Armando Gennaro

Electrochemically Mediated Atom Transfer Radical Polymerization

The structure of a polymer can be fine-tuned by rapidly starting and stopping its synthesis. Atom transfer radical polymerization is a versatile technique for exerting precise control over polymer molecular weights, molecular weight distributions, and complex architectures. Here, we show that an externally applied electrochemical potential can reversibly activate the copper catalyst for this process by a one-electron reduction of an initially added air-stable cupric species (CuII/Ligand). Modulation of polymerization kinetics is thereby tunable in real time by varying the magnitude of applied potential. Application of multistep intermittent potentials successfully triggers initiation of polymerization and subsequently toggles the polymerization between dormant and active states in a living manner. Catalyst concentrations down to 50 parts per million are demonstrated to maintain polymerization control manifested in linear first-order kinetics, a linear increase in polymer molecular weight with monomer conversion, and narrow polymer molecular weight distributions over a range of applied potentials.

Absolute potential of the standard hydrogen electrode and the problem of interconversion of potentials in different solvents.

The absolute potential of the standard hydrogen electrode, SHE, was calculated on the basis of a thermodynamic cycle involving H(2(g)) atomization, ionization of H((g))* to H((g))(+), and hydration of H(+). The most up-to-date literature values on the free energies of these reactions have been selected and, when necessary, adjusted to the electron convention Fermi-Dirac statistics since both e(-) and H(+) are fermions. As a reference state for the electron, we have chosen the electron at 0 K, which is the one used in computational chemistry. Unlike almost all previous estimations of SHE, DeltaG(aq)(theta)(H(+)) was used instead of the real potential, alpha(aq)(H(+)). This choice was made to obtain a SHE value based on the chemical potential, which is the appropriate reference to be used in theoretical computations of standard reduction potentials. The result of this new estimation is a value of 4.281 V for the absolute potential of SHE. The problem of conversion of standard reduction potentials (SRPs) measured or estimated in water to the corresponding values in nonaqueous solvents has also been addressed. In fact, thermochemical cycles are often used to calculate SRPs in water versus SHE, and it is extremely important to have conversion factors enabling estimation of SRPs in nonaqueous solvents. A general equation relating E(theta) of a generic redox couple in water versus the SHE to the value of E(theta) in an organic solvent versus the aqueous saturated calomel electrode is reported.

Ab Initio Evaluation of the Thermodynamic and Electrochemical Properties of Alkyl Halides and Radicals and Their Mechanistic Implications for Atom Transfer Radical Polymerization

High-level ab initio molecular orbital calculations are used to study the thermodynamics and electrochemistry relevant to the mechanism of atom transfer radical polymerization (ATRP). Homolytic bond dissociation energies (BDEs) and standard reduction potentials (SRPs) are reported for a series of alkyl halides (R-X; R = CH 2CN, CH(CH 3)CN, C(CH 3) 2CN, CH 2COOC 2H 5, CH(CH 3)COOCH 3, C(CH 3) 2COOCH 3, C(CH 3) 2COOC 2H 5, CH 2Ph, CH(CH 3)Ph, CH(CH 3)Cl, CH(CH 3)OCOCH 3, CH(Ph)COOCH 3, SO 2Ph, Ph; X = Cl, Br, I) both in the gas phase and in two common organic solvents, acetonitrile and dimethylformamide. The SRPs of the corresponding alkyl radicals, R (*), are also examined. The computational results are in a very good agreement with the experimental data. For all alkyl halides examined, it is found that, in the solution phase, one-electron reduction results in the fragmentation of the R-X bond to the corresponding alkyl radical and halide anion; hence it may be concluded that a hypothetical outer-sphere electron transfer (OSET) in ATRP should occur via concerted dissociative electron transfer rather than a two-step process with radical anion intermediates. Both the homolytic and heterolytic reactions are favored by electron-withdrawing substituents and/or those that stabilize the product alkyl radical, which explains why monomers such as acrylonitrile and styrene require less active ATRP catalysts than vinyl chloride and vinyl acetate. The rate constant of the hypothetical OSET reaction between bromoacetonitrile and Cu (I)/TPMA complex was estimated using Marcus theory for the electron-transfer processes. The estimated rate constant k OSET = approximately 10 (-11) M (-1) s (-1) is significantly smaller than the experimentally measured activation rate constant ( k ISET = approximately 82 M (-1) s (-1) at 25 degrees C in acetonitrile) for the concerted atom transfer mechanism (inner-sphere electron transfer, ISET), implying that the ISET mechanism is preferred. For monomers bearing electron-withdrawing groups, the one-electron reduction of the propagating alkyl radical to the carbanion is thermodynamically and kinetically favored over the one-electron reduction of the corresponding alkyl halide unless the monomer bears strong radical-stabilizing groups. Thus, for monomers such as acrylates, catalysts favoring ISET over OSET are required in order to avoid chain-breaking side reactions.

Controlled Aqueous Atom Transfer Radical Polymerization with Electrochemical Generation of the Active Catalyst

Controlled/living radical polymerizations (C/LRPs) in aqueous media are attractive from both economic and environmental points of view. Aqueous media can be used for the synthesis of a vast array of hydrophilic and hydrophobic polymers, through homogenous and heterogeneous (e.g. suspension and (mini)emulsion) polymerization systems, respectively. Moreover, efficient C/LRPs conducted in aqueous saline buffers can be crucial for the preparation of polymer–biomolecule conjugates under biologically relevant conditions. Atom transfer radical polymerization (ATRP) is one of the most commonly employed C/LRP techniques, enabling the synthesis of polymers with predetermined molecular weights, narrow molecular weight distributions, and specific compositions and architectures. ATRP is often catalyzed by a Cu/Cu system in which the success of this process relies on a rapid and reversible activation/deactivation step. In this dynamic equilibrated system, activators (CuL) react with dormant macromolecular species (RX) to produce propagating radicals (RC) and deactivators (X-CuL; Scheme 1, region delimited by the dashed line). In nonaqueous solvents the equilibrium constant, KATRP= kact/kdeact, is usually small (< 10 ) resulting in a dramatic decrease of [RC] which consequently suppresses bimolecular termination reactions. In contrast to the success of ATRP in organic solvents, aqueous ATRP has been found to suffer from some limitations, specifically with regard to achieving polymerization control and the targeted degree of polymerization (DP). These observed limitations in aqueous ATRPmay result from three main phenomena. First, aqueous ATRP has a relatively large KATRP providing a high [RC] and usually fast polymerizations. Second, the halidophilicity (KX) of Cu L, that is, association of X to CuL, is small therefore diminishing the concentration of deactivator X-CuL. Third, CuL may be unstable in water and may undergo disproportionation. Developing a successful aqueous ATRP requires consideration of all the previously mentioned issues. In fact, improved polymerization control was found by using a high [X ], which helps to suppress deactivator dissociation. Further development of the process has been recently achieved through AGET (activators generated by electron transfer) ATRP. Although this process provides good results in terms of both control and DP, the correct [Cu]/[reducing agent] ratio and appropriate reducing agent are critical for success. The ideal process should have a constant and high Cu/Cu ratio, which is difficult to achieve over the whole polymerization by addition of a single reducing agent. Herein we describe an electrochemical ATRP method (eATRP), aimed to fulfill these criteria. The overall mechanism of eATRP is depicted in Scheme 1. Initially, the reaction mixture contains solvent, monomer, initiator, and CuL (or CuL +X-CuL). Under these conditions, CuL activator is absent in solution, and hence, no polymerization occurs. The onset of polymerization begins only when a sufficient potential (Eapp) is applied to the cathode so that reduction of CuL to CuL occurs at the working electrode. The magnitude of Eapp can be appropriately chosen to achieve a continuous (re)generation of a small quantity of CuL and consequently dictate the [RC]. A living polymerization process is ensured by the combination of a low [RC] and high [CuL]/[CuL] ratio. Furthermore, the polymerization rate and degree of control can be tuned by adjusting the Eapp. The first example of ATRP under electrochemical generation of activator has been recently reported for the successful polymerization of methyl acrylate in acetonitrile. Herein, we describe eATRP of oligo(ethylene glycol) methyl ether methacrylate (OEOMA475) in water. As a catalyst system, CuTPMA (TPMA= tris(2-pyridylmethyl)amine) was selected, which is one of the most active Scheme 1. Mechanism of conventional (delimited by dashed line) and aqueous electrochemical ATRP.

Mechanism of Photoinduced Metal-Free Atom Transfer Radical Polymerization: Experimental and Computational Studies

Photoinduced metal-free atom transfer radical polymerization (ATRP) of methyl methacrylate was investigated using several phenothiazine derivatives and other related compounds as photoredox catalysts. The experiments show that all selected catalysts can be involved in the activation step, but not all of them participated efficiently in the deactivation step. The redox properties and the stability of radical cations derived from the catalysts were evaluated by cyclic voltammetry. Laser flash photolysis (LFP) was used to determine the lifetime and activity of photoexcited catalysts. Kinetic analysis of the activation reaction according to dissociative electron-transfer (DET) theory suggests that the activation occurs only with an excited state of catalyst. Density functional theory (DFT) calculations revealed the structures and stabilities of the radical cation intermediates as well as the reaction energy profiles of deactivation pathways with different photoredox catalysts. Both experiments and calculations suggest that the activation process undergoes a DET mechanism, while an associative electron transfer involving a termolecular encounter (the exact reverse of DET pathway) is favored in the deactivation process. This detailed study provides a deeper understanding of the chemical processes of metal-free ATRP that can aid the design of better catalytic systems. Additionally, this work elucidates several important common pathways involved in synthetically useful organic reactions catalyzed by photoredox catalysts.

Solubility and electrochemical determination of CO2 in some dipolar aprotic solvents

Abstract The solubility of CO 2 in some solvents of electrochemical interest was measured both as a function of its partial pressure and as a function of temperature. Henry constants determined at 298.15 K are reported for DMF, AN, DMSO and THF. The solubility of CO 2 in these solvents is also reported at various temperatures in the range −10 to 25 ° C (20 to 50 ° C for DMSO). The Gibbs energy, enthalpy and entropy of solution at 298.15 K and 1 atm partial pressure of CO 2 were estimated from the temperature dependence of the solubility. The effect of base electrolytes on the solubility of CO 2 was also investigated. Voltammetric investigations of CO 2 reduction showed that, in spite of the large dependence of the peak potential on the nature of the electrode material, the solvent and the background electrolyte, the peak current is proportional to the CO 2 concentration, thus providing a means of quantitative CO 2 determination.

Mechanism of Carbon-Halogen Bond Reductive Cleavage in Activated Alkyl Halide Initiators Relevant to Living Radical Polymerization: Theoretical and Experimental Study

The mechanism of reductive cleavage of model alkyl halides (methyl 2-bromoisobutyrate, methyl 2-bromopropionate, and 1-bromo-1-chloroethane), used as initiators in living radical polymerization (LRP), has been investigated in acetonitrile using both experimental and computational methods. Both theoretical and experimental investigations have revealed that dissociative electron transfer to these alkyl halides proceeds exclusively via a concerted rather than stepwise manner. The reductive cleavage of all three alkyl halides requires a substantial activation barrier stemming mainly from the breaking C-X bond. The activation step during single electron transfer LRP (SET-LRP) was originally proposed to proceed via formation and decomposition of RX(•-) through an outer sphere electron transfer (OSET) process (Guliashvili, T.; Percec, V. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1607). These radical anion intermediates were proposed to decompose via heterolytic rather than homolytic C-X bond dissociation. Here it is presented that injection of one electron into RX produces only a weakly associated charge-induced donor-acceptor type radical anion complex without any significant covalent σ type bond character between carbon-centered radical and associated anion leaving group. Therefore, neither homolytic nor heterolytic bond dissociation applies to the reductive cleavage of C-X in these alkyl halides inasmuch as a true radical anion does not form in the process. In addition, the whole mechanism of SET-LRP has to be revisited since it is based on presumed OSET involving intermediate RX(•-), which is shown here to be nonexistent.

SARA ATRP or SET-LRP. End of controversy?

A debate has proceeded in the literature regarding the mechanism of reversible-deactivation radical polymerization in the presence of Cu0. The two proposed models are supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) and single electron transfer living radical polymerization (SET-LRP). In SARA ATRP, CuI is the major activator of alkyl halides, Cu0 is a supplemental activator and reducing agent of excess CuII through comproportionation, and disproportionation is negligible. In contrast, in SET-LRP, Cu0 is the major activator, CuI does not activate alkyl halides but instead undergoes instantaneous disproportionation. In this article we present a body of experimental and theoretical data, which supports the SARA ATRP mechanism and disagrees with the SET-LRP mechanism.

Mechanism of the electrochemical reduction of carbon dioxide at inert electrodes in media of low proton availability

Direct electrolysis of CO2 in DMF at an inert electrode, such as mercury, produces mixtures of CO and oxalate, whereas electrolysis catalysed by radical anions of aromatic esters and nitriles produces exclusively oxalate in the same medium. Examination of previous results concerning the direct electrochemical reduction and the reduction by photoinjected electrons reveals that there are no significant specific interactions between reactant, intermediates and products on the one hand, and the electrode material on the other, when this is Hg or Pb. These observations and a systematic study of the variations of the oxalate and CO yields with temperature and CO2 concentration, allow the derivation of a consistent mechanistic model of the direct electrochemical reduction. It involves the formation of oxalate from the coupling of two CO2 radical anions in solution. CO (and an equimolar amount of carbonate) is produced by reduction at the electrode of a CO2–CO˙–2 adduct, the formation of which, at the electrode surface, is rendered exothermic by non-specific electrostatic interactions.

Estimation of Standard Reduction Potentials of Halogen Atoms and Alkyl Halides

Standard reduction potentials, SRPs, of the halogen atoms have been calculated in water on the basis of an appropriate thermochemical cycle. Using the best up-to-date thermodynamic data available in the literature, we have calculated E(o)(X•/X-) values of 3.66, 2.59, 2.04, and 1.37 V vs SHE for F•, Cl•, Br•, and I•, respectively. Additionally, we have computed the SRPs of Cl•, Br•, and I• in acetonitrile (CH3CN) and dimethylformamide (DMF) by correcting the values obtained in water for the free energies of transfer of X• and X- from water to the nonaqueous solvent S and the intersolvent potential between water and S. From the values of E(o)(X •/X-) in CH(3)CN and DMF, the SRPs of a series of alkyl halides of relevance to atom transfer radical polymerization and other important processes such as pollution abatement have been calculated in these two solvents. This has been done with the aid of a thermochemical cycle involving the gas-phase homolytic dissociation of the C-X bond, solvation of RX, R•, and X•, and reduction of X• to X- in solution.