Effect of the Nature of Counterion in Cationic (NHC)Pd Complexes on Catalytic Activity in the Addition Polymerization of Norbornene Derivatives

In this work, the influence of a weakly coordinating anion in cationic Pd complexes with N-heterocyclic carbene ligands (NHC ligands) on the activity of these complexes in the addition polymerization of norbornene derivatives with various substituents (alkylidene-, imide-, and ether-containing substituents) was investigated. The key to successful addition polymerization of this type of monomers is the optimal combination of NHC ligand and counterion, which determines the catalytic activity of the complex and the molecular weights of the resulting polymers. The influence of the counterion depends on the nature of the monomer and may differ for norbornenes with different substituents. For example, in the case of polymerization of monomers with nonpolar and low-polarity substituents, (NHC)Pd complexes with the counterion BARF– are more active, whereas monomers bearing highly polar/imide substituents are more efficiently polymerized using complexes with the Sb F₆⁻ counterion. This study also demonstrates the possibility of replacing the counterion in the already formed cationic Pd complex, which allows the catalyst to be tailored to the desired monomer. Ultimately, these results open a versatile platform for the design of efficient catalysts for the addition polymerization of norbornene derivatives by optimizing the nature of both the NHC ligand and the counterion, from ligand selection to counterion exchange, allowing for more targeted catalyst design.