Developing a New Type of Polymer

Northwestern University, Evanston, IL

Researchers at Northwestern University’s Simpson Querrey Institute for BioNanotechnology say that they have developed a polymer that might one day be used in artificial muscles or other lifelike materials; for delivery of drugs, biomolecules, or other chemicals; in materials with self-repair capability; and even for replaceable energy sources. These functions require new hybrid polymers with both rigid and soft nano-sized compartments with extremely different properties that are organized in specific ways.

“We have created a surprising new polymer with nano-sized compartments that can be removed and chemically regenerated multiple times,” said materials scientist Samuel I. Stupp, director of the Querrey Institute. He is a leader in the fields of nanoscience and supramolecular self-assembly, the strategy used by biology to create highly functional ordered structures.

“Some of the nanoscale compartments contain rigid conventional polymers, but others contain the so-called supramolecular polymers, which can respond rapidly to stimuli, be delivered to the environment and then be easily regenerated again in the same locations. The supramolecular soft compartments could be animated to generate polymers with the functions we see in living things,” he explains.

The hybrid polymer combines the two types of known polymers: those formed with strong covalent bonds and those formed with weak non-covalent bonds, well known as “supramolecular polymers.” The integrated polymer offers two distinct compartments with which chemists and materials scientists can work to provide useful features. (See Figure 1)

“Our discovery could transform the world of polymers and start a third chapter in their history: that of the ‘hybrid polymer,’” Stupp says. “This would follow the first chapter of broadly useful covalent polymers, then the more recent emerging class of supramolecular polymers.

“We can create active or responsive materials not known previously by taking advantage of the compartments with weak non-covalent bonds, which should be highly dynamic like living things. Some forms of these polymers now under development in my laboratory behave like artificial muscles,” he explains.

Polymers get their power and features from their structure at the nanoscale. The covalent rigid skeleton of the first hybrid polymer has a cross-section shaped like a ninja star—a hard core with arms spiraling out. In between the arms is a softer material, which can be animated, refreshed, and recharged. These abilities could prove useful in a range of valuable applications.

“The fascinating chemistry of the hybrid polymers is that growing the two types of polymers simultaneously generates a structure that is completely different from the two grown alone,” says Stupp. “I can envision this new material being a super-smart patch for drug delivery, where you load the patch with different medications, and then reload it in the exact same compartments when the medicine is gone.”

Stupp and his research team also discovered that the covalent polymerization that forms the rigid compartment is “catalyzed” by the supramolecular polymerization, thus yielding much higher molecular weight polymers.

The strongly bonded covalent compartment provides the skeleton, and the weakly bonded supramolecular compartment can wear away or be used up, depending on its function, and then be regenerated by adding small molecules. After the simultaneous polymerizations of covalent and noncovalent bonds, the two compartments end up bonded to each other, yielding a very long, perfectly shaped cylindrical filament.

For more information, visit www.northwestern.edu/newscenter .