The spontaneous copolymerization reaction in acetonitrile at 35C by 48 h between N-(2-hydroxyethyl)ethyleneimine with 3,4,5,6-tetrahydrophthalic anhydride under different experimental conditions was studied. The yield and molecular weight ranged between 57 - 78 % and 8.5 - 9.1·103 respectively. The copolymer composition determined from 1H NMR spectra demostrated that as decreases the concentration of HEEI units in the feed, decreases the incorporation of HEEI units in the copolymer. According to the all spectroscopic data a copolymer structure was suggested. It considers the 3,4,5,6-tetrahydrophthalic anhydride units forming part either the backbone and the side chain , and the anhydride bonds are placed only in the side chains