Current status and future perspectives in atom transfer radical polymerization (ATRP) are presented. Special emphasis is placed on mechanistic understanding of ATRP, recent synthetic and process development, and new controlled polymer architectures enabled by ATRP. New hybrid materials based on organic/inorganic systems and natural/synthetic polymers are presented. Some current and forthcoming applications are described.

Atom Transfer Radical Polymerization (ATRP): Current Status and Future Perspectives

This Perspective presents recent advances in macromolecular engineering enabled by ATRP. They include the fundamental mechanistic and synthetic features of ATRP with emphasis on various catalytic/initiation systems that use parts-per-million concentrations of Cu catalysts and can be run in environmentally friendly media, e.g., water. The roles of the major components of ATRP—monomers, initiators, catalysts, and various additives—are explained, and their reactivity and structure are correlated. The effects of media and external stimuli on polymerization rates and control are presented. Some examples of precisely controlled elements of macromolecular architecture, such as chain uniformity, composition, topology, and functionality, are discussed. Syntheses of polymers with complex architecture, various hybrids, and bioconjugates are illustrated. Examples of current and forthcoming applications of ATRP are covered. Future challenges and perspectives for macromolecular engineering by ATRP are discussed.

Macromolecular Engineering by Atom Transfer Radical Polymerization

The use of light to mediate controlled radical polymerization has emerged as a powerful strategy for rational polymer synthesis and advanced materials fabrication. This review provides a comprehensive survey of photocontrolled, living radical polymerizations (photo-CRPs). From the perspective of mechanism, all known photo-CRPs are divided into either (1) intramolecular photochemical processes or (2) photoredox processes. Within these mechanistic regimes, a large number of methods are summarized and further classified into subcategories based on the specific reagents, catalysts, etc., involved. To provide a clear understanding of each subcategory, reaction mechanisms are discussed. In addition, applications of photo-CRP reported so far, which include surface fabrication, particle preparation, photoresponsive gel design, and continuous flow technology, are summarized. We hope this review will not only provide informative knowledge to researchers in this field but also stimulate new ideas and applications to further advance photocontrolled reactions.

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.5b00671

Light-Controlled Radical Polymerization: Mechanisms, Methods, and Applications

Overcoming the challenge of metal contamination in traditional ATRP systems, a metal-free ATRP process, mediated by light and catalyzed by an organic-based photoredox catalyst, is reported. Polymerization of vinyl monomers are efficiently activated and deactivated with light leading to excellent control over the molecular weight, polydispersity, and chain ends of the resulting polymers. Significantly, block copolymer formation was facile and could be combined with other controlled radical processes leading to structural and synthetic versatility. We believe that these new organic-based photoredox catalysts will enable new applications for controlled radical polymerizations and also be of further value in both small molecule and polymer chemistry.

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Metal-Free Atom Transfer Radical Polymerization

Controlled/living radical polymerization techniques have transformed polymer chemistry in the last few decades, affording the production of polymers with precise control over both molecular weights and architectures. It is now possible to synthesize almost an infinite variety of macromolecules using nonspecialized equipment, finding applications in high-tech industry. However, they have several shortcomings. Until recently, living radical polymerizations could not be controlled by an external stimulus, such as visible light, pH, mechanical, chemical, etc. Moreover, they are usually sensitive to trace amounts of oxygen in the system. In this Article, we report a photoinduced living polymerization technique, which is able to polymerize a large range of monomers, including conjugated and unconjugated monomers, using ultralow concentrations of an iridium-based photoredox catalyst (typically 1 ppm to monomers) and a low energy visible LED as the light source (1–4.8 W, λmax = 435 nm). The synthesis of homopolym...

A Robust and Versatile Photoinduced Living Polymerization of Conjugated and Unconjugated Monomers and Its Oxygen Tolerance

Electrochemical reduction of an exemplary ATRP catalyst, C 11 Br 2 /Me 6 TREN, is shown to be an efficient process to mediate and execute an ATRP. The onset of polymerization occurs only through passage of a cathodic current achieved under a reductive potential to form Cu 1 Br 2 /Me 6 TREN, within the reaction medium. Unprecedented control over the polymerization kinetics can be attained through electrochemical methods by modulating the magnitude of the applied potential allowing polymerization rate enhancement or retardation. Additional polymerization control is gained through electrochemical “dials” allowing polymerization rate enhancements achieved by larger applied potentials and the ability to successfully switch a polymerization “on” and “off between dormant and active states by application of multistep intermittent potentials.

https://science.sciencemag.org/content/sci/332/6025/81.full.pdf

Electrochemically mediated atom transfer radical polymerization

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.

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

Electrochemically mediated atom transfer radical polymerization (eATRP) of n-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 eATRP process. Additional electrochemical methods were explored to improve the feasibility of eATRP 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.

Investigation of Electrochemically Mediated Atom Transfer Radical Polymerization

The thermodynamic properties of some copper complexes, among those frequently used as catalysts in controlled/living radical polymerization, has been studied in CH3CN + 0.1 M (C2H5)4NBF4. A combination of different techniques, namely potentiometry, spectrophotometry and cyclic voltammetry, has been used to determine the stability constants of all possible complexes of CuI and CuII present in binary and ternary systems composed of CuI or CuII, a halide ion (X = Cl−, Br−) and a polyamine ligand (L = pentamethyldiethylenetriamine, tris(2-dimethylaminoethyl)amine). The binary Cu−X systems show only mononuclear CuXx complexes, where x = 1, 2, 3, 4 for CuII, and x = 1, 2 for CuI. Conversely, in the case of the binary Cu−L systems, besides the mononuclear complexes CuLl, where l = 1 or 2, also dinuclear complexes Cu2L were found. The ternary systems give rise to a mixture of mononuclear and dinuclear complexes of general formula CumLlXx. Besides the 1:1:1 complex obtained in all combinations, the following speci...

Nicola Bortolamei

Papers

Electrochemically mediated atom transfer radical polymerization

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

Investigation of Electrochemically Mediated Atom Transfer Radical Polymerization

Thermodynamic Properties of Copper Complexes Used as Catalysts in Atom Transfer Radical Polymerization

New insights into the mechanism of activation of atom transfer radical polymerization by Cu(I) complexes.