Copolymers of N-(2-hydroxyethyl) ethyleneimine and succinic anhydride without initiator were prepared under different experimental conditions. The composition of the copolymers was determined by elemental analyses and H-1-NMR spectroscopy. From the results of FT-IR and H-1-NMR spectroscopy a scheme of copolymerization via zwitterion is presented which includes an esterification reaction of the pendant hydroxylic groups.