Design of stable and active (NHC)Pd–phosphine catalysts for the synthesis of high-molecular-weight norbornene-based polymers

A series of cationic palladium complexes of the composition [(NHC)Pd(allyl)(PR3)]+BARF− containing N-heterocyclic carbene (NHC) and phosphine ligands of different nature, and a weakly coordinating borate anion were synthesized. Such Pd complexes were for the first time investigated as single-component catalysts for vinyl-addition polymerization of norbornene and its functionalized derivatives. The complexes exhibit high catalytic activity without the need for co-catalysts (to 7·106 gpolymer/molPd h). Polymers synthesized over these Pd complexes in question are characterized by unimodal GPC curves, narrow molecular weight distributions, and linear dependence of the molecular weight on the monomer conversion, which suggests the occurrence of controlled polymerization. It was demonstrated that both the structure of NHC and the nature of phosphine ligand have strong effect on catalytic activity and initiation efficiency. A correlation between Pd—P bond length and the behavior of the complex in the polymerization reaction was revealed, viz.: the shorter Pd—P bond the lower the lability of the phosphine, the initiation efficiency, and the higher the molecular weights of the resulting polymers. As a result, polymerization on the title complexes allows one to obtain polymers with much higher molecular weights compared to polymers synthesized using similar (NHC)Pd systems with other labile ligands. Besides, the synthesized complexes demonstrate high thermal stability and retain their catalytic activity at elevated temperatures. Generally, the presence of the NHC and phosphine ligands in palladium coordination sphere provides a unique balance between the activity, stability, and possibility to obtain high-molecular-weight products. This makes the complexes in hand promising catalysts for targeted synthesis of functional polymeric materials. © 2026 Elsevier Inc.