Realizing High Refractive Index Thiol-X Materials: A General and Scalable Synthetic Approach

Photopolymers formed from the family of thiol-X “click” reactions notably form sulfur-containing thioether linkages. While sulfur-containing materials are conventionally expected to result in high refractive index (nD/20 °C > 1.6) materials, this expectation is rarely realized with thiol-X photopolymers, because of the lack of monomers with sufficiently high refractive indices. Here, an efficient, modular synthetic strategy to obtain high refractive index thiol-X monomers is presented. The efficacy of the overall approach is demonstrated using only commercially available starting compounds to yield low viscosity (<500 cP) liquid multifunctional thiol and diallyl ether monomers with high refractive indices (nD/20 °C > 1.64). These synthesized monomers underwent rapid thiol-ene photopolymerizations to high conversions, achieving refractive index values (nD/20 °C) up to 1.669 in uniform crosslinked networks exhibiting typical narrow and well-defined loss tangent (tan δ) peaks. The convenience of the low-viscosity, high optical quality thiol-ene resins was demonstrated in the fabrication of two functional optical components. First, a plano-convex lens was aspherized using an overmolding procedure. Second, a two-dimensional grating was fabricated using a two-stage material and a photomask exposure.