On page 944 (DOI: 10.1002/pola.27547), Hyeonuk Yeo, Jiye Lee, Munju Goh, Bon-Cheol Ku, Honglae Sohn, and Nam-Ho You describe the synthesis and characterization of the polyimide (PI) series containing a 2,5-bis(4-aminophenylenesulfanyl)-3,4-ethylenedithiothiophene (APSEDTT) moiety in the main-chain. The APSEDTT monomer with high sulfur content was prepared and polymerized with several aromatic dianhydrides. All PIs exhibit high transparency (>75% at 550 nm for a thickness of ∼20 µm) and have good thermal properties [e.g., thermal decomposition temperatures (T10%) of 409–521 °C]. Additionally, the PIs have extraordinarily excellent optical properties in refractive index and birefringence. In particular, the PI derived from APSEDTT and 3SDEA shows a high refractive index (1.7586), and a low birefringence (0.0087) because of their very high sulfur content (27.7%).