Influence of ligand second coordination sphere effects on the olefin (co)polymerization properties of α-diimine Pd(II) catalysts

A series of alpha-diimine ligands and their corresponding palladium complexes were synthesized and characterized. These alpha-diimine ligands possess unique properties, which could lead to metal complexes with very similar electronic and steric environments around a palladium center. For example, the CO stretching frequencies of the palladium carbonyl complexes are very similar. At a remote position on the second coordination sphere of the ligand structure, substituents (Ph, CF3, NO2 and OMe) are installed to probe their potential influences on olefin polymerization and copolymerization processes. The properties of these palladium complexes in ethylene polymerization, ethylene/methyl acrylate copolymerization, ethylene/norbornene copolymerization and ethylene/5-norbornene-2-yl acetate copolymerization are investigated in detail. Interestingly, most of the metal complexes behaved very similarly to each other except for the complex bearing a Ph substituent, which showed much better activity and generated polymers with much higher molecular weight.