Two series of polynaphthalimides (PNIs), II and III, were prepared from 1,4,5,8-naphthalene tetracarboxylic dianhydride with the usual aromatic diamines (Ia-Ih) or Ia, which was used as a soluble modifying agent, and Ib-Ih via it one-stage process with high-temperature condensation. Series II and III had inherent viscosities of 0.57-1.34 and 1.15-1.63 dL/g, respectively. IIa and most or the III series were soluble in 1-methy-2-pyrrolidone and m-cresol at 5-10 wt % and afforded transparent and tough films by m-cresol solvent casting with a tensile strength in the range of 75-99 MPa. The glass-transition temperature and softening temperature or these PNIs were in the ranges of 360-404 and 188-241 C, respectively, and the 101 weight loss temperatures were above 495 C, with more than it 441 char yield at 800 C in nitrogen. These PNI films had dielectric constants of 4.14-6.46 (1 MHz), with moisture absorption in the range of 0.87-1.91 wt %. (c) 2005 Wiley Periodicals, Inc.