Electron transfer, from radical anions monomers

Reaction (I) involves the initial electron transfer from the metal to the monomer, which leads to the formation of a radical anion which could then participate in the reactions shown in (II) and (III), i.e., by coupling of two radical ions or by transfer of another electron from the metal, respectively, both processes leading to a dianionic species. 

Electron-transfer initiation also occurs in heterogeneous polymerizations involving dispersions of an alkali metal in monomer. Initiation involves electron transfer from the metal to monomer followed by dimerization of the monomer radical-anion to form the propagating... 

Electron transfer from polycyclic aromatic radical anions in polar solvents can also initiate propagation.120 168 169173 One of the early and best understood systems is naphthalene-sodium, a green solution of stable, solvated naphthalene radical anion.176 177 The electron transfer from the radical anion to the monomer yields a new radical anion [Eq. (13.33)]. The dominant reaction of the latter is its head-to-head dimerization to the stabile dimeric dicarbanion [Eq. (13.34)], which is the driving force for the electron transfer even when electron affinity of the monomer is less than that of the polycyclic molecule. Propagation proceeds at both ends of the chain ... 

Photopolymerization of acrylamide by the uranyl ion is said to be induced by electron transfer or energy transfer of the excited uranyl ion with the monomer (37, 38). Uranyl nitrate can photosensitize the polymerization of /S-propiolactone (39) which is polymerized by cationic or anionic mechanism but not by radical. The initiation mechanism is probably electron transfer from /S-propiolactone to the uranyl ion, producing a cation radical which propagates as a cation. Complex formation of uranyl nitrate with the monomer was confirmed by electronic spectroscopy. Polymerization of /J-propiolactone is also photosensitized by sodium chloroaurate (30). Similar to photosensitization by uranyl nitrate, an election transfer process leading to cationic propagation has been suggested. 

As described in the previous chapter, in the work on electrolytic polymerization which has appeared in the literature, the active species were formed by an electrode reaction from the compounds added to the reaction system and thus initiated polymerization. However, the possibility has been considered of direct electron transfer from the cathode to monomer or from monomer to the anode forming radical-anion or -cation, followed by initiating polymerization. Polymerization of styrene initiated by an electron has been observed when the monomer was exposed to the electric discharge of a Tesla coil (74), y-radiation (75, 16) and to cathode rays from a generator of the resonant transformer type (77). 

According to the studies of monomers in the organic glass matrices mentioned so far, the ion radicals formed from solute monomers relate their radiation-induced ionic polymerization to the primary effect of ionizing radiations on matter. It is concievable that the initiating species in the anionic polymerization (caxbanions) are formed by the addition of the monomer molecules to the anion radicals which result from electron transfer from the matrices to the solute monomer. The formation of the cation radicals is necessary also to initiate the cationic polymerization. 

The termination step of a propagating chain could involve proton transfer to monomer, combination with the acceptor radical anion, or electron transfer from the acceptor radical anion to the carbonium ion to yield a terminal radical, as below. 

The sodium dissolves to form an addition compound and, by transferring an electron, produces the green naphthalene anion radical. Addition of styrene to the system leads to electron transfer from the naphthyl radical to the monomer to form a red styryl radical anion. 

The first one resorts to an electron transfer from a metal atom (generally an alkali metal) to a molecule whose electron affinity is sufficiently high. The role of the electrophilic entity can be played by the monomer molecule and, in this case, the transfer of ns electrons from the alkaU metal results in the formation of a radical-anion based on the monomer molecule ... 

Electron transfer processes from an alkali metal to poly(vinylnaphthalene) is another kind of addition reaction. It is easy to perform in THE solution, and yields radical ion sites distributed at random along the chain (Scheme 12). However, initiation of anionic polymerizations by radical ion species proceeds by electron transfer to the incoming monomer. As a consequence this pathway cannot be used for the purpose of grafting, with the single exception of oxirane. The same is true of polymeric radical anions obtained upon metalation of poly(vinylbenzophenone) with alkali metals. ... 

Electron transfer from coinitiator to the excited, photoreducible dye molecule yields radical cations of the coinitiator and radical dye anions. The former can initiate the polymerization. In many cases, however, initiating radicals are formed in subsequent thermal reactions. Species deriving from the dye molecule do not react with monomer molecules. 

The influence of a polar solvent on the initiation mechanism has also been examined. If styrene is reacted with a living (PS)6C ) (Li )6 in THF, no PS chain grows from the fullerene core. Nevertheless, PS with no incorporated Qo is produced [96,100]. This points to an electron transfer from the carbanions located on the fullerene core to the monomer, followed by the classical dimerization of the so-formed ion-radicals to produce a growing di-anionic PS chain. More puzzhng is the hypothesis put forward by these authors to explain some of their observations, that two arms may be released from the core of these living stars upon addition of THF [100]. 

First, we examined the efficiency of the initiation process. A solution of buthyllithium was added to a THF solution of 7 at -70°C. The color of the solution turned to red immediately and a strong ESR signal was observed with a well separated hyperfme structure. The observed radical species was identified as the anion radical of 2-butyl-l,l,2,2-tetramethyldisilanyl-substituted biphenyl by computational simulation as well as by comparison with the spectra of a model compound. The anion radical should be a product of a single electron transfer (SET) process from buthyllithium to the monomer. Since no polymeric product was obtained under the above-mentioned conditions, the SET process is an undesired side reaction of the initiation and one of the reasons why more higher molecular weight polymer was observed than expected. ... 

The second electronic transfer to the oxygen produces the diradical (C) which evolves into monomer formation. The latter possibility (IV) is a homolytlc cleavage giving another anion radical. If the process follows scheme III or IV, we must obtain monomer formation after the oxidation reaction in all cases. We have carried out the oxidation of carbanionic dimers derived from isoprene, crmethylstyrene, styrene, 1,1-diphenylethylene. 

The free-radicals are generated by discharge of proton, peroxides and easily reducible compounds at the cathode according to Eq. (1—4). The radial-anion of monomer is obtained by both direct and indirect electron transfer process [Eq. (5—6)]. The indirect process means that the active oxidizing species is formed from the electrolytes by electrode reaction, followed by interaction with the monomer. An unstable monomer like a,a -2-trichloro-p-xylene is formed and polymerizes instantaneously [Eq. (7)]. The regeneration of ferrous ion from the pool of used up ferric ion in a redox system is electrolytically successful [Eq. (8)] and an... 

When a solution containing a small amount of monomer is irradiated, the cation radicals and electrons are formed primarily from solvent molecules. In this case, cationic intermediates are formed from monomer through positive charge transfer or proton transfer from solvent to solute monomer. Anionic intermediates of monomer are formed by the combination between electrons and monomer molecules. If the solvent has the nature to stabilize the electrons and inhibit succeeding anionic reactions, ionic reactions involving monomer are limited to cationic ones. The situation is the reverse, if the solvent is able to stabilize the cationic intermediates primarily formed. Therefore, the ionic reactions involving monomer may be simple enough in some suitable solvents to be studied. 

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. 

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

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