Low birefringence and low dispersion aliphatic thermosets with a high and tunable refractive index

We herein present a new class of high refractive index polymers prepared through the reaction of bis(2,3-epithiopropyl)sulfide, a series of poly(alkyl polysulfide)s, and elemental sulfur. The addition of an amine catalyst leads to rapid polymerization, resulting in a series of thermosets with high transmittance over the entire visible range. The refractive index of the thermosets can be fine-tuned over the range of 1.700-1.745 (λ = 589 nm) by varying the feed ratios of elemental sulfur and the polysulfide polymers. A wide range of properties, including the birefringence (Δn), optical dispersion, temperature coefficient of refractive index (dn/dT), transmittance, and glass transition temperature (T_g), of the thermosets are presented. The polymers exhibited a very low birefringence (Δn < 0.002), which is attributed to the absence of aromatic functional groups, exceptionally low optical dispersion (V_D > 30), T_g > 60 °C, and dn/dT similar to that of PMMA.