Highly refractive and transparent polyimides (PIs) based on fluorene-bridged and sulfur-containing monomers have been developed. An aromatic dianhydride, 4,4'-[p-thiobis(phenylenesulfanyl)]diphthalic anhydride (3SDEA), was polymerized with several fluorene-containing diamines, including commercially available 9,9'-bis(p-aminophenyl)fluorene (APF), 9,9'-bis[4-(p-aminophenoxy)phenyl]fluorene (OAPF), and newly synthesized 9,9'-bis[4-(p-aminophenyl)sulfanylphenyl]fluorene (ASPF) to afford series A PIs. Meanwhile, series B PIs were obtained from a new dianhydride, 4,4'-[(9H-fluorene-9-ylidene)bis(p-phenylsulfanyl)]diphthalic anhydride (FPSP) and two aromatic diamines, ASPF and 4,4'-thiobis[(p-phenylenesulfanyl)aniline] (3SDA) via a two-step polycondensation procedure. The PIs exhibit good thermal stabilities, such as relatively high glass transition temperatures in the range of 220-270 degrees C and high initial thermal decomposition temperatures (T-10%) exceeding 490 degrees C. The 9,9'-disubstituted fluorene moieties endow the PI films with good optical transparency. The optical transmittances of the PI films at 450 nm are all higher than 80% for the thickness of about 10 mu m. Furthermore, the highly aromatic fluorene moiety and flexible thioether linkages in the molecular chains of the PIs provide them with high refractive indices of 1.6951-1.7258 and small birefringence of 0.0056-0.0070. (c) 2008 Wiley Periodicals, Inc.