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.