A new bioactive diacid monomer, 5-(2-phthalimidoethanesulfonamido) isophthalic acid (6), was synthesized in three steps. This monomer can be regarded as biologically active aromatic diacid and may be used in the design of biodegradable and biological materials. This monomer was polymerized with several aromatic diamines by step-growth polymerization to give a series of biodegradable and highly thermally stable polyamides (PAs) with good yield (70-82%) and moderate inherent viscosity between 0.38-0.68 dL/g in a system of triphenylphosphite/pyridine/N-methyl-2-pyrolidone/CaCl2. The new aromatic diacid 6 and all of the PAs derived from this diacid and aromatic diamines were characterized by Fourier transform infrared, H-1-NMR, C-13-NMR, and elemental analysis techniques. The thermal stability of the PAs was determined by thermogravimetric analysis and differential scanning calorimetry techniques under a nitrogen atmosphere, and we found that they were moderately stable. The soil biodegradability behavior of 6 and all of the PAs derived from this diacid and aromatic diamines were investigated in culture media, and we found that the synthesized diacid 6 and all of the PAs were biodegradable under a natural environmental. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 2818-2827, 2011