1,720,983 research outputs found
Dissociative electron transfer mechanism and application in the electrocatalytic activation of organic halides
Full text linkThe electrochemical dehalogenation of organic halides has been a widely investigated topic in primis for fundamental in- vestigations of dissociative electron transfer (DET) and as a green way for the electrosynthesis of organic compounds and environmental remediation. The main drawback associated with the electrochemical activation of C-X is the very negative potentials required, and this is particularly true in the case of organic chlorides, which represent the most investigated ensemble among organic molecules containing a leaving group. This has boosted over the last decades the research of electrode materials that are active toward C-X bond breaking, and without any doubt, Ag possesses extraordinary electro- catalytic properties, so far unsurpassed by any other electrode material. In this paper, we will attempt to brush up on important concepts related to DET in organic halides, while also evalu- ating recent developments in terms of electrode materials used in electrosynthesis based on organic halides and R-X degra- dation processes
Electrochemical approaches for better understanding of atom transfer radical polymerization
No full textElectrochemistry strongly contributed to deepen the understanding and predictability of atom transfer radical polymerization (ATRP) outcomes. Several electrochemical tools have been used to determine thermodynamic and kinetic parameters that are hardly accessible by other techniques. The electrochemical methods presented in this brief review were applied to systems with extremely different ATRP reactivity, providing a rational database of primary reference for further developments of ATRP
Towards scale-up of electrochemically-mediated atom transfer radical polymerization: Use of a stainless-steel reactor as both cathode and reaction vessel
No full textWell-defined poly (meth)acrylates and polystyrene with predetermined molecular weights and low dispersity were prepared by electrochemically-mediated Atom Transfer Radical Polymerization (eATRP), catalyzed by Cu complexes with amine ligands in a stainless-steel reactor. eATRPs were triggered by feeding a small current through the reactor scaffold used as a cathode. Moderate to high conversions were achieved in a short time. Chain extension confirmed high chain-end fidelity. ICP-MS analysis showed that the stainless-steel reactor does not release ions in solution; if a current is not applied, it cannot trigger polymerization as in zerovalent metal mediated ATRP. Successful laboratory scale-up tests were performed in the same reactor with a higher reaction volume (40 mL instead of 10 mL). The proposed setup is flexible and cost-effective, and in principle could be used for eATRP of a large variety of monomers
Reprint of “Electrochemical reduction of organic bromides in 1-butyl-3-methylimidazolium tetrafluoroborate”
No full textThe electrochemical reduction of a series of aliphatic and aromatic bromides on glassy carbon, silver and copper electrodes has been investigated by cyclic voltammetry in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF4). As in polar aprotic solvents, reductive cleavage of aromatic bromides occurs by a stepwise mechanism with the formation of a transient radical anion. In contrast, concerted electron transfer/bond rupture is the preferred reaction pathway for aliphatic bromides. Both Ag and Cu show remarkable electrocatalytic activities for the activation of the carbon-bromine bond, but the catalytic effect depends on reaction mechanism and molecular structure of RBr. Catalysis is high when reduction occurs by a concerted dissociative electron transfer pathway, which is the case of alkyl bromides. When instead a stepwise mechanism is preferred, i.e., the case of all aromatic bromides, catalysis strongly decreases with the ability of the molecule to delocalize the incoming negative charge. For example, the high positive shift of peak potential (> 0.7 V) observed for bromobenzene on Ag decreases to < 0.1 V for 4-bromobenzonitrile and < 0.02 V for 9-bromoanthracene.Overall, [BMIm]BF4 behaves like molecular solvents such as acetonitrile and dimethylformamide but, in general, both Ag and Cu are less active in the ionic liquid than in polar aprotic solvents
Electrochemically Mediated Atom Transfer Radical Polymerization of Methyl Methacrylate: The Importance of Catalytic Halogen Exchange
No full textElectrochemically mediated atom transfer radical polymerization (eATRP) of methyl methacrylate (MMA) was studied in 1-butyl-3-methylimidazolium triflate ([BMIm][OTf]) and ethanol. When 2-bromopropionitrile and ethyl 2-bromoisobutyrate were used as initiators, poorly controlled or uncontrolled polymerizations yielding polymers with molecular weights largely exceeding the theoretical values were obtained. Poor control was attributed to a reactivity mismatch between initiator and dormant species, which was successfully suppressed by combining eATRP with catalytic halogen exchange. Well-defined polymers (Mn>30000 and Ð<1.2) were obtained in both solvents under optimized conditions. The possibility of using PMDETA as an inexpensive ligand in combination with ethyl α-bromophenylacetate as initiator was also successful. Good chain-end fidelity during eATRP was confirmed by chain extension of PMMA-Cl macroinitiator with MMA in ethanol.Polymers prepared in both solvents were found to be mainly syndiotactic, without any solvent effect on tacticity
Electrochemical study of the effect of Al3+ on the stability and performance of Cu-based ATRP catalysts in organic media
No full textElectrochemically-mediated atom transfer radical polymerization (eATRP) has attracted a great deal of attention as one of the most powerful methods of synthesis of polymers with well-defined architecture and low dispersity. The process is more cost-effective and simpler if performed in undivided cells with cheap sacrificial anodes such as aluminum. In this connection, knowledge of the effect of Al3+ ions released from the anode on the stability and performance of typical Cu catalysts is highly desired. In this study, the stability of Cu complexes with tris(2-(dimethylamino)ethyl)amine (Me6TREN) and tris(2-pyridylmethyl)amine (TPMA), was investigated in DMF, DMSO, and MeCN by cyclic voltammetry, UV-vis-NIR spectroscopy and controlled-potential eATRP. Both ligands form complexes with Al3+, but in DMF and DMSO, TPMA binds more strongly to Cu2+ and Cu+ than Al3+, whereas the opposite is true for Me6TREN in all tested solvents. Therefore, eATRP of n-butyl acrylate catalyzed by [CuITPMA]+ in DMF or DMSO was not affected by Al3+ ions. In contrast, when Me6TREN was used, the reactions could not proceed to high conversions because Al3+ ions displace Cu2+ from the ligand. To suppress the effect of Al3+, use of excess ligand and/or Br− with respect to CuII proved to be efficient
Electrochemically Mediated Aqueous Atom Transfer Radical Polymerization of N,N-Dimethylacrylamide
No full textElectrochemically mediated atom transfer radical polymerization (ATRP) of N,N-dimethylacrylamide (DMAA) catalyzed by copper complexes with polydentate amine ligands was studied systematically in water, investigating several reaction parameters such as applied potential, catalyst concentration, ligand structure, monomer and initiator concentrations. Electropolymerizations were successfully performed under both potentiostatic and galvanostatic conditions; in both eATRP modes, reactions were fast (monomer conversion >90 % in less than 1 h) and well-controlled, providing polymers with narrow molecular weight distributions. Despite the low dispersity, chain extension attempts of the obtained polymer were not successful because of partial loss of C−Br chain-end functionality, due to an intramolecular nucleophilic attack. This is an intrinsic drawback of ATRP of acrylamides and although the electrochemical approach allowed preparation of well-defined polymers in a very short time (down to ca. 15 min), loss of chain-end functionality was unavoidable
Toward Electrochemically Mediated Reversible Addition-Fragmentation Chain-Transfer (eRAFT) Polymerization: Can Propagating Radicals Be Efficiently Electrogenerated from RAFT Agents?
No full textElectrochemistry provides easily tunable parameters for the preparation of well-defined polymers in aspatiotemporal controlled manner under mild conditions. This work discusses the requisites for an electrochemically mediatedreversible addition−fragmentation chain-transfer (eRAFT) polymerization, in which electrochemical stimuli are used to reducethe RAFT agent, either directly or in the presence of a mediator. The redox properties of several RAFT agents were investigatedby cyclic voltammetry and correlated to their structures. The direct electrolysis of RAFT agents in the presence of a monomercaused the loss of RAFT agents, thus leading to uncontrolled polymerizations. These issues could partially be overcome byusing a mediator that shuttles the electrons from the electrode to the RAFT agent in solution. Several compounds were tested todefine the characteristics of suitable mediators
Biocompatible polymers via aqueous electrochemically mediated atom transfer radical polymerization
No full textThe preparation of well-defined polymers via aqueous atom transfer radical polymerization (ATRP) is challenging due to the high activity and limited stability of ATRP catalysts in water. Optimized conditions previously reported for the ATRP of some watersoluble monomers cannot be regarded as universal conditions. Indeed, well-controlled Electrochemically mediated ATRP (eATRP) of oligo(ethylene oxide)methyl ether acrylate (OEOA) and 2-hydroxyethyl methacrylate (HEMA) were achieved by adjusting the conditions in relation to the solubility and reactivity of these monomers and corresponding polymers. Importantly, PEOA with low dispersity and predetermined molecular weight was obtained even with low catalyst loading and high monomer content (up to 50 vol%).Moreover, >90% conversion of HEMA was obtainedwithin 6–7 h, giving PHEMA with dispersity 1.2–1.3. This work confirms eATRP as a versatile, clean technique applicable to a range of water-soluble, biocompatible monomers
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