Polymer structures have traditionally been relatively simple, usually a linear chain made from a single type of monomer or perhaps a copolymer chain made from two or three types of monomers. Sometimes chemists might graft a side chain onto the polymer or cross-link the chains to form polymer networks.
But that old simplicity is falling by the wayside. Advances in polymer synthetic techniques that allow better control over the size and shape of polymers are allowing researchers to think more like architects to dream up exotic new polymer designs. One goal of the work is to create macromolecules with functional properties for drug delivery, catalysis, chemical sensing, and other applications.
Several polymer science experts have now pooled their talents to find out how far they can push the limits of polymer architecture (J. Am. Chem. Soc., DOI: 10.1021/ja400890v).
The research team is led by E. W. (Bert) Meijer of Eindhoven University of Technology, in the Netherlands; Krzysztof Matyjaszewski of Carnegie Mellon University; and Sergei S. Sheiko of the University of North Carolina, Chapel Hill. As a first test, the researchers set out to make a mega-sized copolymer consisting of a bottlebrush polymer with a collapsible tail, combining two of the recently developed advanced structural features in macromolecules.
Bottlebrush polymers are cylindrical in shape and have densely spaced side chains that resemble bristles on a brush. As for the polymer tail, chemists have created polymers with hydrogen-bonding capabilities that reversibly crumple into nanoparticles, a process similar to protein folding.
The team built the copolymer in four steps from methacrylate-based monomers. The process included grafting side chains onto the polymer and a postpolymerization step in which they added a hydrogen-bonding segment to a tail-like side chain. When the researchers shine ultraviolet light on the polymer, the tail sheds a protecting group to expose the hydrogen-bonding units. The end result is an unprecedented bottlebrush polymer with appended nanoparticles.
“This is an amazing example of how precision polymer synthesis techniques can be applied to the preparation of remarkably complex and functional macromolecular architectures,” says Marc A. Hillmyer, director of the University of Minnesota’s Center for Sustainable Polymers. “Combining controlled radical polymerizations, block copolymers, postpolymerization modifications, and triggered hydrogen-bonding interactions certainly pushes the limits of polymer synthesis.”
“A major challenge with synthetic polymers is that they are not as smart as well-defined natural systems, such as proteins,” says Craig J. Hawker, director of the Materials Research Laboratory at the University of California, Santa Barbara. “They do not fold in controllable ways or give unique molecular objects based on their synthetic design. I am convinced this work will spur other researchers to further push the frontiers of polymer synthesis, the ultimate goal being to design synthetic materials with many of the capabilities and properties of natural materials.”