A series of partial ladder copolymers were synthesized by chemically oxidative polymerization of in-phenylenediamine (MPD) and o-ethoxyaniline (EOA) using inorganic oxidants in inorganic acidic aqueous media. The polymerization yield, intrinsic viscosity, solubility, and thermal property of the copolymers were systematically studied by changing the comonomer ratio, initial polymerization temperature, polymerization time, oxidant, monomer/oxidant ratio, and acidic medium. As-prepared fine powder of the MPD/EOA copolymers was characterized by IR, UV-vis, and high-resolution H-1 NMR spectroscopies and DSC. Circular dichroism technique was firstly used to characterize chain structure of the copolymers. The results showed that the oxidative polymerization from MPD and EOA is exothermic and the resulting copolymers exhibit a remarkably enhanced solubility in all of the organic and inorganic solvents chosen as compared with totally insoluble MPD homopolymer. The polymers obtained by the oxidative polymerization are real copolymers containing MPD and EOA units but do not contain MPD and EOA homopolymers based on a careful solubility comparison. The actual MPD/EOA molar ratio calculated based on H-1 NMR spectra of the polymers is different from element analysis results. Element analysis indicated that denitrogenation happens during the polymerization linkage among MPD units and the structure consisting of MPD units is different from that reported. The ladder degree of the copolymers might be monitored by controlling MPD/EOA ratio. The DSC measurement indicates that the copolymers do not exhibit melt transition. (C) 2003 Elsevier Ltd. All rights reserved.