Polymers are an indispensable commodity within modern society found in all sectors of a consumer economy, such as: materials, pharmaceuticals and energy. Typically derived from non-renewable petroleum sources, polymers account for ~7% of the gas oil consumed.1 As of 2013, total US demand for crude oil is approx. 1 billion metric tons per year and as mankind enters into the 21st century the search for renewable energy and materials is at an all-time high.2 To solve this problem, efforts have been focused on utilizing available renewable feedstocks.
There are multiple processes for the polymerization of biorenewable resources ranging from cationic, thermal, radical, and olefin metathesis; however, these efforts in biorenewable polymers have largely been focused on the polymerization of seed oils.3 Surprisingly, not many literature examples on the use of terpenes as monomers for polymerizations via metathesis exist – even though terpenes contain alkenes (Figure 1). The use of terpenes as metathesis monomers is regulated by the propensity to either participate in ring closing metathesis (RCM) as observed with myrcene4 or to act as a chain transfer reagent as in the case with d-limonene.5 Recently, Mecking and coworkers examined two cyclic terpenes, caryophyllene and humulene, as biorenewable monomers for ring opening polymerization (ROMP).
Figure 1. Examples of terpenes used in metathesis
Caryophyllene and humulene are among the cheapest and most readily available sesquiterpenes. These two cyclic terpenes pose an interesting issue because they both contain larger ring systems that lack the typical ring strain found in monomers utilized for ROMP. Furthermore caryophyllene possesses both endo- and exocyclic alkenes that could potentially cause chemoselectivity issues. Initial ROMP screens with a first generation Grubbs catalyst were unsuccessful due to the catalyst’s poor efficiency toward trisubstituted olefins. However, the second generation Grubbs catalysts polymerized the monomers to complete conversion within 12 hours (Figure 2). Furthermore the ROMP of humulene was faster than the polymerization of caryophyllene. Finally, the authors were able to lower the catalyst loading to 0.04 mol% by utilizing a bispyridyl ruthenium catalyst.
Figure 2. Polycaryophyllene and polyhumulene
Upon optimizing ROMP conditions, Mecking and coworkers studied the physical properties of the resulting polymers and their hydrogenated counterparts. The ROMP of caryophyllene yielded a colorless, sticky and viscous material with a Mn between 1.7 to 2.0 x 104 g/mol and a Tg of -32 °C. Interestingly, metathesis reactivity with the exocyclic methylene of caryophyllene was not observed to be problematic. Finally, the hydrogentated polycaryophyllene showed an increase of Tg to ca -16 °C, with the cyclobutane ring intact.
The ROMP polymer from humulene, is also a colorless, sticky material with a Mn of 3.0 x 104 g/mol and a Tg of -48 °C. Furthermore 13C NMR analysis of polyhumulene indicates chemoselectivity of the cyclic alkenes such that metathesis polymerization only occurs on one of the double bonds. Similar to the hydrogenated polycaryophyllene, the hydrogenated polyhumulene provided a material with a slightly higher Tg than the original unsaturated polymer.
Overall, ROMP of these sesquiterpenes yielded soft, non-crosslinked linear polymers which the authors suggest may be particularly suitable for film formation and coatings. As natural resources are becoming depleted, the reliance on renewable resources will continue to grow. The field of biorenewable resources is still in its early stages and has much potential for expansion. This work by Mecking and coworkers is encouraging in demonstrating the potential of a significant biorenewable resource such as terpenes and it will be exciting to see the opportunities this research will lead to in the future.
1 Williams, C. K.; Hillmyer, M. A. Polymers from renewable resources: a perspective for a special issue of polymer reviews. Polymer Reviews 2008, 48, 1–10.
2 Petroleum Supply Monthly http://www.eia.gov/petroleum/supply/monthly/ (accessed Apr 13, 2013).
3 Gandini, A. The irruption of polymers from renewable resources on the scene of macromolecular science and technology. Green Chem. 2011, 13, 1061-1083.
4 Kobayashi, S.; Lu, C.; Hoye, T. R.; Hillmyer, M. A. Controlled Polymerization of a Cyclic Diene Prepared from the Ring-Closing Metathesis of a Naturally Occurring Monoterpene. J. Am. Chem. Soc. 2009, 131, 7960–7961
5 Mathers, R. T.; McMahon, K. C.; Damodaran, K.; Retarides, C. J.; Kelley, D. J. Ring-Opening Metathesis Polymerizations in d-Limonene: A Renewable Polymerization Solvent and Chain Transfer Agent for the Synthesis of Alkene Macromonomers. Macromolecules 2006, 39, 8982–8986.