Electrochemical Atom Transfer Radical Polymerization in Miniemulsion with a Dual Catalytic System

An electrochemical approach was used to control atom transfer radical polymerization (ATRP) of <i>n</i>-butyl acrylate (BA) in miniemulsion. Electropolymerization required a dual catalytic system, composed of an aqueous phase catalyst and an organic phase catalyst. This allowed shuttling the electrochemical stimulus from the working electrode (WE) to the continuous aqueous phase and to the dispersed monomer droplets. As aqueous phase catalysts, the hydrophilic Cu complexes with the ligands <i>N</i>,<i>N</i>-bis­(2-pyridyl­methyl)-2-hydroxy­ethylamine (BPMEA), 2,2′-bipyridine (bpy), and tris­(2-pyridyl­methyl)­amine (TPMA) were tested. As organic phase catalysts, the hydrophobic complexes with the ligands bis­(2-pyridylmethyl)­octadecylamine (BPMODA) and bis­[2-(4-methoxy-3,5-dimethyl)­pyridylmethyl]­octadecylamine (BPMODA*) were evaluated. Highest rates and best control of BA electropolymerization were obtained with the water-soluble Cu/BPMEA used in combination with the oil-soluble Cu/BPMODA*. The polymerization rate could be further enhanced by changing the potential applied at the WE. Differently from traditional ATRP systems, reactivity of the dual catalytic system did not depend on the redox potential of the catalysts but instead depended on the hydrophobicity and partition coefficient of the aqueous phase catalyst

Direct ATRP of Methacrylic Acid with Iron-Porphyrin Based Catalysts

An iron porphyrin catalyst, derived from the active center of proteins such as horseradish peroxidase and hemoglobin, was successfully used for the atom transfer radical polymerizations (ATRP) of methacrylic acid. ATRP of methacrylic acid and other acidic monomers is challenging due to Cu complexation by carboxylates, protonation of the ligand, and displacement of the halogen chain end. A robust mesohemin-based catalyst provided controlled ATRP of methacrylic acid, yielding poly­(methacrylic acid) with <i>M</i>_n ≥ 20000 and dispersity <i>Đ</i> < 1.5. Retention of halogen chain end was confirmed by successful chain extension of a poly­(methacrylic acid)–Br macroinitiator

Electrochemically Mediated Reversible Addition–Fragmentation Chain-Transfer Polymerization

An electrochemically mediated reversible addition–fragmentation chain-transfer polymerization (<i>e</i>RAFT) of (meth)­acrylates was successfully carried out via electroreduction of either benzoyl peroxide (BPO) or 4-bromobenzene­diazonium tetrafluoroborate (BrPhN₂⁺) which formed aryl radicals, acting as initiators for RAFT polymerization. Direct electroreduction of chain transfer agents was unsuccessful since it resulted in the formation of carbanions by a two-electron-transfer process. Reduction of BrPhN₂⁺ under a fixed potential showed acceptable control but limited conversion due to the generation of a passivating organic layer grafted on the working electrode surface. However, by use of fixed current conditions, easier to implement than fixed potential conditions, conversions >80% were achieved. Well-defined homopolymers and block copolymers with a broad range of targeted degrees of polymerization were prepared

Synthesis and Characterization of the Most Active Copper ATRP Catalyst Based on Tris[(4-dimethylaminopyridyl)methyl]amine

The tris­[(4-dimethylaminopyridyl)­methyl]­amine (TPMA^NMe2) as a ligand for copper-catalyzed atom transfer radical polymerization (ATRP) is reported. In solution, the [Cu^I(TPMA^NMe2)­Br] complex shows fluxionality by variable-temperature NMR, indicating rapid ligand exchange. In the solid state, the [Cu^II(TPMA^NMe2)­Br]­[Br] complex exhibits a slightly distorted trigonal bipyramidal geometry (τ = 0.89). The UV–vis spectrum of [Cu^II(TPMA^NMe2)­Br]⁺ salts is similar to those of other pyridine-based ATRP catalysts. Electrochemical studies of [Cu­(TPMA^NMe2)]²⁺ and [Cu­(TPMA^NMe2)­Br]⁺ showed highly negative redox potentials (<i>E</i>_1/2 = −302 and −554 mV vs SCE, respectively), suggesting unprecedented ATRP catalytic activity. Cyclic voltammetry (CV) in the presence of methyl 2-bromopropionate (MBrP; acrylate mimic) was used to determine activation rate constant <i>k</i>_a = 1.1 × 10⁶ M^–1 s^–1, confirming the extremely high catalyst reactivity. In the presence of the more active ethyl α-bromoisobutyrate (EBiB; methacrylate mimic), total catalysis was observed and an activation rate constant <i>k</i>_a = 7.2 × 10⁶ M^–1 s^–1 was calculated with values of <i>K</i>_ATRP ≈ 1. ATRP of methyl acrylate showed a well-controlled polymerization using as little as 10 ppm of catalyst relative to monomer, while side reactions such as Cu^I-catalyzed radical termination (CRT) could be suppressed due to the low concentration of L/Cu^I at a steady state

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

Enhancing Mechanically Induced ATRP by Promoting Interfacial Electron Transfer from Piezoelectric Nanoparticles to Cu Catalysts

A robust mechanically controlled atom transfer radical polymerization (mechano-ATRP) was developed by enhancing the interaction between piezoelectric nanoparticles and ATRP Cu catalysts. The interactions favor a mechano-induced electron transfer from the surface of the nanoparticles to the deactivator Cu^II/L complex under ultrasonic agitation, promoting the formation of the activator Cu^I/L complex, thereby increasing the rate of the polymerization. This mechano-ATRP was carried out with a low loading of zinc oxide nanoparticles, providing a polymer with high end-group fidelity, predetermined molecular weight, and low dispersity. Propagation of the polymer chains was switched on/off in response to the ultrasound. The effects of the nature of the nanoparticle, nanoparticle loading, and targeted degrees of polymerization were investigated to evaluate the mechanism of mechano-ATRP

Benefits of Catalyzed Radical Termination: High-Yield Synthesis of Polyacrylate Molecular Bottlebrushes without Gelation

Catalyzed radical termination (CRT) in atom transfer radical polymerization (ATRP) of acrylates is usually considered as an unfavorable side reaction, as it accelerates termination and decreases chain-end functionality. CRT proceeds via a L/Cu^II–P_n organometallic intermediate and results in saturated chain-ends. Thus, CRT can help to suppress gelation in the synthesis of densely grafted poly­(<i>n</i>-butyl acrylate) molecular bottlebrushes using the “grafting-from” method by decreasing the fraction of chains terminated by conventional bimolecular radical combination. Molecular bottlebrushes by ATRP are typically prepared slowly in low yield and to limited monomer conversion to prevent radical combination, cross-linking, and gelation. Under conditions promoting CRT with highly active ATRP catalysts, a relatively high monomer conversion (>70%) was achieved without macroscopic gelation. CRT was favored using conditions that favored the formation of the L/Cu^II–P_n intermediate such as lower temperature and higher concentration of increasingly more active L/Cu^I catalysts. These conditions were beneficial for the fast and high-yield synthesis of polyacrylate molecular bottlebrushes, since they reduced the fraction of chains terminated by combination and prevented cross-linking of molecular bottlebrushes. High grafting density (>85%) and wormlike structures of molecular bottlebrushes were confirmed by side-chain cleavage and by molecular imaging via atomic force microscopy (AFM), respectively

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

Impact of Organometallic Intermediates on Copper-Catalyzed Atom Transfer Radical Polymerization

In atom transfer radical polymerization (ATRP), radicals (R^•) can react with Cu^I/L catalysts forming organometallic complexes, R–Cu^II/L (L = N-based ligand). R–Cu^II/L favors additional catalyzed radical termination (CRT) pathways, which should be understood and harnessed to tune the polymerization outcome. Therefore, the preparation of precise polymer architectures by ATRP depends on the stability and on the role of R–Cu^II/L intermediates. Herein, spectroscopic and electrochemical techniques were used to quantify the thermodynamic and kinetic parameters of the interactions between radicals and Cu catalysts. The effects of radical structure, catalyst structure and solvent nature were investigated. The stability of R–Cu^II/L depends on the radical-stabilizing group in the following order: cyano > ester > phenyl. Primary radicals form the most stable R–Cu^II/L species. Overall, the stability of R–Cu^II/L does not significantly depend on the electronic properties of the ligand, contrary to the ATRP activity. Under typical ATRP conditions, the R–Cu^II/L build-up and the CRT contribution may be suppressed by using more ATRP-active catalysts or solvents that promote a higher ATRP activity