Boosting the Thermal Stability of α-Diimine Palladium Complexes in Norbornene Polymerization from Construction of Intraligand Hydrogen Bonding and Simultaneous Increasing Axial/Equatorial Bulkiness.

Increasing the thermostability of α-diimine late-transition-metal complexes and therefore rendering them more active at higher temperatures is of great importance, yet challenging for the olefin polymerization field. In the present research, a new family of α-diimine palladium complexes that can promote norbornene polymerization at high temperatures (up to 140 °C) is disclosed. Because of the conformational restriction caused by increasing the axial and equatorial bulkiness as well as the presence of intraligand H···F hydrogen bonds, -aryl rotations can be efficiently restricted, therefore circumventing the deactivation of the active species at high temperatures. At 80-140 °C, these complexes can efficiently catalyze norbornene homopolymerizations, giving high catalytic activities up to 5.65 × 10 g of PNB per mole Ni per hour and polymers with high molecular weights up to 37.2 × 10 g/mol, which are highly superior to catalytic systems mediated by CF-free complexes. Moreover, these complexes could also afford medium catalytic activities in the presence of polar 5-norbornene-2-carboxylic acid methyl ester (NB-COOCH).