Spotlights Cite This: J. Am. Chem. Soc. 2019, 141, 6775−6775
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J. Am. Chem. Soc. 2019.141:6775-6775. Downloaded from pubs.acs.org by 93.179.90.242 on 05/01/19. For personal use only.
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MAY THE (MECHANICAL) FORCE BE WITH YOU Over the past decade, researchers have discovered several molecules that undergo complex chemical reactions prompted by mechanical force, rather than bond scission or other chemical drivers. When embedded into polymers, these mechanophores have demonstrated a variety of mechanochemical responses, including coloration, luminescence, crosslinking, depolymerization, small-molecule release, catalysis, and mechanically gated activation. So far most examples of this phenomenon have involved only sparse incorporation of mechanophores into mechanochemically inert polymers, limiting the magnitude of these materials’ mechanical responses. Now, Yan Xia and co-workers report the scalable synthesis of mechanophores known as benzoladderenes that can be effectively used to create polymechanophore systems (DOI: 10.1021/jacs.9b01736). These new compounds overcome limitations of previous mechanophores by optimizing and expanding on a previously known benzoladderene structure, allowing gram quantity synthesis of these materials. Through ring-opening methathesis polymerization of these novel benzoladderenes, the researchers created homopolymers and block copolymers with controlled molecular weights and low dispersity. They demonstrated that, with sonification, these polymers can be mechanochemically transformed into conjugated poly(o-phenylene-hexatrienylene). These new compounds expand the unique family of polymechanophores with dramatic response to mechanical force. Christen Brownlee
such as seeding of clouds and cryopreservation of biological materials. Dalia Yablon, Ph.D.
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BRIDGING THE GAP IN CARBENE POLYMERIZATION METHODS Carbene polymerization methods provide access to highmolecular-weight and densely functionalized polymers. These reactions are typically initiated by Pd or Rh species and utilize diazoalkanes, such as ethyl diazoacetate (EDA), as inexpensive carbene precursors. Despite the progress that has been made in this area, controlled and living polymerization methods suffer from low yield, broad dispersity, and poor control over number-average molecular weight. F. Dean Toste and coworkers have overcome many of these challenges by identifying (π-allyl)palladium acetate dimer as an initiator for the polymerization of EDA in a controlled and quasi-living manner (DOI: 10.1021/jacs.9b01532). While the polymerization of EDA has been achieved through initiation with (π-allyl)palladium dimers, Toste and co-workers hypothesized that yields could be improved through the use of a dimer with a more nucleophilic and Lewis basic carboxylate bridge. Indeed, the use of (π-allyl)palladium acetate dimer as an initiator in the polymerization of EDA improves the polymer yield by 77% compared to the reaction with the chloride-bridged dimer. This polymerization occurs in a controlled and quasi-living manner and can be used in the block copolymerization of EDA and 2,2,2-trifluoroethyl diazoacetate. Extensive experimental and computational studies support a dinuclear mechanism, providing valuable insight that may inform future initiator development. Elizabeth Meucci
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HOW BACTERIA MAXIMIZE FREEZING EFFICIENCY Nucleation of ice from tiny water droplets normally requires temperatures lower than 35 degrees below freezing. But bacteria that thrive in cold environments are able to nucleate ice at the comparatively red-hot temperatures of 2 degrees above freezing through ice nucleating proteins (INPs). In a computational study, Valeria Molinero and co-workers elucidate the role of these proteins’ size and aggregation on the temperature at which they nucleate ice (DOI: 10.1021/ jacs.9b01854). These bacterial proteins can either nucleate ice or prevent its growth through an ice-binding surface to control the kinetics of water crystallization. Using molecular simulations that are compared with experimental results, the chemists study a particular antifreeze protein that both stops ice growth and promotes its nucleation through the same surface. This protein’s nucleation temperature increases with length of the ice-binding site until a plateau is reached. This study also gives the means to predict the number of protein monomers that need to aggregate in order to maximize freezing efficiency. This study provides tools to guide surface optimization that can be applied to other areas where ice nucleation plays a role, © 2019 American Chemical Society
Published: May 1, 2019 6775
DOI: 10.1021/jacs.9b04330 J. Am. Chem. Soc. 2019, 141, 6775−6775
