Two series of rigid and semi-rigid polyesters and polyamids with flexible side chains were prepared by solution and melt as well as interfacial polycondensation of 2,5-dialkoxyterephthaloyl chloride with various diols and diamines. Thermal behaviors of the polymers were studied by DSC, TGA and polarizing microscopy. The results indicated that only rigid polymer systems form nematic mesophase. However, none of the semi-rigid polymers formed nematic melts. The structures of both polyesters and polyamides were also examined by the wide angle X-ray scattering. Analysis of sharp reflection, appeared at the lower angle of X-ray diffractograms of rigid polymers, suggested that rigid polymers crystallized to form a layered structure in the solid state.