Design and Synthesis of Colorless Cyclic Olefin Polymers with High Refractive Index, Transparency, and Thermal Stability

High-refractive-index polymers are critical materials for optical applications. Cyclic olefin polymers (COPs) are among the most promising optical materials, but achieving high refractive indexes remains challenging. In this study, a series of high-refractive-index COPs was synthesized via ring-opening metathesis polymerization and subsequent hydrogenation, using three carbazole-based monomers (HM1–HM3), with HM2 and HM3 featuring naphthalene-fused structures. The resulting polymers exhibited high refractive indices (up to 1.697), Abbe numbers of 14.5–25.2, superior optical transparency (>90%), excellent thermal and processing stability (glass transition temperature: 140–180 °C; Td5% > 400 °C), and low water absorption (<0.01%). Density functional theory (DFT) calculations and X-ray diffraction (XRD) analysis revealed that both molecular polarizability and chain packing contribute to the enhancement of refractive index. This study first incorporates carbazole–naphthalene-fused units into high-refractive-index polymers, offering a novel design strategy for optical materials.