A new series of alkoxy-substituted poly(p-phenylene 1,3,4-oxadiazole)s modified by the insertion of small percentages of various comonomers were synthesized through the precursor polyhydrazides. The comonomers used contained trans double bonds or meta-alkoxy-substituted aromatic rings to improve the solubility of the final polymers. The synthesized copolymers were chemically characterized by H-1 NMR and Fourier transform infrared spectroscopy. In some cases, the copolymers really showed improved solubility in organic solvents. The N-15 solid-state NMR technique was applied to examine the degree of conversion from the precursor polyhydrazides to the final polymers, which determined the effective conjugated length in the target polyoxadiazoles. Thermal stability and structural characteristics of all the polymers as well as a preliminary investigation on the optical properties of polyoxadiazoles are also reported. The copolymers retained high absorbance in the UV region and high transmission in the whole telecommunication range. (C) 2003 Wiley Periodicals, Inc.