Thionation of Poly(2-ethyl-2-oxazoline) and Its Effect on Solubility and Cloud Point

Many water-soluble polymers undergo phase separation above their lower critical solution temperature (LCST) in aqueous solutions. Generally, the LCST is controlled via the polymer–solvent interactions, which, in turn, are highly structure dependent and typically tuned via the polymer side chain. For example, the LCST for the poly(2-oxazoline) family can be tuned via the length and branching of the polymer side chain. In particular, the LCST of poly(2-ethyl-2-oxazoline) varies between 60 and 100 °C, depending on its molar mass, polymer architecture, and concentration. Here, we introduce LCST adjustment via modification of the carbonyl of the amide moiety of poly(2-oxazoline)s. We introduce poly(2-ethyl-2-oxazoline-co-N-propiothioacetyl ethylene imine), synthesized from poly(2-ethyl-2-oxazoline) using Lawesson’s reagent. The degree of thionation of the polymers was varied and characterized by using spectroscopic methods and elemental analysis. While the glass transition temperatures of the copolymers increase with increasing thionation degrees, the decomposition temperatures decrease. Interestingly, polymers with less than 20% thionation degree were soluble in water with LCST behavior. The cloud point temperatures decrease with increasing thionation degree due to the weaker hydrogen bonding ability of thioamides and decreased polarity.