Synthesis of novel sustainable optical poly(isosorbide thioethers) with high refractive indices and good biocompatibility using functional ionic liquid catalysts

The synthesis of intraocular lenses (IOLs) with a high refractive index (RI) and good transparency to create patient-specific eye power is of primary importance, while the development of a novel IOL material, especially with outstanding optical characteristics and biocompatibility, is still an ongoing challenge. Herein, a bio-based poly(isosorbide thioether) (PITE) with excellent optical properties was developed as a novel IOL material and a new strategy for synthesizing high molecular weight PITE using ionic liquid catalysts was proposed. Compared to traditional IOLs (RI = 1.45–1.54), the synthesized bio-based PITEs showed the highest RI of 1.64 and an optical transparency of 93%, indicating that they were excellent optical materials. Furthermore, the resultant PITEs also exhibited ideal glass transition temperatures (Tg = 7.5–50.9 °C), good thermal stability (Td-5% > 312 °C) and excellent biocompatibility for L929 and HeLa cells. In order to achieve efficient synthesis of PITE, a series of functional ionic liquid catalysts were designed and synthesized. Among them, 1,8-diazabicyclo[5.4.0]-undec-7-en-8-ium lactate ([DBU][Lac]) in a trace amount (0.5 mol%) exhibited the highest catalytic performance, presenting a Mw value of up to 63.3 kg mol−1, higher than 7.4 kg mol−1 without an IL. Detailed mechanistic investigations using nuclear magnetic resonance spectroscopy and density functional theory calculations revealed that the “amphiphilic” character of ILs showed a cation–anion synergistic catalytic mechanism and then promoted nucleophilic hydrothiolation. The present study could provide enormous opportunities for the design and synthesis of bio-based optical polymers with high performance for IOLs, corneal contact lenses and other optical uses.