Norbornene Copolymerization with Polar Monomers Catalyzed by Palladium Catalysts Containing Imidazolidin-2-imine/Guanidine Ligands.

The development of a palladium catalyst that has enhanced catalytic performance, such as low aluminum cocatalyst loading, good copolymerization ability, high molecular weight, and excellent solubility of the (co)polymers, is still a challenge in norbornene copolymerizations. Here, a series of PdCl 2 and PdMeCl complexes containing differently substituted anilines and imidazolidin-2-imine/guanidine ligands was successfully synthesized and characterized. X-ray diffraction analysis results revealed that these Pd complexes adopted an almost square-planar geometry, and the six-membered chelate ring showed structural distinctions as compared to traditional N^N-based α-diimine and β-diimine Pd complexes. These Pd complexes were activated by EtAlCl 2 and then exhibited moderate activity (10 4 -10 5 g mol -1 h -1 ) and good thermal stability (up to 90 °C) for norbornene polymerization to produce high-molecular-weight PNBs ( M n up to 96.4 kg mol -1 ) with narrow polydispersities (PDI as low as 1.39). These Pd complexes also exhibited good polar group tolerance in the copolymerization of norbornene with methyl 5-norbornene-2-carboxylate and methyl 10-undecenoate, in which the activity was achieved up to 7.04 × 10 4 g mol -1 h -1 . It furnished polar functionalized norbornene-based copolymers with high molecular weight ( M n up to 63.1 kg mol -1 ), narrow PDI, reasonable polar monomer incorporation, and good solubility. These Pd catalysts exhibited an enhanced copolymerization ability to produce PNB or NB-based copolymers, representing significant progress in this field.