Thermosensitive polymers play an important role in controlled drug delivery and other biomedical and biotechnology applications. Thermosensitive polymers and copolymers that contain free reactive groups provide further possibilities in the field of targeted therapy. A high concentration of free amino groups in copolymers allows for the immobilization of various biosubstances, e.g., drugs, proteins or polysaccharides. Copolymers containing free amino groups were prepared via the cationic copolymerization of 2-ethyl-2-oxazoline with 2-(4-aminophenyl)-2-oxazoline. The mechanism of copolymerization was studied via a model polymerization of 2-ethyl-2-oxazoline in the presence of ethyl 4-aminobenzoate (EAB) as the source of the aromatic amino group. These model polymerizations showed that EAB incorporated into the polymer via a termination step. The studies also showed that the molar mass of the formed polymers depended on the ratio of initiator to EAB. The composition of the prepared copolymers was determined by NMR and absorbance spectroscopy. The prepared copolymers exhibited decreasing phase transition temperatures with increasing concentrations of aromatic units, which were confirmed by optical transmittance measurements. The influence of the concentration of copolymers, the presence of surfactants and the presence of an inorganic salt on the thermosensitive properties of the prepared copolymers was also demonstrated.