Polymer Chemistry: Alive with Light

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Polymer Chemistry: Alive with Light research highlights most organic reagents are not very soluble polymerization) and RAFT (reversible in it. This property, however, can actually addition fragmentation chain transfer be used to accelerate reaction rates in what polymerization) are now commonplace are known as ‘on water’ reactions. These in the chemical literature. Some relatively can occur for certain transformations when recent developments in this area have water-insoluble reactants are vigorously focused on how to control these reactions stirred in water to give aqueous emulsions or with an external stimulus, such as light or an suspensions. Such reactions are thought to applied electrical current. occur at the interface between the immiscible But is it art? phases, but they are not fully understood. Practical synthetic chemistry shows an For example, the complicated structure of artistic side with synthesized doodles, emulsions makes it difficult to quantify the IrIII* logos and colouring-book molecules. effects of important reaction parameters such as water surface area and surface-to- It’s been a busy summer in the blogosphere! IV ● R volume ratio. Pn–Br Visible Ir Br Pn Between ‘arsenic life’ and the newly Now, Wilhelm Huck and colleagues light discovered Higgs boson, science blogs at Radboud University Nijmegen and the seem more relevant than ever. In all the fuss, University of Cambridge, have used a fluidic IrIII though, they may have overlooked some approach to control the interfacial features promising chemical artists in their midst. of two model ‘on water’ reactions to better © 2012 WILEY First up, let’s turn our attention to the understand and quantify the role played 2012 London Olympic Games. Chemists by the water surface. The reaction set-up Previous attempts to photoregulate at the University of Warwick decided to consisted of a cross-junction fed by tubes living radical polymerization processes have commemorate the event by synthesizing containing either water or reagents dissolved been somewhat limited, either giving rise a new molecular version of the famed in toluene. Monodisperse plugs of toluene to poor molecular weight control or only Olympic rings. ChemConnector’s Antony and water form at the junction and, by enabling the initiation step — rather than Williams explains (http://go.nature.com/ controlling the flow rate of the immiscible the subsequent chain-growth steps — to oCerSJ) that a chalkboard ‘doodle’ evolved phases or changing the diameter of the be controlled. Now, Brett Fors and Craig into a synthesis of olympicene; the RSC’s tubing, Huck and colleagues could control Hawker from the University of California, ChemSpider SyntheticPages later published the interfacial surface-to-volume ratio of the Santa Barbara, have shown that the living (http://go.nature.com/N4OM3p) the step- reacting plugs. Using this set-up they studied radical polymerization of methacrylates can by-step details. IBM-Zurich (of pentacene- the cycloaddition reaction of quadricyclane be turned on and off at the flick of a (light) imaging fame) posted a series of finely with diethyl azodicarboxylate (DEAD), switch by using a commercially available resolved atomic images to Flickr (http:// and the ene reaction between β-pinene and iridium-based photredox catalyst. The go.nature.com/uPSHll). DEAD, reactions known from previous Ir(iii) catalyst absorbs visible light to give an Next we’ll talk high fashion. At studies to exhibit ‘on water’ behaviour. excited-state Ir(iii)* species that can react Chemistry World, Phillip Broadwith noted Huck and colleagues observed a linear with an alkyl bromide initiator to produce an (http://go.nature.com/VYtBww) a neat relationship between the percentage alkyl radical and an Ir(iv)Br complex. The undergraduate synthesis project: making conversion of reactants into products (as alkyl radical initiates the polymerization of a a hydrocarbon to match a clothing label’s analysed by NMR spectroscopy) and the methacrylate monomer to give a propagating logo. After just three steps, Plymouth interfacial surface area between the plugs, polymer radical, which can then react University’s Mark Cockerton and Simon strongly supporting a role for the surface with the Ir(iv)Br species to give a dormant Belt had their target…but still no word on in enhancing the rate. They also calculated polymer chain with a bromide end group, at whether they’ve tied up a sponsorship deal a decrease in the activation energy of the the same time as regenerating the original with the apparel company in question! reactions when ‘on water’ in comparison Ir(iii) complex. Speaking of synthetic art, we’d be to in toluene, which supports a mechanism In the presence of light, this process remiss not to wish a happy belated birthday involving a hydrogen-bond-stabilized (pictured) continues to cycle in this fashion, to organic chemist K. C. Nicolaou. Blogger transition state. GA although the alkyl bromide is no longer BRSM prepared a special tribute (http:// a small-molecule initiator but a polymer go.nature.com/jkKFt8) to his penchant for POLYMER CHEMISTRY chain. When the light is turned off, however, flashy colours and ancient mythology in Alive with light the propagation reaction stops almost the figures that adorn his papers and the Angew. Chem. Int. Ed. http://doi.org/fz5hsn (2012) immediately — importantly, Fors and Hawker graphical abstracts that accompany them. show that no chain termination occurs during A small group of Blogroll’s ‘usual suspects’ The properties of any given polymeric this resting period. When the light is turned — including Adam Azman, Chemjobber material are influenced significantly by back on an hour later, the polymerization and Dr Freddy — re-imagined K.C.’s classic the molecular weights of the individual reaction resumes and continues until the molecules with new artistic twists. Perhaps polymer chains and also the distribution of light source is removed once more. Further these entries will finally prompt publication those values. Methods that offer very good evidence for the living nature of the system is of that chemical colouring book (http:// control over these parameters are therefore provided through the efficient formation of a go.nature.com/dWIUMI). prized in polymer synthesis. In particular, diblock copolymer. SC controlled living radical polymerization Written by See Arr Oh, who blogs at has attracted much attention and acronyms Written by Gavin Armstrong, Stuart Cantrill, http://justlikecooking.blogspot.com such as ATRP (atom transfer radical Stephen Davey and Anne Pichon. NATURE CHEMISTRY | VOL 4 | SEPTEMBER 2012 | www.nature.com/naturechemistry 691 © 2012 Macmillan Publishers Limited. All rights reserved.
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