Sulfone Bonding in High-Refractive-Index Poly(dithioacetal)s: Molecular Design toward Improved Transparency and Thermostability

Aromatic poly(dithioacetal)s (PDTAs) are functional optical polymers with high refractive indices (nD ∼ 1.7), colorlessness, and chemical recyclability, although the high flexibility of the dithioacetal unit has resulted in poor glass transition temperatures (Tg < 100 °C). In this study, we present a rational approach to improving the Tg and transparency of PDTAs by introducing polar S=O-containing side chains to leverage intermolecular dipole–dipole interactions. PDTAs with 4-(methylsulfonyl)phenyl (SO2MePh) side groups, in particular, exhibit much improved Tg (up to 133 °C) and transparency (%T ≥ 98) while retaining their high refractive indices (nD = 1.72–1.74) compared to methylthio-containing PDTAs. Density measurements revealed that introducing S=O groups into PDTA side chains induces sulfone-bonding interactions that lead to denser interchain packing in the bulk, thereby enhancing thermostability. Strikingly, combining SO2MePh side chains and a rigid, polarizable biphenyl unit in the PDTA scaffold increases Tg by up to 177 °C while maintaining a comparably high refractive index (nD = 1.74). Overall, these findings demonstrate the role of polar S=O groups in aromatic PDTAs as a key structure that balances thermal stability, transparency, and high-refractive-index properties.