Four new polypyromellitimides (MCm‐PPIs and MCm‐OPIs, m = 6, 8) that are singly substituted with a flexible n‐alkyloxy side branch at the pyromellitimide ring were prepared by the two‐step polycondensation of 3‐[4‐(n‐alkyloxy)phenyloxy]pyromellitic dianhydrides (MCm‐PMDAs, m = 6, 8) with p‐phenylenediamine (PDA) and 4,4′‐oxydianiline (ODA), respectively. The dianhydride monomers were synthesized from durene via several reaction steps. Inherent viscosities of the precursor poly(amic acid)s ranged from 0.57 to 1.58 dl/g. After chemical structures of the polyimides had been characterized, their thermal properties, crystalline structures, and liquid crystal (LC) aligning abilities on their rubbed thin films were determined and discussed in comparison to the polypyromellitimides that are doubly substituted at the pyromellitimide ring with the same side branches. For all polymers thermogravimetric analysis (TGA) programs showed a typical two‐step degradation behavior with onset temperatures in the 430–455°C range. In X‐ray scattering studies all the samples were found to be amorphous, but the presence of a loosely developed layer structure could be confirmed, in which two main chains gather together to form a double‐strand backbone layer and n‐alkyl branches fill the space between the layers. On the rubbed surfaces of the polyimide thin films LCs uniformly aligned parallel to the rubbing direction with the pre‐tilt angles 5–7° in MCm‐OPIs and 18–32° in MCm‐PPIs. Copyright © 2006 John Wiley & Sons, Ltd.