A role for terpenoid cyclization in the atom economical polymerization of terpenoids with sulfur to yield durable composites

Renewably-sourced, recyclable materials that can replace or extend the service life of existing technologies are essential to accomplish humanity's quest for sustainable living. In this contribution, remeltable composites were prepared in a highly atom-economical reaction between plant-derived terpenoid alcohols (10 wt% citronellol, geraniol, or farnesol) and elemental sulfur (90 wt%). Investigation into the microstructures led to elucidation of a mechanism for terpenoid polyene cyclization initiated by sulfur-centered radicals. The formation of these cyclic structures contributes significantly to understanding the mechanical properties of the materials and the extent to which linear versus crosslinked network materials are formed. The terpenoid-sulfur composites can be thermally processed at low temperatures of 120 °C without loss of mechanical properties, and the farnesol-sulfur composite so processed exhibits compressive strength 70% higher than required of concrete for residential building. The terpenoid-sulfur composites also resist degradation by oxidizing acid under conditions that disintegrate many commercial composites and cements. In addition to being stronger and more chemically resistant than some commercial products, the terpenoid-sulfur composites can be used to improve the acid resistance of mineral-based Portland cement as well. These terpenoid-sulfur composites thus hold promise as elements of sustainable construction on their own or as additives to extend the operational life of existing technologies, while the cyclization behaviour could be an important contributor in other polymerizations of terpenoids.