The ATRP Equilibrium

ATRP can be approached from both sides of the equilibrium, that is, beginning from an alkyl halide and a low oxidation state metal, or from a radical and the higher oxidation state metal this latter approach is termed reverse ATRP (rATRP) [81,189,190]. Qiu et al. used this technique to prepare block copolymers, also of MMA and St [ 191 ]. They used a hexasubstituted ethane thermal in-iferter, diethyl 2,3-dicyano-2,3-di (p-tolyl)succinate, which decomposes reversibly to form two radicals when heated. The new radical is either deactivated by the CuCl2/bpy complex or adds MMA monomer, followed by deactivation, both of which will produce the dormant species in the ATRP equilibrium. The rATRP... 

ATRP is the most versatile CRP technique and has already attracted considerable interest from industry.(13) It relies on establishing an equilibrium between an alkyl halide initiator (RX, X = Br, Cl) and a low oxidation state metal complex, generating a radical and a higher oxidation state complex with a coordinated halide ion. The atom transfer equilibrium is presented schematically on the left hand side of Figure 1.(14) The ATRP equilibrium constant, which is the ratio of the rate constants of RX activation and of radical deactivation, can be formally presented as a product of several contributing equilibrium constants shown in Figure 1 and eq. 1. 

Another component of the ATRP equilibrium constant, the halidophilicity, also depends upon the nature of the transferable atom (and the ligand). In systems where the halidophilieity is low, the XCu Ln eomplex can easily dissociate to halide ions and Cu°L whieh cannot deactivate radieals, and the dissociative loss of deaetivator ultimately leads to fast reaetions (eq. 2) that are poorly controlled (eq. 3). It has been shown that halidophilieity is low in protic media, and it decreases signifieantly in mixed organie-aqueous solvents as the amount of water is inereased. ("25,24) The amount of deaetivator present in the reaction system depends on the halidophilieity and on the total eoneentrations of Cu species and halide ions, aeeording to eq. 4. Therefore, to minimize the dissociation of deaetivator in systems where the halidophilieity is low, extra Cu eomplex and / or halide salts should be added. This has been shown to improve the polymerization control.("25,25,2d) Halidophilicity is discussed in more detail in the following sections. 

To verify the model presented in this paper, it is interesting to calculate the values of Katrp and compare them with the experimental values. For instance, the values of the contributing equilibrium constants to the ATRP equilibrium between MeBriB and CuBr / bpy in a mixed solvent MeCN-H20 (9 1 v/v) are logKBH = -38.96 (value in pure MeCN),(i6) IngKeromido = 3.70,(2<) logKeA =... 

A detailed model is provided and justified that represents the ATRP equilibrium constant as a combination of contributing equilibria C-X bond homolysis, electron affinity of the halogen atom, reduction of Cu, and the... 

The following discussion highlights facets of the ATRP equilibrium that should be considered when conducting polymerization with the catalyst at very low concentration. After a brief discussion of the factors that determine this equilibrium, particular emphasis is paid to concurrent reactions that may occur during ATRP and will affect the efficiency of the technique, including dissociation of the catalyst, competitive complexation reactions, and equilibria involving electron transfer. Cu-mediated ATRP is described here but most of the concepts can be applied to polymerizations catalyzed by other metal complexes. 

For convenience, it has been proposed that the ATRP equilibrium constant (represented in Scheme 1 as Kj trp = oJ kdeact) is expressed as a combination of four reversible reactions oxidation of the metal complex, or electron transfer (Ket), reduction of a halogen to a hahde ion, or electron affinity (Kb/, alkyl halide bond homolysis (Kbh), and association of the halide ion to the metal complex, or halogenophilicity (Xx) (Scheme... 

The relationship between several of these individual reactions and the overall position of the ATRP equilibrium has been clearly demonstrated in several studies. Alkyl hahde bond dissociation energies for... 

Several of the side reactions encountered in ATRP, such as catalyst dissociation and competitive monomer complexation, become more pronounced when the catalyst is used at very low concentration. These and other undesirable reactions, such as catalyst disproportionation or radical coordination to the metal center, can often be avoided with the appropriate choice of transition metal and complexing ligands. Still other side reactions, such as electron transfer between alkyl radicals and the metal catalysts, can actually be minimized by using low catalyst concentrations. This work aimed to demonstrate that with a thorough knowledge of the components of the ATRP equilibrium and a general awareness of potential side reactions under certain conditions, ATRP catalysts can be rationally selected and conditions optimized for very diverse polymerization systems. 

The general meehanism of ATRP has been shown in Fig. 11.16, which indicates a radical pathway. The radical nature of the reaetive or propagating species in eopper-mediated ATRP is proposed on the basis of several experimental observations, which have been sutmnarized by Matyjaszewski and Xia (2001) as (1) The ATRP equilibrium ean be approached from both sides, i.e., either... 

The control of the polymerization reaction afforded by ATRP is the result of the formation of dormant alkyl (pseudo)halides. This reduces the instantaneous concentration of the active radicals and thereby suppresses bimolecular termination reactions. The reversible deactivation and activation leads to a slow, but steady growth of the polymer chain with a well defined end group (Scheme 27). Control and properties of the synthesized polymers depend on the stationary concentration of active radicals and the relative rates of propagation and deactivation. When one or less than one monomer unit is incorporated into the polymer chain during one activation step, the polymerization is well controlled. The ATRP equilibrium can be approached from both directions in Scheme 27. Beginning with an alkyl halide and the lower valent metal complex, the process is called direct ATRP. If a conventional thermal initiator like AIBN and the higher valent metal complex are the starting materials, the polymerization process is named reverse ATRP [287]. 

Scheme 2 Representation of the ATRP equilibrium (Note /fact < Adeact)-...

The rate of polymerization is ultimately governed by the position of the ATRP equilibrium, as illustrated in eqn [1] for a particular monomer, M. 

Tliere are presently several ways to set up the ATRP equilibrium, shown in Scheme 2, and they will now be addressed in greater detail. 

In 1995, it was established that the ATRP equilibrium can be approached from both sides ... 

The ATRP equilibrium was shown in Scheme 2. One approach to developing a deeper understanding of the overall ATRP equilibrium is to examine the reactions that formally contribute to the overall equilibrium ... 

The ATRP equilibrium (Katrp in Scheme 7) can be expressed as a combination of several formally contributing reversible reactions. The ATRP equilibrium can be defined as the product of the equilibrium constants describing homolytic bond dissociation of R-X and Gu -X, that is, the halogenophulidty of the initiator and deactivator. Thus, Kbh describes equilibrium... 

The ATRP process is characterized by the ATRP equilibrium constant, which can be defined as either the ratio of the rate constants of activation and deactivation or as the ratio of concentrations of all involved species. 

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