Effects of N-Substituents on the Solution Behavior of Poly(sulfobetaine methacrylate)s in Water: Upper and Lower Critical Solution Temperature Transitions

A series of zwitterionic poly(sulfobetaine methacrylate)s (PSBMAs) with various N-substituents, including n-alkyl, cyclohexyl, 2-hydroxyethyl, and phenyl, was synthesized in order to elucidate the effects of N-substituents on the solution behavior of the PSBMAs in water. When the lengths of two symmetric N-n-alkyl substituents were increased simultaneously, the PSBMAs exhibited an array of behavior in water, from upper critical solution temperature transitions (UCST) to soluble, lower critical solution temperature transitions (LCST), and insoluble, which presumably stemmed from the competition between the decrease in electrostatic zwitterionic interactions and the increase in the hydrophobic effect with increasing alkyl length. The replacement of N-n-hexyl with cyclohexyl while keeping the N-methyl substituent diminished the hydrophobic effect and made the LCST vanish. The introduction of a 2-hydroxyethyl or a phenyl N-substituent enhanced the attractive interactions between zwitterionic groups, resulting in UCST behavior. The insight gained from this work can facilitate the design of UCST- and LCST-type thermoresponsive zwitterionic PSBMAs by varying N-substituents and will open up more opportunities for potential applications of already widely used zwitterionic materials (e.g., thermoresponsive zwitterionic polymer brushes and nanogels for use in bioseparation and drug delivery).