A rigid rod polymer with strong hydrogen bonds between the polymer chains was found in polypyridobisimidazole based on 2,3,5,6-tetraaminopyridine and 2,5-dihydroxyterephthalic acid (D. J. Sikkema and V. Lishinsky, WO 94/25506). The strong interchain forces translate into the highest compressive strength found in polymer fibres by far, and the polarity of the polymer seems to contribute to easy adhesion in advanced composite manufacture. The modulus and tenacity are as expected for a rigid rod polymer in the general class of the earlier PBZT and PBO fibres.