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

## TL;DR

A new method creates counterion-free ionic polymers with enhanced properties by in situ ionization and coupling of neutral oligomers.

## Contribution

A novel base-free salt metathesis route for synthesizing counterion-free ionic associating polymers with high chain-end fidelity.

## Key findings

- Reactive melt blending of A2 and B2 leads to a 2-fold increase in polymer melt viscosity.
- The Dη value deviates from Rouse model predictions, indicating a transient dynamic network.
- The method enables systematic tuning of ionic interactions for dynamic polymeric materials.

## Abstract

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.