New highly solution-processable aniline/butylthioaniline copolymers were prepared via oxidative copolymerization (OCP) and by concurrent reduction and substitution (CRS). Butylthio-substituted polyaniline obtained via the CRS route (Pan-SBu), being in line with the expected property changes after the addition of an electrondonating substituent to an aromatic ring, displayed a lowered redox potential (E-0) and a redshifted maximum wavelength (lambda(max); ultraviolet-visible) in comparison with its parent unsubstituted polyaniline (Pan). However, copolymers CP1-CP4 (obtained via the OCP method) displayed opposite behaviors, showing higher E-0 values and blue-shifts in lambda(max), than the unsubstituted Pan. The results suggested that CP1-CP4 had shorter conjugation lengths than the unsubstituted Pan, possibly because of their chain conjugation defects (e.g., 1,3-ring linkage structures), as evidenced by IR studies. The results of H-1 NMR studies also indicated that Pan-SBu had much higher structural homogeneity than copolymer CP4. Because the CRS synthetic route involved no backbone alternations, the resultant copolymer (Pan-SBu) should have maintained the same backbone structure and hence the high conductivity of the parent unsubstituted Pan. (c) 2005 Wiley Periodicals, Inc.