Cardo structures are applied extensively for the preparation of high refractive index polymers (HRIPs) with intrinsically low optical anisotropy. While such design allows for the achievement of low birefringence, the refractive indices of cardo-based polymers have been largely limited to below 1.70. We report herein a modular cardo design based on 9,9-diarylfluorene bearing sulfur-containing aromatic substituents at the C-9 position and polymerizable handles at the 2,7-positions. Applying various polymerization methods, including thia-Michael polyaddition, SNAr polycondensation, and Cu-catalyzed C–S coupling, allowed for the modular translation of the cardo design into polyesters, poly(arylene ether)s, and poly(arylene thioether)s. The structural design allowed for property tuning through variations in the side chain and backbone structures, affording a library of 12 HRIPs with refractive indices of up to 1.752 (λ = 632.8 nm), birefringence (Δn) on the order of 10–3 to 10–4, and glass transition temperatures (Tg) spanning from 45 to 260 °C. The structure–property relationships established from these HRIPs constitute generalizable design rules for cardo-based HRIPs beyond conventional 9,9-linking strategies.
Yoo et al. (Sun,) studied this question.