Polyterpenes by ring opening metathesis polymerization of caryophyllene and humulene

Polymers with their myriad functions are indispensable to any modern technology. Today, their production is largely based on fossil fuels. In view of their limited range, other sources are desirable on the long term. Beyond such considerations, renewable resources are also attractive as they can contain unique chemical structures. Thus, carbohydrates, fatty acids or terpenes contain a wealth of different structural elements. Particularly, terpenes and terpenoids feature thousands of different scaffolds. However, except for pinene, limonene and myrcene, there are few examples of other terpenes used for polymerization. To translate the structural elements of a given renewable resource into useful material properties, appropriate synthetic methods are required. Caryophyllene and humulene are among the most abundant and cheapest sesquiterpenes. They are found in many plants and fungi. For example, more than 10 tons of clove oil are produced annually by the clove tree Eugenia caryophyllata (Syzygium aromaticum). Clove oil contains 7–12% caryophyllene and 1–4% humulene. Another potential source is hop oil, with up to 25% caryophyllene and 45% humulene. Concerning the utilization of caryophyllene or humulene as a source of polymers, only ill-defined polymeric materials have been obtained from such sesquiterpenes as undesired sideproducts during episulfidation. The unsaturated cyclic nature of caryophyllene and humulene in principle makes them suitable for ring opening metathesis polymerization (ROMP). Thus, (−)-trans-caryophyllene (or β-caryophyllene) features a cyclobutane ring fused in a trans fashion to a ninemembered ring containing a 1,5-diene with the intra-ring double bond in E arrangement. Humulene, (1E,5E,8E)-1,4,4,8tetramethyl-cycloundeca-1,5,8-triene, possesses an elevenmembered ring with all three double bonds in a trans arrangement. ROMPs of such motifs are unusual, and there are only a few reports of ROMP of nineand eleven-membered rings, namely of the parent unsubstituted cyclononene or cycloundecene. There is also only scarce evidence of ROMP of 1-methyl-substituted cycloalkenes. We now report on the reactivity of caryophyllene and humulene in metathesis polymerization, and on the properties and the reactivity of the polymers obtained (Scheme 1). Five catalyst precursors differing in the nature of the active species and the initiation reactivity were screened for their behaviour towards caryophyllene, namely [(PCy3)2Cl2RuvCHPh] (1), [(PCy3)(η-C–C3H4N2Mes2)Cl2RuvCHPh] (2), [(3-BrPyr)(η-C–