An aromatic sulfur-containing diamine 4,4'-thiobis[(p-phenylenesulfanyl) aniline] (3SDA) was synthesized and polymerized with a sulfur-containing dianhydride 4,4'-[p-thiobis(phenylenesulfanyl)ldiphthalic anhydride (3SDEA) and three nonsulfur aromatic tetracarboxylic dianhydrides, respectively to afford four poly(amic acid)s (PAAs) with the inherent viscosities of 0.54-1.04 dL/g. Flexible and tough polyimide (PI) films obtained from the PAA precursors showed good thermal, mechanical, and optical properties. The glass transition temperatures (T(g)s) of the PIS ranged from 179.1-227.2 degrees C determined by differential scanning calorimetry (DSC), and 173.8-227.3 degrees C by dynamic mechanical analysis (DMA), depending on the dianhydride used. The 10% weight loss temperatures were in the range of 500-536 degrees C, showing high intrinsic thermal-resistant characteristics of the PI films. The PI films also showed good optical transparency above 500 nm, which agreed well with the calculated absorption spectra using the time-dependent density functional theory. The average refractive indices (n(av)) measured at 632.8 nm. were 1.7191-1.7482, and the in-plane/out-of-plane birefringences (Delta n) were 0.0068-0.0123. The high refractive indices originate from the high sulfur contents, good molecular packing, and the absence of bulky structures. The relatively small birefringence mainly results from the flexible thioether linkages structures of the diamine. (c) 2007 Wiley Periodicals, Inc.