Thermally Responsive 2-Hydroxyethyl Methacrylate Polymers: Soluble–Insoluble and Soluble–Insoluble–Soluble Transitions

2-Hydroxyethyl methacrylate (HEMA)-based (co)polymers showed soluble to insoluble (S–I) thermoresponses, as measured by turbidimetry, when heated in aqueous solutions of appropriate ionic strength and/or pH. Surprisingly, many of the polymers showed a second insoluble to soluble (I–S) thermoresponse at higher temperatures, probably as a result of breaking of polymer–polymer H-bonding that allowed redissolution of the polymer chains. The thermoresponse of the charged copolymers was very sensitive to the polymer concentration, pH, and ionic strength likely due to the role of charge screening in the chain collapse and aggregation necessary to observe cloud points. Urea, a commonly used “H-bond breaker”, raised the cloud point (and decreased the clearing point for systems that showed I–S transitions); however, subsequent cooling or heating runs in the presence of urea often showed dramatically different thermoresponses as a result of urea hydrolysis, leading to a pH change and/or polymer hydrolysis at high temperature. Marked hysteresis or changes from run to run were seen for some systems because of polymer precipitation or slow rehydration of phase-separated material or, in some cases, slow hydrolysis of HEMA units.