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.

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.

Reversible-deactivation radical polymerization (RDRP) in the presence of Cu0 is a versatile technique that can be used to create well-controlled polymers with complex architectures. Despite the facile nature of the technique, there has been a vigorous debate in the literature as to the\ud mechanism of the reaction. One proposed mechanism, named supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP), has CuI as the major activator of alkyl halides, Cu0 acting as a supplemental activator, an inner-sphere electron transfer occurring during the activation step, and relatively slow comproportionation and disproportionation. In SARA ATRP slow activation of alkyl halides by Cu0 and comproportionation of CuII with Cu0 compensates for the small number of radicals lost to termination reactions. Alternatively, a mechanism named single electron transfer living radical polymerization (SET-LRP) assumes that the CuI species do not activate alkyl halides, but undergo instantaneous disproportionation, and that the relatively rapid polymerization is due to a fast reaction between alkyl halides and “nascent” Cu0 through an outer-sphere electron transfer. In this article a critical assessment of the experimental data are presented on the polymerization of methyl acrylate in DMSO with Me6TREN as the ligand in the presence of Cu0, in order to discriminate between these two mechanisms. The experimental data agree with the SARA ATRP mechanism, since the activation of alkyl halides by CuI species is significantly faster than Cu0, the activation step involves inner-sphere electron transfer rather than an outer-sphere electron transfer, and in DMSO comproportionation is slow but occurs faster than disproportionation, and activation by CuI species is much faster than disproportionation. The rate of deactivation by CuII is essentially the same as the rate of activation by CuI, and the system is under ATRP equilibrium. The role of Cu0 in this system is to slowly and continuously supply CuI activating species and radicals, by supplemental activation and comproportionation, to compensate for CuI lost due to\ud the unavoidable radical termination reactions. With the mechanistic understanding gained by analyzing the experimental data in the literature, the reaction conditions in SARA ATRP can be tailored toward efficient synthesis of a new generation of complex architectures and functional materials

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Polymerizations and mechanistic studies have been performed to understand the kinetic pathways for the\ud polymerization of the monomer oligo(ethylene oxide)\ud monomethyl ether acrylate (OEOA) in aqueous media.\ud Typically, the medium consisted of 18 wt % OEOA in\ud water, in the presence of Cu catalysts coordinated by tris[2(dimethylamino)ethyl]amine (Me6TREN). Well-controlled\ud polymerization of OEOA can be achieved in the presence of\ud halide anions and Cu wire with≲600 ppm of soluble CuII\ud species, rather than previously reported ca. 10 000 ppm of CuII and Cu0 particles formed by predisproportionation of CuI prior to monomer and initiator addition. The mechanistic studies conclude that even though disproportionation is thermodynamically favored in aqueous media, the SARA ATRP, not SET-LRP,\ud mechanism holds in these reactions. This is because alkyl halides are much more rapidly activated by CuI than by Cu0\ud (contribution of Cu0 to activation is <1%). Because of the high activity of CuI species toward alkyl halide activation,\ud [CuI/Me6TREN] in solution is very low (<5μM) and classical ATRP equilibrium between CuI and CuII species is maintained.\ud Although in aqueous media disproportionation of CuI/Me6TREN is thermodynamically favored over comproportionation, unexpectedly, in the presence of alkyl halides, i.e., during polymerization, disproportionation is kinetically minimized.\ud Disproportionation is slow because its rate is proportional to [CuI/Me6TREN]2 and [CuI/Me6TREN] is very small. Thus, during polymerization, comproportionation is 104 times faster than disproportionation, and the final thermodynamic equilibrium between disproportionation and comproportionation could be reached only after polymerization is completed. Activation of alkyl\ud halides by CuI/Me6TREN in aqueous media occurs 8 orders of magnitude faster than disproportionation