Investigation of Electrochemically Mediated Atom Transfer Radical Polymerization

Electrochemically mediated atom transfer radical polymerization (<i>e</i>ATRP) of <i>n</i>-butyl acrylate was systematically investigated using diminished catalyst concentrations (≤300 parts per million) under a variety of formulations and electrochemical conditions. Critical polymerization parameters, including the applied potential, catalyst concentration, and ligand, were explored and correlated with polymerization rates, polymer properties, and currents during the <i>e</i>ATRP process. Additional electrochemical methods were explored to improve the feasibility of <i>e</i>ATRP under galvanostatic conditions. Copper electrodeposition and stripping experimentation proved to be an effective strategy for catalyst recycling allowing sequential controlled polymerizations to be possible utilizing one catalyst charge

Miniemulsion ARGET ATRP via Interfacial and Ion-Pair Catalysis: From ppm to ppb of Residual Copper

It was recently reported that copper catalysts used in atom transfer radical polymerization (ATRP) can combine with anionic surfactants used in emulsion polymerization to form ion pairs. The ion pairs predominately reside at the surface of the monomer droplets, but they can also migrate inside the droplets and induce a controlled polymerization. This concept was applied to activator regenerated by electron transfer (ARGET) ATRP, with ascorbic acid as reducing agent. ATRP of <i>n</i>-butyl acrylate (BA) and <i>n</i>-butyl methacrylate (BMA) was carried out in miniemulsion using Cu^II/tris­(2-pyridylmethyl)­amine (TPMA) as catalyst, with several anionic surfactants forming the reactive ion-pair complexes. The amount and structure of surfactant controlled both the polymerization rate and the final particle size. Well-controlled polymers were prepared with catalyst loadings as low as 50 ppm, leaving only 300 ppb of Cu in the precipitated polymer. Efficient chain extension of a poly­(BMA)-Br macroinitiator confirmed high retention of chain-end functionality. This procedure was exploited to prepare polymers with complex architectures such as block copolymers, star polymers, and molecular brushes

Harnessing the Interaction between Surfactant and Hydrophilic Catalyst To Control <i>e</i>ATRP in Miniemulsion

Harnessing the Interaction between Surfactant and Hydrophilic Catalyst To Control <i>e</i>ATRP in Miniemulsio

Reversible-Deactivation Radical Polymerization in the Presence of Metallic Copper. A Critical Assessment of the SARA ATRP and SET-LRP Mechanisms

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 mechanism of the reaction. One proposed mechanism, named supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP), has Cu^I 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 Cu^II 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 Cu^I 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 Cu^I 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 Cu^I species is much faster than disproportionation. The rate of deactivation by Cu^II is essentially the same as the rate of activation by Cu^I, and the system is under ATRP equilibrium. The role of Cu⁰ in this system is to slowly and continuously supply Cu^I activating species and radicals, by supplemental activation and comproportionation, to compensate for Cu^I lost due to 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

Single and Multiple Doping in Graphene Quantum Dots: Unraveling the Origin of Selectivity in the Oxygen Reduction Reaction

Singly and multiply doped graphene oxide quantum dots have been synthesized by a simple electrochemical method using water as solvent. The obtained materials have been characterized by photoemission spectroscopy and scanning tunneling microscopy, in order to get a detailed picture of their chemical and structural properties. The electrochemical activity toward the oxygen reduction reaction of the doped graphene oxide quantum dots has been investigated by cyclic voltammetry and rotating disk electrode measurements, showing a clear decrease of the overpotential as a function of the dopant according to the sequence: N ∼ B > B,N. Moreover, assisted by density functional calculations of the Gibbs free energy associated with every electron transfer, we demonstrate that the selectivity of the reaction is controlled by the oxidation states of the dopants: as-prepared graphene oxide quantum dots follow a two-electron reduction path that leads to the formation of hydrogen peroxide, whereas after the reduction with NaBH_4, the same materials favor a four-electron reduction of oxygen to water

Reversible-Deactivation Radical Polymerization in the Presence of Metallic Copper. Comproportionation–Disproportionation Equilibria and Kinetics

This article is the first in a series of papers, describing reversible-deactivation radical polymerization (RDRP) in the presence of metallic copper. The aim of these papers is to determine the proportions and roles of Cu⁰, Cu^IBr/L, and Cu^IIBr₂/L, and the overall reaction mechanism. This paper is focused on the comproportionation and disproportionation equilibrium between Cu⁰, Cu^IBr/L and Cu^IIBr₂/L in dimethyl sulfoxide (DMSO) for various surface areas of Cu⁰ and different ligand concentrations, in both the absence and presence of methyl acrylate (MA). Comproportionation dominated disproportionation when there was enough ligand present in the reaction medium to stabilize all soluble copper species. The relative amount of Cu^I at comproportionation/disproportionation equilibrium increased with ligand concentration. Cu^I represents approximately 99.95% of all soluble Cu species in MA/DMSO = 2/1 (v/v) at the ratio [Me₆TREN]₀:[Cu^IIBr₂]₀ = 6:1. Under typical polymerization conditions, there is essentially no disproportionation, since the ratio [Me₆TREN]:[Cu^II] is very large, starting from infinity and decreasing down to 6.7, for ∼3% terminated chains under the initial conditions [MA]₀:[MBrP]₀:[Me₆TREN]₀ = 222:1:0.1, in 33.3% (v/v) DMSO, with excess Cu⁰. The kinetics of comproportionation and disproportionation were both slow, requiring hours to reach equilibrium. The apparent rate coefficients for comproportionation and disproportionation were calculated as <i>k</i>_comp^app = 9.0 × 10^–4 cm s^–1 and <i>k</i>_disp^app = 2.0 × 10^–5 cm s^–1 in DMSO, as well as 3.5 × 10^–3 cm s^–1 and 3.1 × 10^–6 cm s^–1 in MA/DMSO = 2/1 (v/v), respectively. The results of this study invalidate the assumption of instantaneous and complete disproportionation, proposed in single-electron transfer living radical polymerization (SET-LRP). These findings agree with Cu⁰ acting as a supplemental activator and reducing agent in atom transfer radical polymerization (SARA ATRP)

Aqueous RDRP in the Presence of Cu⁰: The Exceptional Activity of Cu^I Confirms the SARA ATRP Mechanism

Polymerizations and mechanistic studies have been performed to understand the kinetic pathways for the polymerization of the monomer oligo­(ethylene oxide) monomethyl ether acrylate (OEOA) in aqueous media. Typically, the medium consisted of 18 wt % OEOA in water, in the presence of Cu catalysts coordinated by tris­[2-(dimethylamino)­ethyl]­amine (Me₆TREN). Well-controlled polymerization of OEOA can be achieved in the presence of halide anions and Cu wire with ≲600 ppm of soluble Cu^II species, rather than previously reported ca. 10 000 ppm of Cu^II and Cu⁰ particles formed by predisproportionation of Cu^I 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, mechanism holds in these reactions. This is because alkyl halides are much more rapidly activated by Cu^I than by Cu⁰ (contribution of Cu⁰ to activation is <1%). Because of the high activity of Cu^I species toward alkyl halide activation, [Cu^I/Me₆TREN] in solution is very low (<5 μM) and classical ATRP equilibrium between Cu^I and Cu^II species is maintained. Although in aqueous media disproportionation of Cu^I/Me₆TREN is thermodynamically favored over comproportionation, unexpectedly, in the presence of alkyl halides, i.e., during polymerization, disproportionation is kinetically minimized. Disproportionation is slow because its rate is proportional to [Cu^I/Me₆TREN]² and [Cu^I/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 halides by Cu^I/Me₆TREN in aqueous media occurs 8 orders of magnitude faster than disproportionation