Recent mechanistic developments in atom transfer radical polymerization

doi:10.1016/j.molcata.2006.01.076

Journal of Molecular Catalysis A: Chemical

Volume 254, Issues 1–2, 19 July 2006, Pages 155-164

https://doi.org/10.1016/j.molcata.2006.01.076 Get rights and content

Abstract

Recent mechanistic innovations concerning catalyst development in atom transfer radical polymerization (ATRP) are described. The following topics will be discussed: structure–reactivity relationships of the polymerization catalyst, including correlating reaction parameters with catalyst, alkyl halide, and monomer structure; concurrent reactions that may occur during ATRP and will affect its efficiency, including oxidation/reduction of radicals to radical cations/anions, solvent, monomer, and radical coordination to the active catalyst, and side reactions particular to aqueous media. In addition, novel methods of fine tuning initiation, activation, and deactivation processes, including simultaneous reverse and normal initiation ATRP, activators generated by electron transfer ATRP, hybrid catalyst systems, and bimetallic ATRP will be presented.

Graphical abstract

Recent mechanistic innovations concerning catalyst development in ATRP are discussed and include the finer components of the ATRP equilibrium, concurrent reactions that may occur during ATRP and will affect its efficiency, and novel methods for fine tuning catalyst systems to alleviate handling problems and enhance efficiency.

Introduction

Recent developments in controlled/“living” radical polymerization (CRP) processes have resulted in unprecedented control over the synthesis of many new well-defined (co)polymers with predictable molecular weights and narrow molecular weight distributions [1], [2]. Among the available CRP techniques, atom transfer radical polymerization (ATRP) has proven particularly invaluable as a synthetic tool [3], [4], [5]. In addition to the extraordinary control that this technique has provided over polymeric materials with a plethora of topologies, compositions, microstructures, and functionalities [6], [7], [8], [9], precise supramolecular control has been realized with ATRP that has led to the self-organization of many copolymers into regular nano-structured morphologies that in turn affects the macroscopic properties of these materials [10], [11], [12], [13].

The basic working mechanism of ATRP involves homolytic cleavage of an alkyl halide bond R single bond X by a transition metal complex Mtⁿ to generate (with a rate constant k_act) the corresponding higher oxidation state metal halide complex Mtⁿ⁺¹X and an alkyl radical R radical dot (Scheme 1) [14], [15]. R can then propagate with a vinyl monomer (k_p), terminate by either coupling or disproportionation (k_t) [16], or be reversibly deactivated in this equilibrium by Mtⁿ⁺¹X (k_deact). Radical termination is diminished as a result of the persistent radical effect [17] that ultimately strongly shifts the equilibrium towards the dormant species (k_act ≪ k_deact).

The efficient ATRP catalyst consists of a transition metal species which can expand its coordination sphere and increase its oxidation number, a complexing ligand, and a counterion which can form a covalent or ionic bond with the metal center. ATRP has been successfully mediated by a variety of metals, including those from Groups 4 (Ti [18]), 6 (Mo [19], [20], [21]), 7 (Re [22]), 8 (Fe [23], [24], [25], [26], Ru [27], [28], Os [29]), 9 (Rh [30], Co [31]), 10 (Ni [32], [33], Pd [34]), and 11 (Cu [3], [14]). Cu has proven by far the most efficient metal as determined by the successful application of its complexes as catalysts in the ATRP of a broad range of monomers in diverse media. Nitrogen-based ligands that are commonly used in conjunction with Cu include derivatives of bidentate bipyridine (bpy) [3], [35] and phenanthroline (phen) [36], tridentate diethylenetriamine (DETA) [37] and terpyridine (tpy) [38], and tetradentate tris[2-aminoethyl]amine (TREN) [39], tetraazacyclotetradecane (CYCLAM) [40] and other branched multidentate ligands [41], [42] (Fig. 1).

Control over polymerization molecular weight and molecular weight distribution in all CRP techniques is established through a dynamic equilibrium between dormant species and propagating radicals. One advantage of ATRP over other CRP processes is that this equilibrium can be easily and appropriately adjusted for a given system by modifying the complexing ligand of the catalyst [14]. In this way, control has been established over the polymerization of a wide variety of monomers, including styrenics [43], (meth)acrylates [44], [45], [46], [47], acrylonitrile [48], [49], acrylamides [46], [50] and others [14], over a broad range of temperatures.

This review will focus on mechanistic innovations concerning catalyst development in ATRP and will highlight those efforts which have optimized the overall catalytic process. Several facets are considered: finer components of the ATRP equilibrium; concurrent reactions that may occur during ATRP and will affect its efficiency; novel methods for fine tuning catalyst systems to alleviate handling problems and enhance efficiency.

Section snippets

Confirming the radical nature of ATRP

ATRP originates from and is mechanistically similar to atom transfer radical addition (ATRA), a widely used reaction in organic synthesis [51], [52]. ATRA exploits atom transfer from an organic halide to a transition metal complex to generate reacting radicals, followed by back-transfer of the atom from the transition metal to the product radical species. Although the most plausible mechanism for this reaction based upon experimental evidence involves free radicals, it has been debated as to

Conclusions

Most ATRP research is commercially, environmentally and/or economically driven. Much research has been done to correlate reaction parameters with catalyst, alkyl halide, and monomer structure, identify concurrent reactions that may occur during ATRP and will affect its efficiency, and fine tune methods of initiation, activation, and deactivation in ATRP. But whatever the motivation or incentive – whether it be developing more powerful catalysts that allow the polymerization of less reactive

Acknowledgements

The financial support from the Carnegie Mellon University CRP Consortium and the NSF grant (CHE-0405627) are greatly appreciated.

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Control over the molecular weight and molecular weight distribution (MWDs) in all controlled radical polymerization (CRP) techniques was established through dynamic equilibrium between the dormant species and propagating radicals. Thus far, a wide range of catalysts have been used to catalyze ATRP, including the most frequently used systems based on copper [6–12], ruthenium [13–16] and iron [17–27]. Among the catalyst systems reported thus far, iron-based catalyst systems are particularly attractive owing to their relative non-toxicity, potential activity, biocompatibility, and relatively low cost.
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An example where the linear polymer is directly employed in the cyclization reaction is a dianionic vinyl polymer being joined by consecutive SN2 reactions with a bifunctional electrophile, the second SN2 reaction closing the ring [4,7,19–22] (see Scheme 1). Atom transfer radical polymerization (ATRP) and the closely related atom transfer radical coupling (ATRC) offer chemists an easy route to well defined vinyl polymers [23–32]. While mechanistically similar to each other, the conditions of the ATRC reaction are altered to favor higher concentrations of polymer radicals to induce coupling, which is an undesired termination step in an ATRP system (Scheme 2) [29,33,34].
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However, one of the demerits in ATRP technique is the use of transition metal catalysts, which often exhibit high toxicity. Recently, several novel catalytic systems for ATRP have been developed, for example, by using Cu (II) species and reducing agents such as tin (II) ethylhexanoate and ascorbic acid (so-called activators regenerated by electron transfer (ARGET) ATRP) [4,7–9]. Although the use of reducing agents allows the ATRP systems initially started from the oxidatively stable Cu (II) species and diminishes their amounts because the reducing agents constantly regenerate active Cu (I) species from the stable Cu (II) species, the presence of the metal catalyst is still demanded.
In this paper, we report atom transfer radical polymerization of N-isopropylacrylamide (NIPAAm) by an enzyme mimetic catalyst, that is, hematin of a ferriprotoporphyrin complex. The hematin-catalyzed polymerization of NIPAAm was carried out in the presence of an alkyl halide as an initiator and a reducing agent in DMF/H₂O solvent. The reaction proceeded at room temperature and the M_ns increased with increasing monomer conversions. The M_n values were relatively close to the theoretical ones. The reaction did not take place in the absence of any one of the initiator, catalyst, and reducing agent. The ¹H NMR spectrum of the low molecular weight polyNIPAAm indicated the presence of the moieties from the initiator at the polymer ends. These results suggested that the reaction was initiated from radical species, which were produced by bromide abstraction from the initiator by the reduced catalyst.
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It is important to note that the ternary complex [XCuIL] is not a good activator even though on thermodynamic grounds it is a stronger reducing catalyst than [CuIL]+ [11]. The mechanism of carbon-halogen bond activation by Cu(I) complexes has been extensively investigated in relation to the initiation step in ATRP [13–20]. It is believed that in ATRP the halogen atom transfer from RX to Cu(I) occurs by an inner-sphere electron transfer (ISET) process involving a transition state in which RX and Cu(I) are bound through a halogen bridge (Scheme 2).
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Since its discovery in 1994, the transition metal-catalyzed atom transfer controlled/living radical polymerization has been developed over two decades by a number of catalysts predominantly consisting of late transition metals (Groups 8–11). The underlying notion for this living process is judicious choice of central metals and their ligands, which activate dormant carbon–halogen covalent bonds originating from the initiator reversibly into growing radical via one-electron redox of the transition metal catalysts, to suppress bimolecular termination reactions inherent in free radical propagation. Now that transition metal-catalyzed atom transfer controlled/living radical polymerization has been developed for a wide range of monomers, many researchers have adopted them as a tool for preparing well-defined structure polymers. This chapter discusses the up-to-date status of the transition metal catalysts for the controlled/living radical polymerizations, especially other than copper, which is discussed in another chapter of this book. The present review covers the following topics with 398 references with respect to each central metal: (a) scope of mechanism: catalysts, initiators, and monomers; (b) catalyst design: metal complexes, ligands, additives, and initiators; and (c) processes: to improve the controllability of polymerization. The metal catalysts are classified by the groups in the periodic table including not only late (Groups 8–11) but also early transition metals (Groups 3–7), among which Group 8 metals (Ru and Fe) are specifically mentioned in detail. The relationships between the structure and reactivity of the catalysts are described as well as the versatility of applicable monomers and experimental conditions.
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Recent mechanistic developments in atom transfer radical polymerization

Abstract

Graphical abstract

Introduction

Section snippets

Confirming the radical nature of ATRP

Conclusions

Acknowledgements

Prog. Polym. Sci.

Prog. Polym. Sci.

Prog. Polym. Sci.

Prog. Polym. Sci.

Prog. Polym. Sci.

J. Organomet. Chem.

Polyhedron

Polymer

Prog. Polym. Sci.

Prog. Polym. Sci.

Coord. Chem. Rev.

J. Organomet. Chem.

Handbook of Radical Polymerization

Am. Chem. Soc.

Acc. Chem. Res.

Chem. Eur. J.

Chem. Mater.

J. Mater. Chem.

Macromol. Symp.

Eur. Phys. J. E: Soft Matter

Chem. Rev.

Chem. Rev.

Chem. Rev.

Polym. Bull.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Macromolecules

Macromolecules

Macromolecules

Macromolecules

Macromolecules

Angew. Chem. Int. Ed.

Macromolecules

Macromolecules

Macromolecules

Macromolecules

Macromolecules

Macromolecules

Science

Macromol. Rapid Commun.

Macromolecules

Macromol. Rapid Commun.

Macromolecules

Macromolecules

ACS Symp. Ser.

Macromolecules

Macromolecules

Macromolecules

Macromolecules