All‐Hydrocarbon High‐Refractive‐Index Cyclic Olefin Polymer via ROMP

Comprehensive Summary In the rapidly evolving field of advanced materials science, the development of optical polymers with high refractive index has attracted much attention. Among these optical polymers, cyclic olefin polymers (COPs) have stood out as a versatile platform owing to exceptional transparency, low birefringence, good heat resistance, low moisture absorption, and superior chemical resistance. However, they suffer from a relatively low refractive index, ranging from n = 1.52 to 1.54, which poses a challenge. In this contribution, we demonstrate that the incorporation of aromatic groups into the polymer backbone has opened up new horizons, particularly in the pursuit of materials with high refractive index. COPs with high refractive index and high glass‐transition temperature ( T g ) are prepared via ring‐opening metathesis polymerization (ROMP) and subsequent hydrogenation. Notably, these all‐hydrocarbon COPs are amorphous, have a wide T g range of 30–282 °C, exhibit high thermal stability ( T d,5% = 414–471 °C), and possess significantly high refractive index (1.5815–1.6748 at 589 nm) and excellent optical transmittance (89.0%–95.3%), making them promising candidates for advanced optical applications.