Biobased Thiol-ene Networks with High Optical Transparency and Abbe Number Derived from Citric Acid

Biobased polymers with excellent thermostability, mechanical properties, and optical transparency are desired in many demanding areas. In this study, a highly rigid tetravinyl compound (4V) was synthesized starting from citric acid. 4V was further subjected to a permethylation reaction, affording another tetravinyl compound (4MV) with improved rigidity and thermostability. Two series of networks were developed through the solvent-free thiol-ene coupling of 4V (or 4MV) and three multifunctional thiol monomers. The 4V series exhibited similar glass transition temperature (Tg) values as compared to the networks based on 2,2′-diallyl bisphenol A, highlighting the prominent rigidity of 4V. The 4MV series exhibited higher thermostability, Tg values, and mechanical properties compared to the 4V analogues based on the same thiol. Highly transparent and colorless films with approximately 90% transmittance in the visible region (>500 nm) were obtained for all polymers. These materials demonstrated moderate refractive indices (n) within the range 1.5075–1.5779, along with remarkably high Abbe numbers (νD) ranging from 101 to 235. As a result, these materials hold great potential for various applications such as encapsulation resins for light-emitting diodes, optical materials for antireflective coatings, and display devices. This research paves the way for the development of biorenewable thiol-ene networks with desired properties including high Tg, transparency, mechanical robustness, and balanced n and νD values.