polymerizing styrene in the presence of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) radicals. [10] Later, Wayland et al. used a cobalt(II) porphyrin complex to reversibly trap radical species during the polymerization of acrylate monomers, which showed living features: i) linear increase of polymer MW with monomer conversion, and ii) narrow MWD. [11] These techniques set the basis for the development of nitroxide-mediated polymerization (NMP) [12,13] and organometallic mediated radical polymerization (OMRP). [14][15][16] Within the past three decades, controlled radical polymerization (CRP) has been established as a new field in polymer chemistry, in which exceptional control was achieved over polymer architectures, thus enabling the preparation of commercially relevant polymer-based materials for advanced applications. Following IUPAC recommendations, CRP should be termed as reversible deactivation radical polymerization (RDRP). [17] Besides the aforementioned NMP and OMRP, the most affirmed RDRPs are atom transfer radical polymerization (ATRP) [18][19][20] and reversible additionfragmentation chain transfer (RAFT) polymerization. [21] RDRP ensures comparable degree of control as living ionic polymerization, while retaining the versatility and the scope of conventional radical polymerization. The fraction of terminated chains in RDRP is small, typically below a few mol%. Polymers and copolymers prepared by RDRP methods can have defined topologies (stars, brushes, networks, combs), compositions (block, gradient, graft, alternate) and chain-end functionalities. [22] This review will focus on ATRP, with particular attention to the design and application of progressively more active and selective copper-based catalysts, and the development of novel, more benign initiation systems. The correlation between the structure of Cu complexes and their catalytic activity will be discussed in detail, also taking into account the effect of solvent and, when present, surfactants and other additives. Mechanisms of Reversible Deactivation Radical Polymerization ProcessesThe core of all RDRP systems is the increase of chain lifetime by reversibly deactivating the propagating radical species, thus forming dormant species that can be subsequently reactivated. As opposed to conventional RP in which the 25 Years of ATRP Approaching 25 years since its invention, atom transfer radical polymerization (ATRP) is established as a powerful technique to prepare precisely defined polymeric materials. This perspective focuses on the relation between structure and activity of ATRP catalysts, and the consequent choice of the initiating system, which are paramount aspects to well-controlled polymerizations. The ATRP mechanism is discussed, including the effect of kinetic and thermodynamic parameters and side reactions affecting the catalyst. The coordination chemistry and activity of copper complexes used in ATRP are reviewed in chronological order, while emphasizing the structure-activity correlation. ATRP-initiating systems are described, from ...

In contrast with previous accounts, it is reported that a single, strongly hydrophilic Cu complex can control an electrochemically mediated atom transfer radical polymerization (eATRP) in oil-in-water miniemulsion in the presence of anionic surfactants, such as sodium dodecyl sulfate (SDS). The anionic surfactant interacted strongly with cationic copper complexes, enabling controlled polymerization by a combination of “interfacial” and “ion-pair” catalysis, whereby ion pairs transport the catalyst to the monomer droplets. The ion-pair system was assembled in situ by mixing commercially available reagents (NaBr, SDS, and traditional hydrophilic copper complexes). Polymer purification was very facile because after reaction >99% of the hydrophilic copper complexes spontaneously left the hydrophobic polymer particles.

Using the power of light to drive controlled radical polymerizations has provided significant advances in synthesis of well-defined polymers. Photoinduced atom transfer radical polymerization (ATRP) systems often employ UV light to regenerate copper activator species to mediate the polymerization. Taking full advantage of long-wavelength visible light for ATRP would require developing appropriate photocatalytic systems that engage in photoinduced electron transfer processes with the ATRP components to generate activating species. Herein, we developed conjugated microporous polymers (CMP) as heterogeneous photocatalysts to exploit the power of visible light in promoting copper-catalyzed ATRP. The photocatalyst was designed by cross-linking phenothiazine (PTZ) as a photoactive core in the presence of dimethoxybenzene as a cross-linker via the Friedel−Crafts reaction. The resulting PTZ-CMP network showed photoactivity in the visible region due to the extended conjugation throughout the network because of the aromatic groups connecting the PTZ units. Therefore, photoinduced copper-catalyzed ATRP was performed with CMPs that regenerated activator species under green or red light irradiation to start the ATRP process. This resulted in efficient polymerization of acrylate and methacrylate monomers with high conversion and wellcontrolled molecular weight. The heterogeneous nature of the photocatalyst enabled easy separation and efficient reusability in subsequent polymerizations.

Since its inception, atom transfer radical polymerization (ATRP) has seen continuous evolution in terms of the design of the catalyst and reaction conditions; today, it is one of the most useful techniques to prepare well-defined polymers as well as one of the most notable examples of catalysis in polymer chemistry. This Perspective highlights fundamental advances in the design of ATRP reactions and catalysts, focusing on the crucial role that mechanistic studies play in understanding, rationalizing, and predicting polymerization outcomes. A critical summary of traditional ATRP systems is provided first; we then focus on the most recent developments to improve catalyst selectivity, control polymerizations via external stimuli, and employ new photochemical or dual catalytic systems with an outlook to future research directions and open challenges.

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 n-butyl acrylate (BA) and n-butyl methacrylate (BMA) was carried out in miniemulsion using CuII/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.

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.

Atom transfer radical polymerization (ATRP) is a staple technique for the preparation of polymers with well‐defined architecture. In ATRP, the catalyst governs the equilibrium between propagating radicals and dormant species, thus affecting the polymerization control for a range of monomers and transferable atoms employed in the process. The design and the use of highly active catalysts could diminish the amount of transition metal complexes, extend ATRP to less active monomers and give access to new chain‐end functionalities. At the same time, very active catalysts can be involved in formation of organometallic species. Herein, the role of the catalyst on the ATRP equilibrium is carefully elucidated, together with recent observations on the impact of the catalyst nature on formation of organometallic species and relevant side reactions. Based on this knowledge, a perspective on the benefits and challenges that derive from the use of highly active ATRP catalysts is presented.

Depolymerization of vinyl polymers into monomers is energy-intensive due to the high thermal and chemical stability of the backbone. Depolymerizations of methacrylic polymers are typically conducted above the ceiling temperature and thermal degradation temperature to degrade polymers by bond scission. This work investigates the catalyzed depolymerization of a Clcapped poly(poly(dimethylsiloxane) methacrylate) (P-(PDMS 11 MA-Cl)) polymer mediated by an atom transfer radical polymerization catalyst: copper(II) chloride/tris(2-pyridylmethyl)amine (CuCl 2 /TPMA) at 170 °C. The depolymerization yield, rate, and selectivity were improved by increasing the ratio of [TPMA]/[CuCl 2 ]. Electron transfer from the ligand contributed to the Cu(I) activator (re)generation at high temperature (T > 130 °C), as proven by ultraviolet−visible spectroscopy. The bottlebrush could be partially depolymerized and repolymerized over a few cycles.