Researchers at Martin Luther-University Halle-Wittenberg (MLU) and the Leibniz Institute of Plant Biochemistry (IPB) in Germany have discovered that enzymes are capable of degrading synthetic polyisoprene.
The specific conditions needed for this to occur were created and exploited by the researchers. Polyisoprene is the principal component of natural rubber, a material used for the production of car tires. Until now, the degradation of polyisoprene has only been possible with a composition that is similar to naturally occurring rubber. The MLU and the IPB believe that the research could provide important insights for a move toward a circular economy.
Natural rubber is utilized for the production of polyisoprene, which is then used for the manufacture of different types of rubber. Polyisoprene is a long-chain molecule formed by linking hundreds or thousands of smaller isoprene molecules.
“Various bacteria are able to degrade natural polyisoprene with the help of enzymes,” explained MLU chemist Vico Adjedje.
With the global demand for rubber products surpassing the existing stock of natural rubber, the starting materials used for such products are produced mainly by using chemical synthesis. Both natural and synthetic versions share similar properties but show several differences in the structure of the molecules of which they are composed.
With the help of their research teams, professor Dr Wolfgang Binder at MLU and junior professor Dr Martin Weissenborn at IPB have discovered a method to decompose artificially produced polyisoprene using an enzyme called LCPK30.
“We are the first to have succeeded in getting the polyisoprene into a form that the enzyme can also work with,” said Binder. In doing so, the researchers took their inspiration from nature.
“Our assumption was that synthetic polyisoprene should be present in an emulsion so that the enzyme can work properly,” added Adjedje.
The researchers explain that milk, which consists mostly of water and fat, provides a typical example of an emulsion. It forms globules a few micrometers in size and its fine distribution in water makes the milk appear cloudy. Just like fat, polyisoprene is virtually insoluble in water. Nature manages to distribute it evenly in water as a milky-white latex milk, which is harvested on rubber plantations and then processed into natural rubber.
Researchers took inspiration from the latex milk and subsequently succeeded in distributing synthetically produced polyisoprene evenly in water, using a specific solvent. The enzyme complied with the artificial emulsion and thus remained intact over the reaction time, breaking down the long molecular chains of the polyisoprene into much smaller fragments.
“A lot happens to the starting material before it becomes a finished tire: the molecule chains are chemically cross-linked to change the mechanical properties,” said Adjedje. “Plasticizers and antioxidants are added. The latter in particular presents a problem for the enzyme because they attack its structure.”
In the future, the researchers aim to break down other similar substances found within car tires and state that the results could also encourage a recycling economy.
“We could further process the degradation products into fine chemicals and fragrances – or [to]reproduce new plastics,” said Binder.
The researchers used LCPK30 as found in nature. Weissenborn and his research team are currently working to optimize the enzyme so that it becomes less sensitive to solvents and triggers further reactions.
The study appeared in the Green Chemistry journal.