A series of organic-soluble polynaphthalimides (PNI) bearing flexible ether links and phthalide cardo group were synthesized from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) with 3,3-bis[4-(4-aminophenoxy)-3-methylphenyl]phthalide (BAMP) and its equimolar mixtures with other dietheramines by high-temperature solution polycondensation in m-cresol. The PNIs had moderate to high inherent viscosities in the range of 1.24-2.25 dL/g and could afford flexible and tough films with tensile strengths of 97-138 MPa by casting their m-cresol solutions. These PNIs exhibited high thermal stability, with glass transition temperature of 291 degrees C-321 degrees C, 10% weight-loss temperatures above 542 degrees C, and char yields at 800 degrees C in nitrogen higher than 56%. In comparison with PNIs without the BAMP component, these BAMP-modified PNIs revealed an enhanced solubility and film-forming capability. (c) 2007 Wiley Periodicals, Inc.