An FT-i.r. method was used to investigate the thermal cyclization process in synthesis of polyesterimidazopyrrolones from diesterdianhydrides and 3,3＇-diaminobenzidine. It was proved that the structure of diphenols in the diesterdianhydrides influences the cyclization which goes via an amino-imide to the imidazopyrrolone structure. The presence of an electron-donating group in the diphenol causes the beginning of cyclization at a lower temperature than in the case when in the diesterdianhydride there is a diphenol with electron-withdrawing group. The cyclization process was investigated by dynamic and isothermal heating.