Even though the libraries of cyclic imino ethers, that is, 2-oxazoline and 2-oxazine monomers with different side chain substituents, have increased over the last years, there are still unsolved questions. Especially the less-studied kinetic properties, which are influenced by the nucleophilic potential of monomeric cyclic imino ethers interacting with terminal groups of growing polymer chains, are addressed. Here, the focus is on the kinetic principles governing 2-oxazolines and 2-oxazines with identical cyclopropyl side chains during homo- and copolymerization. We identified that the monomeric interaction with polymeric, terminal 2-oxazolinium species is favored over 2-oxazinium ions in both the homo- and cross-propagation. Moreover, temperature-dependent measurements in water–ethanol mixtures proved the upper critical solution temperature behavior of poly(2-cyclopropyl-2-oxazine)25 compared to the 2-oxazoline-/2-oxazine-based variants of higher polymerization degree and copolymers, which possess lower critical solution temperature characteristics. The low glass transition temperature of poly-2-oxazines is increased by the co-synthesis with 2-oxazolines. This study provides information on the nucleophilic attack during the cationic ring-opening polymerization of 2-oxazolines and 2-oxazines and highlights how the combination of 2-cyclopropyl-2-oxazoline and 2-cyclopropyl-2-oxazine can be used to manipulate the temperature-dependent properties of the polymers.
Schumann et al. (Wed,) studied this question.