In this study, we show for the first time the post-polymerization modification of PEtOx with LR to obtain poly(2-ethyl-2-oxazoline-co-N-propiothioacetyl ethylene imine) copolymers comprising thioamide repeating units. Increased thionation of polymers results in increased chain rigidity, and therefore Tg values increased, while a decreased thermal stability was observed. Thionation was found to be an alternative way to tune the hydrophilic/lipophilic balance of these water-soluble polymers in general and the Tcp of PEtOx solutions with Flory Huggins type I behavior in particular.