Acid‐Promoted ROMP of a Pyridine‐Fused Norbornene Toward Main‐Chain Cationic Polymers

Cationic polymers have emerged as a versatile class of materials with multiple applications spanning from biomedicine to catalysis and membranes for ion transport. To circumvent solubility challenges and enable straightforward characterization, quaternization of pnictogen‐containing neutral polymer offers an attractive strategy toward well‐defined cationic macromolecules. However, the Lewis basicity of such cationic progenitor is often detrimental to controlled polymerizations. Herein, we report the ring‐opening metathesis polymerization (ROMP) of pyrido[2,3‐ b ]norbornadiene—a pyridine‐fused norbornene monomer—under acidic conditions as a general platform for the synthesis of cationic polymers. The addition of HCl effectively prevented deleterious coordination of the pyridine motif to the Ru catalyst through protonation, yielding well‐defined polymers with predictable molar masses, narrow dispersities, and good chain‐end fidelity. Kinetic studies revealed that the presence of HCl significantly accelerated the polymerization relative to a conventional ROMP of an analogous monomer under neutral conditions. This approach provided a streamlined route to structurally tunable main‐chain pyridinium‐containing macromolecules via quaternarization of the repeating pyridine nitrogen using diverse alkylating agents.