Polymer Folding

BORROWING FROM nature's strategy of using intramolecular interactions to fold and unfold proteins, chemists in the Netherlands have created the first examples of single chains of synthetic polymers that use hydrogen bonding to reversibly fold themselves into well-defined nanoparticles.

Previously only biopolymers such as proteins and nucleic acids were capable of performing such ordered folding. The ability to control the internal structure of the nanoparticles and their aggregation into organized supramolecular structures in thin films without establishing permanent covalent bonds could open up vast new possibilities in materials science and nanotechnology, the researchers say.

The work, by E. Johan Foster, Erik B. Berda, and E. W. (Bert) Meijer of Eindhoven University of Technology, "is a nice example showing the potential of single-chain polymer nanoparticles in the development of new materials through sophisticated control of supramolecular interactions," observes polymer chemist Sang Youl Kim, of the Korea Advanced Institute of Science & Technology, in Daejeon, South Korea.

Meijer and coworkers fashioned the nanoparticles from poly(norbornene) diblock copolymers in which the minor block has either a urea or urethane pendant group containing an ureidopyrimidinone moiety (J. Am. Chem. Soc., DOI: 10.1021/ja901687d). Chemists often use ureidopyrimidinones to assemble supramolecular structures, in part because the groups can dimerize by forming a string of four hydrogen bonds.

When the researchers shine ultraviolet light on dilute solutions of the polymers, a nitrophenyl protecting group drops off the ends of the pendant groups, freeing the ureidopyrimidinones to engage in hydrogen bonding that forms the nanoparticles, which are approximately 20 nm in diameter. Adding a little acid disrupts the hydrogen bonds and permits the polymer chains to expand back to their original random coil form. When the researchers cast films of the nanoparticles, they observed that the nanoparticles can organize into a cross-linked network under the right conditions, just as some proteins can aggregate into large structures such as fibrils.

Craig J. Hawker, director of the Materials Research Laboratory at the University of California, Santa Barbara, says the discovery "is a great example of pushing the expanding field of single-chain polymer nanoparticles to the next level."

By making the nanoparticles metastable, controlled transitions from weakly interacting nanoparticles to highly entangled linear macromolecules are now possible, Hawker says. These structural transitions can be exploited to deliver small molecules or expose internal functional groups. Such capability exists in natural systems like proteins but is lacking in traditional covalently locked systems, such as cross-linked polymer nanoparticles, he explains.

The Eindhoven research team has an unlimited list of new applications to try, ranging from powder coatings to single-chain enzymelike catalysts to drug delivery, Meijer says. "We are really aiming at closing the gap between polymer random coils and well-defined biomacromolecules," he adds.