The persistent radical effect PRE

The use of this phenomenon to control carbon-carbon bond-forming reactions relies on R being rapidly converted into another transient radical which, in the case of a polymerisation, occurs by repetitive addition to a monomer double bond to give the propagating polymer radical, P Thus, the PRE prevents dead polymer (P—P ) formation and the dormant concentration of P—T remains effectively constant. It follows that the excess of T ensures that reversible termination and addition of P to monomer are dominant reactions allowing all polymer chains to grow practically simultaneously (Section 10.5.4). 

Alkoxyamines have been promoted as alternative initiators to metal hydrides in radical cyclisations since metal hydride reactions often lack regioselectivity and produce mixtures of reduced uncyclised and cyclised products. Further, the nitroxide portion of the alkoxyamine 

4 Nitroxide-mediated living/controlled radical polymerisations (NMP) 

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Scheme 10.10 The persistent radical effect (PRE) as a function of reversible and irreversible termination, i.e. cross-coupling and self-termination, respectively.

While these three systems possess different components, the general similarities between the CRP processes are in the use of initiators, radical mediators (i.e., persistent radicals or transfer agents), and in some cases, catalysts (Fig. 2). It is important to note that while SFRP/NMP and ATRP are subject to the Persistent Radical Effect (PRE) [56] the degenerative processes, such as RAFT, do not conform to the PRE model due to the transfer dominated nature of the reaction. 

In general, CRP techniques are based on either an SFR that exhibits the persistent radical effect (PRE) [52-55] (e.g., N in NMRP), an inactive species HalTp... 

The ACOMP continuously computed conversion data for experiments at 90 °C with different concentration of Cu Br (0,0.025, and 0.05 equiv vs. Cu Br) made possible to observe the persistent radical effect (PRE) kinetics at the early polymerization stages. 

The mentioned work of SFR to adjust the radical concentrations for a high preference of aoss-combination was originally recognized in the chemistry of low-mass compounds and termed the persistent radical effect (PRE). In the field of polymerization, such a work of X" was clearly recognized by Johnson et al. in their computer simulation work, and subsequently by Fukuda et al. and Greszta and Matyjaszewski, and clear experimental evidence for the inequality [X"] [P ] and the quasi-equilibrium in eqn [2] was first presented for a nitroxide-mediated LRP of styrene by Fukuda et al. Subsequently, Fischer made a detailed theoretical analysis of the PRE in polymerization. For more details about PRE, the readers are referred to the review by Fischer. ... 

NMP is controlled by the persistent radical effect (PRE) [6] that can be described as follows If we consider a diamagnetic compound (RY) that decomposes homolytically to generate two radical species at both the same time and the same rate, the PRE occurs when one of the radical species (Y) is more persistent than the other (R). Typically, at the early stages of the reaction, the concentrations of the... 

It is important to note that irreversible termination (kt,ac) of the C-radicals (R, RMi, ... RM ) also occurs in NMP. However, the extent of such termination reactions is small in comparison to reactions between C-radicals and R R NO which produce the desired dormant chains and eventually the living and controlled polymer R R NOM R. This surprising selectivity can be explained by the persistent radical effect (PRE), which is briefly treated in the next section of this chapter. 

The persistent radical effect (PRE) is a kinetic effect observed when transient and persistent radicals are formed with equal or similar rates. The cross-coupling of these radicals proceeds with high selectivity and dominates the homo-coupling of the transient radicals. The first example of PRE was most likely observed in 1936 by Bachmann and Wiselogle during experiments on thermolysis of pentaarylethanes. 

In the course of NMP, the persistent radical effect (PRE) leads to a steady increase in excess nitroxide. This slows the polymerization rate down and leads to longer polymerization times. As shown by Matyjaszewski, Fukuda and Miura/ introduction of a conventional radical initiator which slowly decomposes under the reaction conditions considerably enhances the conversion rate. Even a low rate of external initiation ( 1% of the initial internal initiation, reaction 1, Scheme 4.5) leads to a considerable reduction in the polymerization time while the livingness, polydispersity and controlled degree of polymerization remain virtually unchanged. The extra radicals reduce the concentration of persistent nitroxides rather than initiating new chains. The kinetic aspects of additional initiation were studied by Fukuda et alP and Fischer et In summary, if the rate R of generation of additional... 

In the group of Fischer, a considerable amount of work was dedicated to the kinetic description of NMP. They introduced the concept of the so-called persistent radical effect (PRE) (49,50). The PRE relies on the limited buildup of the concentration of deactivator, which in the case of NMP is the nitroxide, the persistent radical. On the basis of the PRE, it can be predicted which conditions will lead to narrow MMDs, and to a small fraction of irreversibly terminated dead chains. 

Hanns Fischer elaborated kinetics describing at least some CRPs in terms of persistent radical effect (PRE) (the term coined by Finke). This is an application of the theory explaining the phenomena of self-regulation of the radical reactions that are related to Scheme 21. The principle, as Fischer says, is simple. 

The first kinetic analysis of this phenomenon was made in 1986 by Fischer, who also highlighted its importance in living radical polymerization. The first numerical simulation of PRE in the context of NMP was performed by Johnson et al The term persistent radical effect was proposed in 1992 by Daikh and Finke. " Several excellent reviews eovering the applications of PRE in organic and polymer chemistry are available. ... 

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