Thionation\nof Poly(2-ethyl-2-oxazoline) and Its Effect\non Solubility and Cloud Point

Many water-soluble polymers undergo phase separation\nabove their\nlower critical solution temperature (LCST) in aqueous solutions. Generally,\nthe LCST is controlled via the polymer–solvent interactions,\nwhich, in turn, are highly structure dependent and typically tuned\nvia the polymer side chain. For example, the LCST for the poly(2-oxazoline)\nfamily can be tuned via the length and branching of the polymer side\nchain. In particular, the LCST of poly(2-ethyl-2-oxazoline) varies\nbetween 60 and 100 °C, depending on its molar mass, polymer architecture,\nand concentration. Here, we introduce LCST adjustment via modification\nof the carbonyl of the amide moiety of poly(2-oxazoline)s. We introduce\npoly(2-ethyl-2-oxazoline-<i>co</i>-<i>N</i>-propiothioacetyl\nethylene imine), synthesized from poly(2-ethyl-2-oxazoline) using\nLawesson’s reagent. The degree of thionation of the polymers\nwas varied and characterized by using spectroscopic methods and elemental\nanalysis. While the glass transition temperatures of the copolymers\nincrease with increasing thionation degrees, the decomposition temperatures\ndecrease. Interestingly, polymers with less than 20% thionation degree\nwere soluble in water with LCST behavior. The cloud point temperatures\ndecrease with increasing thionation degree due to the weaker hydrogen\nbonding ability of thioamides and decreased polarity.