Activation/deactivation equilibrium

The reversible chain transfer process (c) is different in that ideally radicals are neither destroyed nor formed in the activation-deactivation equilibrium. This is simply a process for equilibrating living and dormant species. Radicals to maintain the process must be generated by an added initiator. 

A wide range of nitroxidcs and derived alkoxyamincs has now been explored for application in NMP. Experimental work and theoretical studies have been carried out to establish structure-property correlations and provide further understanding of the kinetics and mechanism. Important parameters are the value of the activation-deactivation equilibrium constant K and the values of kaa and (Scheme 9.17), the combination disproportionation ratio for the reaction of the nilroxide with Ihe propagating radical (Section 9.3.6.3) and the intrinsic stability of the nitroxide and the alkoxyamine under the polymerization conditions (Section 9.3.6.4). The values of K, k3Cl and ktieact are influenced by several factors.11-1 "7-"9 ... 

The activity of initiators in ATRP is often judged qualitatively from the dispersity of the polymer product, the precision of molecular weight control and the observed rates of polymerization. Rates of initiator consumption are dependent on the value of the activation-deactivation equilibrium constant (A") and not simply on the activation rate constant ( acl). Rate constants and activation parameters are becoming available and some valuable trends for the dependence of these on initiator structure have been established.292"297... 

Optimal conditions for ATRP depend strongly on the particular monomer(s) to be polymerized. This is mainly due to the strong dependence of the activation-deactivation equilibrium constant (A ), and hence the rate of initiation, on the type of propagating radical (Section 9.4.1.3). When using monomers of different types, polymer isolation and changes in the catalyst are frequently necessary before making the second block... 

This redefinition establishes the effective activation/deactivation equilibrium constant K=k k 2 = k lk2. (Note that in the cellular automata models, the rate constants k are expressed as transition probabilities per iteration Pi) Using the above redefinition, the mechanism of Eq. (9.1) becomes a set of first-order reactions... 

In thermal reactions, the reaction rate depends on the activation-deactivation equilibrium and on the decomposition rate of the activated complex ... 

ATRP a transition metal complex is needed for the activation of the alkyl-halide-ended macromolecules, and a wider range of temperatures can be applied. In both cases, the polymerization kinetics are governed by the activation-deactivation equilibrium and by the persistent radical effect [6]. The number-average degree of polymerization DP ) is calculated by the ratio of the initial monomer concentration to the initiator (i.e., alkoxyamine or alkyl halide) concentration, multiplied by monomer conversion. 

In NMP, the nitroxide deactivator should be sufficiently oil-soluble to remain within the particles and participate in the activation-deactivation equilibrium. In case of favorable partitioning towards the aqueous phase or chemical degradation due to side reactions, an increase in the polymerization rate is observed at the expense of the molar mass distribution, which broadens. In ATRP, the transition metal complexes (mainly copper-based activator and deactivator) should be stable enough in the presence of water and should not interact with the various components of... 

Klumperman and coworkers [259] observed that while it is lately quite common to treat living radical copolymerization as being completely analogous to its radical counterpart, small deviatiOTis in the copolymerization behavior do occur. They interpret the deviations on the basis of the reactions being specific to controlled/living radical polymerization, such as activation—deactivation equilibrium in ATRP. They observed that reactivity ratios obtained from atom transfer radical copolymerization data, interpreted according to the conventional terminal model deviate from the true reactivity ratios of the propagating radicals. 

Theoretical expression of the average activation-deactivation equilibrium constant in controHed/living free-radical copolymerization operating via reversible termination. Application to a strongly improved control in nitroxide-mediated polymerization of methyl methacrylate, Macromolecules 2005, 38, 5485-5492. 

Figure 2 Activation-deactivation equilibrium in nitroxide-mediated polymerization, (a) Bicomponent initiating system and (b) monocomponent initiating system.

Nevertheless, it must be noted that the equilibrium exists providing that the activation-deactivation equilibrium constant, K, obeys eqn [5] ... 

These equations, and in particular eqn [6], could then be used to determine the rate coefficients such as the activation-deactivation equilibrium constant. Nevertheless, this was experimentally difficult to obtain when high conversions are reached. This is based on the inaccurate assumption that the initiator concentration (RY or dormant species in polymerization) does not change as the reaction proceeds. Tang et then derived new equations to take into accoimt the consumption of the initiator. 

Kinetic and theoretical studies The nitroxide-mediated copolymerization was far less studied than the homopolymerization although a large number of polymers produced via a radical polymerization mechanism are actually random copolymers. Early kinetic and mechanistic studies were published by Zaremski et al. for the TEMPO-mediated copolymerization of styrene with various comonomers. They discussed various regimes depending on the ability or disability of the second monomer to undergo a controlled/living NMP and determined experimentally the activation-deactivation equilibrium constants for many of those systems. 

In a more recent study, Charleux et studied the theoretical features of the activation-deactivation equilibrium in nitroxide-mediated copolymerization and applied it to the SGl-mediated copolymerization of methyl methacrylate with a low percentage of styrene (typically in the 4-9 mol.% range). They actually demonstrated that the system exhibited all the characteristics of a living/controlled polymerization, which was explained by the following features (1) the overall concentration of propagating radicals was strongly reduced by the copolymerization effect and the irreversible termination reactions undergone by the MMA/SGf system were hence slowed down (2) isolated styrene subunits were incorporated into the chains and the terminal one promoted the reversible deactivation by the SGI nitroxide and (3) the MMA penultimate unit effect enhanced deactivation of the so-formed styryl-SGf... 

Both oil- and water-soluble initiators have been investigated in miniemulsion NMP with TEMPO and SGI. Bicomponent systems were the first to be applied as they only required the addition of a nitroxide to a classical radical initiator for the establishment of the activation-deactivation equilibrium. 

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