Intrinsic Sulfur-Containing Thermoplastic Cyclic Olefin Polymers with High Refractive Index, Colorless Transparency, and Thermal Stability

High-refractive-index polymers are essential materials for advanced optical applications. Cyclic olefin polymers (COPs) are particularly attractive optical materials; however, conventional COPs typically exhibit refractive indices below 1.55, and achieving higher values without sacrificing transparency remains challenging. Here we report a series of intrinsic sulfur-containing COPs prepared by ring-opening metathesis polymerization followed by hydrogenation using a sulfur-containing heteroaromatic monomer (HSM) and rigid cycloolefin comonomers. Without external sulfur-rich additives, the resulting polymers exhibit high refractive indices (n589 = 1.612–1.673), relatively high Abbe numbers (νd = 20.4–30.6), and colorless optical transparency (visible-light transmittance of >90%). In addition, the polymers show good thermal stability, with Tg values ranging from 132 to 175 °C and Td5% values above 300 °C. This work presents an effective molecular design strategy for colorless, high-refractive-index, and thermally robust thermoplastic COPs.