AbstractAromatic heterocyclic polymers are exceedingly attractive high performance materials because of their unusually high mechanical strength, chemical resistance and thermal stability, but are very difficult to process unless “swivel” atoms or groups are inserted along the chains to increase conformational flexibility. The present theoretical investigation employs the CNDO/2 method with direct geometry optimization to calculate such flexibility for the wholly aromatic swivels biphenyl, 2,2′‐bipyridyl, 2‐phenylpyridine, 2,2′‐bipyrimidyl, and 2‐phenylpyrimidine. The most important result is the prediction that both flexibility and accessibility of coplanar conformations should increase significantly with the number ofortho‐CH groups replaced by N‐atoms. The calculations also provide information on other conformation‐dependent properties such as optimized geometries, charge distributions, and dipole moments, and these results are found to be in satisfactory agreement with the results of previous theoretical and experimental studies. Finally, since these chains are soluble only in strongly acidic media, the species mono‐ and diprotonated 2,2′‐bipyridyl, 2,2′‐bipyridyl · H3O+, and 2,2′‐bipyrimidyl · 2H2O were also investigated with regard to some of