High catalytic activities in the norbornene polymerization with neutral palladium complexes containing N4-type tetradentate chelating ligands

Norbornene polymerization catalyzed by new Pd(II) complexes bearing N4-type tetradentate ligands obtained from the reaction between a 6-methyl-2-picolinic acid or picolinic acid and appropriate diamines has been studied. A class of new palladium complexes, [Pd(X1X2bpb)] and [Pd(X1X2-6-Me2bpb)] (X1 = Me, X2 = Me (1 and 4); X1 = H, X2 = H (2 and 5); X1 = H, X2 = NO2 (3 and 6); bpb = N,N′-(o-phenylene)bis(pyridine-2-carboxamidate); 6-Me2bpb = N,N′-(o-phenylene)bis(6-methylpyridine-2-carbox-amidate)) were synthesized and characterized. The molecular structure of Pd complex 5 was determined by X-ray crystallography, showing distorted square planar configurations. Using modified methylaluminoxanes (MMAO) as an activator, the palladium complexes exhibited high catalytic activities for the polymerization of norbornene. The catalytic activities up to 4.0 × 106 g of PNBEs/molPd·h and Mw up to 8.34 × 105 g/mol with PDI < 2.53 were observed. Amorphous polynorbornenes (PNBEs) were obtained with good solubility in halogenated aromatic solvents. Interestingly, the structural modification with the methyl groups of pyridyl rings and the strong electron-withdrawing substituents induced improvement in solubility, thermal stability and catalytic activity. FT-IR, 1H, and 13C NMR analyses of the polymers suggest that the catalytic polymerization occurs via vinyl addition mechanism.

Graphical abstractPalladium complexes with N4-type tetradentate ligands exhibited high catalytic activities for the polymerization of norbornene. The structural modifications with the methyl groups of pyridyl rings and the strong electron-withdrawing substituents induced the improvement in solubility, thermal stability and catalytic activity.Figure optionsDownload full-size imageDownload as PowerPoint slide