Electron spin resonance propagating radicals

Electron spin resonance (ESR) spectroscopy can be advantageously used to measure the radical concentrations of the nitroxide radicals (XV and XVI) produced, since these are much more stable then the R- radicals. Of greater importance, ESR can be used to determine the structure of R% since the ESR of the nitroxide radical is quite sensitive to the structure of R. (For this purpose, nitroso spin traps are more useful, since the R group in the nitroxide radical is nearer to the lone electron.) This can allow a determination of the structures of radicals first formed in initiator decomposition, the radicals that actually initiate polymerization (if they are not identical with the former) as well as the propagating radicals [Rizzardo and Solomon, 1979 Sato et al 1975],... 

That this reaction occurs is shown by electron spin resonance measurements, which indicate the complete disappearance of radicals in the system immediately after the addition of monomer. The dimerization occurs to form the styryl dicarbanion instead of - CH2CH4>CH4>CH2 -, since the former is much more stable. The styryl dianions so-formed are colored red (the same as styryl monocarbanions formed via initiators such as n-butyl-lithium). Anionic propagation occurs at both carbanion ends of the styryl dianion... 

The formation of ion radicals from monomers by charge transfer from the matrices is clearly evidenced by the observed spectra nitroethylene anion radicals in 2-methyltetrahydrofuran, n-butylvinylether cation radicals in 3-methylpentane and styrene anion radicals and cation radicals in 2-methyltetrahydrofuran and n-butylchloride, respectively. Such a nature of monomers agrees well with their behavior in radiation-induced ionic polymerization, anionic or cationic. These observations suggest that the ion radicals of monomers play an important role in the initiation process of radiation-induced ionic polymerization, being precursors of the propagating carbanion or carbonium ion. On the basis of the above electron spin resonance studies, the initiation process is discussed briefly. 

This initiation process is thus similar to alkali metal initiation in (a). That this reaction occurs is shown by electron spin resonance measurements, which indicate the complete disappearance of radicals in the system immediately after the addition of monomer. The monomer in these systems often has a lower electron affinity than the polycyclic hydrocarbon, but dimerization of the monomeric radical anion [Eq. (8.15)] drives the equilibrium of reaction (8.14) to the right. Dimerization of radical centers is highly favored by their high concentrations, typically 10 -10 M and the large rate con-stants (10 -10 L/mol-s) for radical coupling. (Note that the dimerization occurs to form the styryl dicarbanion instead of CH2CH0CH0CH2 , since the former is much more stable.) The styryl dianions are colored red (the same as styryl monocarbanions formed via initiators such as n-butyllithium). Anionic propagation occurs at both carbanion ends of the styryl dianion ... 

This leads to a hydroperoxide and an alkyl radical, which can again react with oxygen according to Reaction (4.2). The rate of Reaction (4.3) is slow, ks = 10 -10 1 mor s at 30°C, dependent on the type of hydrocarbon, when compared with Reaction (4.2), and is therefore the rate-determining step for chain propagation. Due to their low reactivity, peroxy radicals are present in relatively high concentration in the system when compared with other radicals, determined via electron spin resonance. 

Electron Spin Resonance (ESR) spectroscopy can contribute to understanding both the kinetics and the mechanism of radical polymerizations.f Propagation rate constants (fep) of various kinds of monomers have been estimated using ESR spectroscopy. Indeed ESR is one of the most effective methods for estimating values for kp and it is a mutually complementary method to the Pulsed Laser Polymerization (PLP) method. Usually equation (1), and its integrated form (2), have been used to... 

Electron Spin Resonance Spectroscopy—Stationary Polymerization. The experimental determination of kp data usually proceeds via the lUPAC recommended PLP-SEC procedure (see above). However, imder certain circumstances, p data are also available by direct determination of the concentration of propagating free radicals via ESR spectroscopy, accompanied by the measurement of the overall pol3rmerization rate (481) (see Electron Spin Resonance). The calculation of p then proceeds via either the differential (eq. 55) or the integrated form of the propagation rate law expression ... 

This chapter describes the application of electron spin resonance (ESR) spectroscopy and controlled radical polymerization techniques to basic research on the chemistry of radical polymerizations. This combination can provide information on the chain length of propagating radicals, chain-transfer reactions to polymers, and penultimate unit effects in copolymerization, topics that have been difficult or impossible to study by direct detection of radicals. 

Zetterlund PB, Yamazoe H, Yamada B. Propagation and termination kinetics in high conversion free radical copolymerization of styrene/divinylbenzene investigated by electron spin resonance and Eourier-transform near-infrared spectroscopy. Polymer 2002 43 7027-7035. 

Electron spin resonance detection of the propagating and mid-chain radicals involved in the polymerization of phenyl acrylate (PhA) and the determination of the rate constants for this monomer using the ESR method were reported by Azukizawa et al. [114]. Absolute values of kJiM were determined at infinitely low conversion, to minimize the factors affecting the radical concentration. 

Buback M, Kowollik C, Kamachi M, Kajiwara A. Free-radical propagation rate coefficients of dodecyl methacrylate deduced from electron spin resonance experiments. Macromolecules 1998 31 7208-7212. 

Three different types of radicals were identified by electron spin resonance (ESR) spectroscopy. In the first case, isolated poly(maleic anhydride) contained one radical identifiable as the MA propagating radical. This same radical was shown to be capable of polymerizing styrene and copolymerizing styrene-methyl methacrylate mixtures. It was also shown that the two other radicals were transformed into active species capable of the cationic polymerization of isobutyl vinyl ether or vinyl ethers in the presence of monomeric... 

Because of the respective values of rate constants of termination (kt 10 to 10 L-mor -s ) and propagation (kp 10 to 10" L mor -s at 60°C) reactions, it is recommended to work with particularly low instantaneous concentrations in free radicals ([RM ] 10 M), in order to favor propagation over termination reactions. It is difficult to measure such low value of [RM ], except by using a spectrometric technique as sensitive as electron spin resonance (ESR). Assuming that the number of active chains remains constant—which is true only during short intervals of time—, one can calculate the rate of polymerization even if [RM ] is experimentally inaccessible and thus unknown. This assumption implies that the rate of appearance of RM is equal to their rate of disappearance, which corresponds to steady-state conditions one can accordingly write Ri = Rt, which corresponds to... 

Carlsson D, Dobbin C, Wiles D. Direct observations of macroper-oxyl radical propagation and termination by electron spin resonance and infrared spectroscopies. Macromolecules 1985 18 2092-94. 

PLP-SEC and (iii) single-pulse pulsed-laser polymerization coupled with online time-resolved electron-spin resonance spectroscopy (SP-PLP-EPR). The propagation rate coefficient for MCRs may be obtained via ft-PLP-SEC and SP-PLP-EPR. Termination rate coefficients kt , and kt are only accessible from SP-PLP-EPR," in which different types of radicals can simultaneously be traced as a function of time. Remaining kinetic coefficients can then be obtained via computer modeling. Table 1.5 collates kinetic coefficients for butyl acrylate polymerization as an example. 

The existence of RT (Scheme 7.3a) was examined in three ways, (a) If RT exists, A is mediated. Thus, we attempted to detect A by electron spin resonance (ESR) in the St polymerizations. The Sn, Ge, P, and N-centered A radicals could not be detected due to their low equilibrium concentrations, while TI (O deactivator), BHT (O precursor), and CHD (C precursor) derived A radicals were in fact observed (Figure 7.11). The spectrum for TI was broad, probably due to the aggregation of TI in the St medium, and that for BHT clearly split into four peaks (1 3 3 1 ratio) by the methyl group at the para position and further split into three peaks (1 2 1 ratio) by the protons at the meta positions. The spectrum for CHD was somewhat noisy due to the lower concentration of A than those for TI and BHT. The spectrum split into three peaks (1 2 1 ratio) by the protons at the meta positions and further split into two peaks (1 1 ratio) by the proton at the central position. In these experiments, a large amount (100 00 mM) of the catalyst was used to facilitate the detection of A. Nevertheless, the concentration of A was low, in the order of 10 M, in the three cases, indicating that in typical polymerization conditions (with 3-10 mM of the catalyst), it is very low, on the order of 10 -10 M (nanomolar), and the activation process effectively occurs at such a low A concentration (as indicated above). Polymer was not apparently detected (Figure 7.11) due to its much lower concentration (10 -10 M estimated from the polymerization rate) than that of A (10 M). (b) If RT exists, the propagating species is a free radical (Polymer ). This was confirmed from the tacticity of the product polymer, namely, the tacticity was virtually the same as that without the catalyst (IMP) in all studied cases (St and MMA with various... 

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