Free radical reactions nitroxide concentrations

Figure 3 shows the CL - time profile for samples of PP containing different concentrations of the profluorescent nitroxide TMDBIO. The CL curve for unstabilized PP shows that after a very short time, there is an exponential increase in CL corresponding to rapid oxidation and embrittlement of the polymer. The effect of the added nitroxide is not to decrease totally the CL from the PP, but rather to retard the emission so that there is a slower development of the CL emission intensity. There is thus an increase in the time taken to see the exponential increase in emission intensity, but the emission is not reduced to zero in this retardation period. In contrast, if a peroxy-radical scavenging, hindered phenol such as Irganox 1010 were to be added to the PP then the increase with time of the CL intensity would be totally suppressed and there would be an apparent induction period 8), This result may be interpreted within the framework of the free-radical reactions in Figures 1 and 2 above. 

Stable free radicals are frequently employed as inhibitors (436,437). The most commonly used species are nitroxides, eg, 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO) 18. They are far too stable to be able to initiate polymerization, but they are reactive enough to imdergo reaction with other free radicals (438). Nitroxides are very efficient inhibitors, being capable of producing induction periods when present in concentrations of less than 10 mol L" Nitroxides, such as TEMPO, react with carbon-centered radicals at close to diffusion controlled reaction rates (439-441). The stoichiometry between the number of the chains... 

The key feature of the use of a dormant species may be seen in the following general scheme (Scheme 1.32) that involves complexation of the propagating species by means of a stable nitroxide radical (Hawker et al, 2001). The P -0 bond of the alkoxy amine P -0-NR is thermally labile at the polymerization temperature, so this becomes the site for the insertion of monomer. Propagation then occurs at a rate that is much slower than for a simple free-radical addition reaction since the propagating radical concentration (which is governed by the position of the equilibrium with the alkoxy amine... 

The equilibrium of Equation 3.75 dictates that the concentration of active free radicals in the system, [Ptot]> remains low. For the system to remain living, the reversible deactivation reaction with nitroxide must be dominant compared to irreversible radical-radical termination (Equation 3.5). However, as it is impossible to totally eliminate the loss of radicals through termination, an imbalance between [X] and [Ptot] arises ... 

Many such compounds are very stable under normal conditions, and heterolytic reactions can be carried out on other functional groups in the molecule without destroying the paramagnetic nitroxide group. Scheme 12.1 includes other examples of free radicals that can exist in appreciable concentrations. 

The first step, R6, converts the HALS initially added to the clearcoat, parent HALS, into inhibition cycle, R7 and R8, products. These reactions compete with R2 and R3 lowering the stationary radical concentration, which in turn lowers the hydroperoxide concentration and the photooxidation rate. The rate constants and radical concentrations are such that only a small fraction (—5%) of the HALS stabilizer is in the form of nitroxide. Although nitroxides are thermally stable, they are not pho-tolytically stable. Nitroxides absorb light, and excited-state nitroxides can abstract hydrogen atoms to initiate free-radical formation. These reactions have been discussed in detail. "Reactions R9 and RIO are important both for the nitroxide decay measurement of free radical formation and in limiting the ultimate effectiveness ofHALS.i° i5... 

If all these conditions are fulfilled, samples exhibiting both controlled molar mass and narrow molar mass distribution (Dm 1) can be obtained. However, taking into consideration the respective rate constant of free radical capture and that of irreversible termination, one can wonder how the condition of a rate of reversible termination notably higher than that of their irreversible deactivation could be reasonably satisfied. The only way to favor the former reaction against termination is to increase the nitroxide concentration in the reaction medium to a level higher... 

If one does not pay attention to this aspect and care to introduce into the reaction medium a concentration in nitroxide higher than that of the transient free radicals generated by the initiator, the system by itself will produce the excess of nitroxide necessary to control the polymerization by consenting the irreversible termination of a certain percentage of its chains. By doing so, it limits the deactivation of further chains and exhibits a controlled character during the rest of the polymerization. Elucidated and theorized by Fisher (name of the researcher who proposed this mechanism), this phenomenon is termed persistent radical effect or Fischer effect (see Figure 8.4). 

All the CRP methods have strengths that can be exploited in particular systems. TEMPO is essentially useful only for the polymerization of styrene-based monomers, whether for the preparation of statistical or block copolymers [38]. The radicals generated through the self-initiation of St help to moderate the rate of polymerization by consuming any excess TEMPO generated by termination reactions, which will not occur with other monomers. Acrylate monomers, for example, are very sensitive to the concentration of free TEMPO and therefore its build-up causes the polymerization to stop. The use of different nitroxides and alkoxyamines like DEPN [73] and TMPAH [71], which provide higher equilibrium constants and allow for faster polymerization rates, has also enabled the homo- and copolymerizations of acrylate monomers, as well as for St at lower temperatures. Block order is important, however, and chain end functionality is reduced when TMPAH functional polymers are chain extended with BA. This may... 

The mechanism of SFRP [266] (Scheme 25) involves an equilibrium between nitroxide-capped polymer chains and uncapped polymer chains. Its success relies on the retention of the suitable amount of free nitroxide in the reaction to keep the propagating polymer radical chains at a concentration which allows the polymerization to proceed at a sufficient rate but avoids bimolecular termination by coupling. 

In the presence of the free nitroxide, [R] quickly reaches a quasi-steady state and with a small amount, [MA- SGl], during the reaction. Numerical simulations indicate b, that is, only a small fraction of the initial radical concentration is terminated during the course of monomer conversion. Then,... 

For other nitroxides, in particular TEMPO and other cyclic ones, side reaction of p-hydrogen transfer was shown to totally impede the achievement of a controlled polymerization and led to 100% of dead chains, all with a vinylidene-type terminal unsaturation. However, this difficulty did not prevent the synthesis of block copolymers, providing the first block was well controlled (i.e., PS) and the methacrylate monomer was polymerized in a second step. With SGI, the simation was more contrasted and acmally depended strongly on the concentration of free nitroxide in the system extensive self-terminations of the propagating radicals at low SGI concentration, predominant p-hydrogen transfer at high SGI concentration. In consequence, appropriate experimental conditions could not be found for the controlled homopolymerization of methacrylic esters. 

With SGI as a mediator, the preformed alkoxyamine used in miniemulsion was an oil-soluble low molar mass compound with the 1 -(methoxycarbonyl)eth-1 -yl initiating radical (the so-called MONAMS A5). This type of well-defined initiator allowed good control over the initiation step, the concentration of living chains and the concentration of free nitroxide. Both molar mass and rate of polymerisation were similar to that when the reaction was carried out in bulk (Farcet et al, 2003). The target M could be larger than in the previously presented bicomponent initiating system with persulphate and metabisulphite and the PDIs were systematically lower. 

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