A new kind of aromatic unsymmetrical diamine monomer containing thiazole ring, 2-amino-5-[4-(4'-aminophenoxy)phenyl]-thiazole(APPT), was synthesized. A series of novel polyimides were prepared by polycondensation of APPT with various aromatic dianhydrides via one-step process. The resulting polyimides held inherent viscosities of 0.40-0.71 dL/g and were easily dissolved in strong dipolar solvents. Meanwhile, strong and flexible polyimide films were obtained, which had thermal stability with the glass transition temperatures (T-g) of 268.2-328.8 degrees C in nitrogen, the temperature at 5% weight loss of 452-507 degrees C in nitrogen and 422-458 degrees C in air, and the residue at 800 degrees C of 54.18-63.33% in nitrogen, as well as exhibited outstanding mechanical properties with the tensile strengths of 105.4-125.3 MPa, elongations at breakage of 6-13%. These films also held dielectric constants of 3.01-3.18 (10 MHz) and showed predominantly amorphous revealed by wide-angle X-ray diffraction measurements. (c) 2007 Elsevier Ltd. All rights reserved.