Upper critical solution temperature (UCST) behavior of pyroglutamate-functionalized non-ionic homopolymers: cosolvency in water/alcohol binary solutions

Abstract

The properties of biomacromolecules lead to the development of synthetic mimics to emulate their functions. In an earlier report, we presented L-pyroglutamic acid (PGA)-functionalized methacrylate-based polymers demonstrating a typical upper critical solution temperature (UCST)-type phase transition behavior and an intriguing liquid–liquid phase separation (LLPS) phenomenon in pure water, provided the degree of polymerization was restricted to 40 (P. Chowdhury, B. Saha, K. Bauri, B. S. Sumerlin and P. De, J. Am. Chem. Soc., 2024, 146, 21664–21676). In this contribution, PGA-based well-defined polymers with two different backbone structures (methacrylate and styrene) and varied chain lengths are synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. Despite differences in backbone hydrophobicity, both polymers featured cosolvency phenomenon, as evident from the phase diagrams of ternary polymers/water/alcohol systems, and displayed sharp and reversible UCST-type transitions in water/alcohol solutions with negligible hysteresis. In addition, their phase transition temperature could be modulated in a wide temperature range from 10 to 80 °C by varying the molar mass, polymer concentration, the composition of the binary water/alcohol mixture, the identity of the alcohol, and also the nature and concentration of the foreign substances. Interestingly, the higher molar mass methacrylate polymers formed coacervate droplets that underwent macroscopic LLPS in a binary water/ethanol (80/20, vol/vol) mixture. Overall, this study advances the fundamental understanding of PGA-tethered uncharged, thermoresponsive homopolymers.