Polymer Dipole Engineering Enables Ultra-Sensitive and Ion-Specific Tuning of Lower Critical Solution Temperature

This study demonstrates a "polymer dipole engineering" strategy to achieve ultrasensitive and ion-specific modulation of the lower critical solution temperature (LCST) in thermoresponsive polymers. Guided by this strategy, we synthesized poly2-(ethylsulfonyl)ethyl acrylamide (PESEAm) featuring a highly dipolar sulfone group in its repeat unit. PESEAm exhibits an LCST-type phase transition, driven by the disruption of sulfone-water hydrogen bonds upon heating. Remarkably, the polymer demonstrates an ultrasensitive and linear response to both NaSCN and Na2SO4, enabling precise tuning of its cloud point across a broad temperature range from 10 to 90 °C. The slopes of the cloud point-concentration plots reach 647 °C M-1 for NaSCN and -562 °C M-1 for Na2SO4, representing the highest sensitivity ever reported for nonionic polymers. This exceptional sensitivity originates from the highly dipolar sulfone group, which engages in strong ion-dipole interactions with SCN- to sharply elevate the cloud point. In the case of Na2SO4, polarization of the hydrogen-bonded water works in synergy with enhanced interchain dipole-dipole interactions to lower the LCST. The dipole engineering strategy demonstrated here provides a rational design route to thermoresponsive polymers with ultrasensitive and broadly tunable phase transitions, offering new insight into the development of ion-responsive and thermoresponsive polymers.