AbstractA series of rigid rod polyimides 1a–f derived from pyromellitic anhydride (PMDA) and 2,5‐di‐n‐alkoxy‐1,4‐phenylenediamine containing flexible alkoxy side chains with 4 to 16 carbon atoms was prepared by solution polycondensation. As demonstrated by model experiments full conversion to the imide structure is achieved by reaction of PMDA with 2,5‐dialkoxyphenylene diisocyanate, the latter compound being blocked by imidazole. The best solvent for the polycondensation, found in the course of the model reaction, is 1,3‐dimethyl‐2‐oxo‐perhydro‐pyrimidine together with 4‐dimethylaminopyridine as an accelerator. Wide‐angle X‐ray diffractograms showed that all polyimides under consideration here are highly crystalline at room temperature. The occurrence of a sharp reflection at the region of low angles together with its higher orders demonstrates the presence of a layered structure. The layer spacing was found to increase linearly with the length of the alkyl side chains. Polyimides bearing long side chains (n ≥ 10) exhibit two broad endotherms in the DSC analysis. As is revealed by wide‐angle X‐ray analysis the first transition is related to a disordering process of the side chains. The second transition leads to a layered high‐temperature phase akin to the modification (B) found in a recent study of a similar rigid polyester bearing flexible side chains.