Crystal-packing, optical, and electrical properties of poly(2,5-dihydroxy-1,4-phenylene benzobisthiazole) (DiOH-PBZT) and copolymers of DiOH-PBZT/poly(1,4-phenylene-benzobisthiazole) (PBZT) were examined. Intramolecular hydrogen bonds between the hydroxyl units and the neighboring nitrogen atoms, as evidenced by the IR spectra, led to the formation of a pseudoladder chain structure and changed the chain packing. The (200) and (010) planes were both affected by the copolymer composition, with the (200) plane spacing increasing from 5.895 to 6.482 Angstrom and the (010) plane spacing decreasing from 3.539 to 3.404 Angstrom with the transition from the unsubstituted PBZT homopolymer to the DiOH-PBZT homopolymer. The cell dimensions of the copolymers were simple averages of those of the individual homopolymers, suggesting the isomorphic crystal structure formation of the two units. The c-axis spacing, how ever, remained unchanged. The increase in the conjugation length of the copolymers as the dihydroxy content increased was confirmed by the bathochromic shift of the absorption band in the ultraviolet-visible spectra. The intrinsic conductivities of the copolymers were 3 orders of magnitude higher than that of the unsubstituted PBZT. (C) 2001 John Wiley & Sons, Inc.