Temperature-Responsive Zwitterionic Polymers That Undergo UCST-Type Liquid–Liquid Phase Separation under Physiological Ionic Strength

Associative phase separation (complex coacervation) in liquid-liquid phase separation (LLPS) involves the separation of multiple substances into concentrated and dilute phases by electrostatic interactions. Simple phase separation (simple coacervation) occurs when the hydrophilicity and hydrophobicity of a single molecule change dramatically in response to a specific stimulus. Simple coacervation arises from the lower critical solution temperature (LCST)- and upper critical solution temperature (UCST)-type phase separations in aqueous media containing temperature-responsive polymers. LCST- and UCST-type LLPSs induce droplet formation at temperatures above the LCST and below the UCST, respectively. Although there have been several studies on the UCST-type LLPS of temperature-responsive polymers in water, only a few temperature-responsive polymers that exhibit UCST-type LLPS in aqueous media with physiological ionic strength have been reported. In this study, we synthesized temperature-responsive zwitterionic polymers, exhibiting UCST-type LLPS in physiological ionic strength, by copolymerizing two types of zwitterionic monomers─sulfabetaine (SaB) having ammonium and sulfate groups and sulfobetaine (SB) having ammonium and sulfonate groups─in aqueous media with various ionic strengths. The resulting zwitterionic copolymers, P(SaB-co-SB)s, exhibited a cloud point (CP) characterized by a transition from turbidity to transparency as the temperature increased in a buffer with physiological ionic strength. The CP of P(SaB-co-SB) shifted from lower to higher temperatures as the SaB content increased. Microscopic observation showed that P(SaB-co-SB) underwent UCST-type LLPS to form coacervate droplets even in a buffer solution with physiological ionic strength at temperatures lower than the CP; however, the coacervates dissolved above the CP, unlike general UCST-type temperature-responsive polymers. The CPs of the P(SaB-co-SB)s under physiological ionic strength varied with the SaB content and ionic strength of the copolymerization medium. UCST-type LLPSs were induced by strong dipole-dipole interactions between SaB units at physiological ionic strength.