Synergistic Iodination-Hydrogen Bonding Design Enables Thermostable and Ultrahigh Refractive Index Poly(dithioacetal)s.

High refractive index polymers (HRIPs) serve as cutting-edge materials for lightweight, miniaturized, and high-resolution optical devices, although balancing ultrahigh refractive index, transparency, and thermostability remains challenging. Here, poly(dithioacetal)s with aromatic main chains, heavy and polarizable iodine groups, and additionally incorporated hydroxy units exhibit a high glass transition temperature (Tg = 112 °C) and an ultrahigh refractive index (nD = 1.81), while maintaining visible-light transparency (%T ≥ 96 at 1 μm thickness). Density measurements confirmed that the synergistic incorporation of iodine atoms and hydrogen bonds provides both a high packing coefficient and high polarizability, resulting in the formation of dense PDTA networks that simultaneously enhance nD and Tg. These findings rationalize a simple yet effective design for HRIPs to balance high thermostability and ultrahigh refractive indices without compromising transparency, through maximizing the polarizability density of repeating units.