Counterion-Free Ionic Associating Polymers: In Situ Ionization and Coupling of Alkyl Sulfonate Precursors

Mixing oppositely charged cationic and anionic polymer salts (poly⁺X^- and poly^-Y⁺) typically yields ionic associating polymers (IAPs) coexisting with counterions (X^-/Y⁺). These counterions screen interchain Coulombic interactions and weaken polymer association. Herein, we present an innovative and straightforward strategy to synthesize counterion-free IAPs based on two charge-neutral telechelic oligomers A2 and B2, bearing imidazole and ethyl sulfonate end groups, respectively. Notably, we have developed a novel base-free salt metathesis route to synthesize B2 with nearly quantitative chain-end fidelity (>97%). It successfully overcame issues of unstable intermediates and basic conditions encountered in the conventional route. Reactive melt blending of A2 and B2 results in in situ ionization and chain coupling, producing a polymer melt characterized by a 2-fold increase in viscosity due to aprotic and reversible ionic associations. The viscosity and self-diffusion of the IAP were quantified by rheology and pulsed-field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy, respectively. Notably, the product of diffusion coefficient and viscosity (Dη) positively deviates from the Rouse model prediction, consistent with the formation of a transient dynamic network in which chain mobility is partially decoupled from macroscopic viscosity. We anticipate that this modular synthesis approach can be readily extended to other synthetic polymer systems, where the strength of ionic interactions can be systematically tuned. Such control would guide the design of dynamic polymeric materials that assemble and disassemble on demand, offering enhanced recyclability and sustainability.