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REACTION DYNAMICS
Researchers get the first detailed look at how catalysts crank out individual polymer chains ence 2017, DOI: 10.1126/science.aan6837). When chemists think about polymerizaThese new findings are “very cool,” says tion, they typically envision a wormlike Suzanne A. Blum of the University of Calipolymer growing smoothly and continufornia, Irvine. Blum’s group has used fluoously from a catalyst. But the actual view rescently labeled molecules to study sinof how polymer growth unfolds has regle-molecule dynamics and recently used mained murky because of the limitations this approach to watch catalytic activity as of analytical techniques. labeled monomers randomly got added to Using a pair of magnetic tweezers, a growing polymer chain (Angew. Chem. Int. optical microscopy, and spectroscopic techniques, Cornell University researchers Ed. 2017, DOI: 10.1002/anie.201708284). The newfound wait and jump steps of led by Peng Chen, Geoffrey W. Coates, polymer lengthening were previously oband Fernando A. Escobedo have achieved scured by “ensemble averaging,” Blum exthe first real-time visualization of single polymer chain growth. What they report is plains, in which researchers use techniques such as dynamic light scattering to observe startling: Individual polymer chains don’t all the molecules in a sample at once, and increase steadily but instead undergo coninformation about size distributions and secutive wait and jump steps. other polymer parameters are extracted With the aid of molecular dynamics from the data. Single-molecule approaches computer simulations, the researchers atavoid limitations of ensemble averaging, tribute this jerky mechanism to formation of polymer tangles—which they call hair balls—that By attaching a polymer chain to a glass slide (gray) and form around the catato a magnetic particle (orange) bound to the ruthenium lyst as thousands of new catalyst (linkage shown at right) and pulling the chain monomer units are added out with a pair of magnetic tweezers, researchers are to the growing chain. The able to watch the wait and jump dynamics of ringhair balls sporadically opening polymerization. unravel after a couple of minutes, and a new hair ball starts to form. Besides helping researchers better understand catalyst activity, polymerization rates, and bulk polymer properties, the researchers suggest their discovery of the growth spurts may be relevant to how cells produce biopolymers such as proteins, nucleic acids, and polysaccharides (Sci-
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C&EN | CEN.ACS.ORG | OCTOBER 23, 2017
Blum says, but these measurement techniques are often a double-edged sword because the resulting data are a challenge to interpret without corroborating spectroscopic methods. By including molecular dynamics simulations, the Cornell team was able to get a clearer picture of conformational changes in the growing polymer. “The ability to see dynamics in an important reaction like polymerization and to understand them through modeling is an exciting technological advance,” Blum says. In their single-molecule experiment, the Cornell researchers attached the free end of a polymer chain to a glass surface using a silane linkage and attached the ruthenium catalyst at the growing end of the polymer to a magnetic particle held in place by a pair of magnetic tweezers. By tracking the position of the magnetic particle, the team achieved real-time visualization of a single chain’s growth during ring-opening polymerization. “This is just a superb piece of science,” says Craig J. Hawker of the University of California, Santa Barbara. Hawker’s group recently reported the one-pot synthesis of block copolymers using five different monomers with widely varying properties (Angew. Chem. Int. Ed. 2017, DOI: 10.1002/ anie.201707646). The new polymer-growth monitoring process could help researchers understand how to better control such exotic polymerizations and allow them to tune the macroscopic properties of polymer networks to design new functional materials, he says. “The Cornell team’s tantalizing view of how polymer chains grow sheds light on many synthetic challenges dating from the earliest days of polymer chemistry,” Hawker adds. “This work will have major implications far beyond single-chain dynamics.”—STEVE RITTER
C R E D I T: A DA PT E D FRO M S CI E NC E
Polymer growth spurts revealed
