Semi-crystalline poly(thioether) prepared by visible-light-induced organocatalyzed thiol-ene polymerization in emulsion

A photocatalytic thiol-ene aqueous emulsion polymerization under visible-light is described to prepare linear semicrystalline latexes using 2,2’-dimercaptodiethyl sulfide as dithiol and various dienes. The procedure involves low irradiance (3 mW cm–2), LED irradiation source, eosin-Y disodium as organocatalyst, low catalyst loading (< 0.05% mol) and short reaction time scales (< 1 h). The resulting latexes has molecular weights of about 10 kg mol–1, average diameters of 100 nm and a linear structure consisting only of thioether repeating units. Electron-transfer reaction from a thiol to the triplet excited state of the photocatalyst is suggested as the primary step of the mechanism (type I), whereas oxidation by singlet oxygen generated by energy transfer has a negligible effect (type II). Only polymers prepared with aliphatic dienes such as diallyl adipate or di(ethylene glycol) divinyl ether exhibit a high crystallization tendency as revealed by differential scanning calorimetry, polarized optical microscopy and X-ray diffraction. Ordering and crystallization are driven by molecular packing of poly(thioether) chains combining structural regularity, compactness and flexibility.