cationic palladium complexes were confirmed by X-ray crystal diffraction to have trans forms. Palladium dichloride complexes and methylpalladium complexes after activation with MMAO show high activity for norbornene polymerization, whereas cationic palladium complexes can polymerize norbornene alone without any cocatalysts and exhibit a high thermostability. Norbornene polymerization with the cationic palladium catalyst was proven to proceed through a coordination–insertion mechanism by NMR studies. Analysis of oligomers obtained by polymerizing the monomer in the presence of H2 reveals the existence of a C7 linkage in the polynorbornene (PNB) by σ-bond metathesis, which may be the reason for the insolublity of polynorbornenes obtained by palladium catalysts.

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