Iron-free mechanochemical limonene inverse vulcanization†

An iron-free mechanochemical-assisted limonene inverse vulcanization is reported. The process makes use of only limonene and sulphur, industrial waste by-products, under mild conditions (ca. 40 °C) and short time (2 h) using a zirconium oxide reactor and a planetary ball mil. The obtained high value products are light yellow solids, readily soluble in chloroform, optically active oligosulfides, which are different from polysulfides reported under conventional conditions (ca. 185 °C), as confirmed by NMR spectroscopy and mass spectrometry. A general reaction mechanism is proposed, initiated by homolytic sulphur ring opening triggered by mechanical stress, and involving thiirane intermediates, via an addition–elimination reaction of sulphur to the limonene double bonds.