From imidazole-blocked 2,5-bis[(n-alkyloxy)methyl]-1,4-benzene diisocyanates and pyromellitic dianhydride a series of new rigid-rod polyimides (C-n-PY-PI; n = 4, 6, 8) having linear and flexible (alkyloxy)methyl (-CH2OCnH2n+1; n = 4, 6, 8) side chains were prepared and characterized and their properties were measured and discussed with regard to effects of side chains. Incorporation of the side chains onto the rigid main chain greatly enhanced the solubility and fusibility of the polymers, and melting point of C-8-PY-PI was determined to be 277 degrees C. The UV-VIS absorption behavior was independent of side-chain length. TGA thermograms revealed a two-step pyrolysis behavior, in which the side chains split off separately at lower temperatures. X-ray diffractograms showed that all the polyimides are crystalline at room temperature. Sharp reflections in small-angle region obviously indicated the presence of a layered crystal structure.