Initiation by radical anions

In some cases it may be useful not to separate initiation from propagation and to work in homogeneous medium. It is then necessary to create conditions for the rapid transfer of an electron from the metal to a suitable molecule, and to use the generated radical ion for initiation. The simplest adical ion is the solvated electron. 

The resulting complexes can initiate polymerization [186] of vinyl and heterocyclic [187] monomers. 

Electron transfer from alkali metals to aromatics is very easy in suitable solvents [182]. The radical anions produced in this way do not dimerize. The formation of a covalent C—C bond would be accompanied by the loss of resonance stability in the aromatic system. Paul et. al. [188] have shown that the unpaired electron is placed in the lowest unoccupied orbital of the molecule, and that the stability of the radical anion increases in the order diphenyl naphthalene phenanthrene anthracene. 

The formation of radical anions of this type can be schematically described as follows (with Na and naphthalene as examples of donor and acceptor) [189], 

The course of the reaction strongly depends on solvent. In dimethyl ether, the equilibrium (45) is shifted strongly to the right in the less basic diethyl ether, interaction does not occur. The most used solvents are again THF, dimeth-oxyethane, etc. The electron is transferred to the monomer by the green solution of naphthalenesodium 

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Monomers, such as ethylene, propylene, isobutylene, and isoprene, containing the carbon-carbon double bond undergo chain polymerization. Polymerization is initiated by radical, anionic or cationic catalysts (initiators) depending on the monomer. Polymerization involves addition of the initiating species R, whether a radical, cation, or anion, to the double bond followed by its propagation by subsequent additions of monomer... 

Electron-transfer to monomer is not the only mode of initiation by radical-anions, although it is unique for non-polar monomers. With some polar monomers, especially cyclic ones, the initiation resembles protonation. For example, the reaction of sodium naphthalenide with ethylene oxide follows the route (9)... 

In many cases, homopolymerization can be initiated by the anion-radicals of the monomers themselves. Of course, such monomers must have pronounced electron affinity (EA) and be stabilized by delocalization of an unpaired electron. Typical examples are represented by the anion-radicals of 1,1-dicyanoethylene (EA = 1.36 eV) and methyl or ethyl 2-cyanoacrylates (EA = 1.08 eV). In all of these anion-radicals, an unpaired electron is primarily localized on C atom of the CH2 segment and characterized by appreciable resonance stabilization (Brinkmann et al. 2002). These anion-radicals are nucleophilic and attack the neutral monomers to initiate polymerization. 

Whether a vinyl monomer polymerizes by radical, anionic, or cationic initiators depends on the inductive and resonance characteristics of the substituent(s) present. The effect of the... 

The reduction of the tetrasilacyclohepta-1,2-diene with a sodium mirror in Et20 gave dark red crystals that were shown by an X-ray structure determination to be the sodium salt of the allyl anion 61 produced by a series of intramolecular rearrangements of the initially formed radical anion (Fig. 37). The Na-C distances are 265.6(5)-288.2(5) pm.98... 

Enholm has reported tandem cyclizations induced by RsSnH (Scheme 38)101. The initial cyclization is a A5-hexeny 1-type cyclization of the initially formed radical anion (61 — 62). After hydrogen transfer from R3SnH, the resulting tin enolate 63 undergoes nucleophilic addition to the nitrile. 

The electron transfer rates kt range in the order of 107 to 108M"-1 sec 1. Furthermore the radical anion may transfer its electron to each suitable acceptor which affords an energetically more favorable radical anion. As an example, sodium biphenylene reacts with anthracene to form the anthracene radical anion exclusively 111 a Polymerizations by radical anions may be preceded by an electron transfer to the monomer (Eq. (248) ). Thus the monomer radical anion may initiate anionic and/or mixed anionic-radical polymerization. 

The product ratio in electrochemical reduction of benzalacetone is significantly altered by surfactants and various cations, which cause micellar and/or ion-pairing effects. Using these additives, it is possible to control the partitioning of the initially formed radical anion between the two main reaction pathways either dimerization or further reduction to the saturated ketone.Additionally, micellar surfactants allow the use of aqueous media without cosolvents. 

The photochemical step Is initiation of radical anion formation by electron transfer from the nucleophile to the aryl halide, one of the two being In the excited state. Thus amino acid substituted diaryl thioethers have been prepared In high... 

Formation of 4- or 5-membered rings follows reduction of dialkyl bromoalkylidenemalo-nates by cyclization onto the 6-carbon of the diactivated double bond [Eq. (48)] [225]. A plausible mechanism may involve C-Br cleavage through inter- or intramolecular electron transfer from the initially formed radical anion to the a orbital. The radical so fonned may add intramolecularly to the activated alkene function. An alternative, 8 2 reaction between the alkene radical anion and the bromide would be expected to occur from the a-carbon of the activated alkene, which bears the higher charge density. 

The reaction is most probably initiated by radical attack of a reduced carbonyl function on the aromatic ring in the adjacent system. The product is formed as the radical anion but reoxidized by air during work-up. In the presence of proton donors, or in alcoholic solvents, reduction of 98 gives a mixture of acyclic and partly hydrogenated cyclic products [288]. Substituted 98, such as the 4,4, 5,5 -tetracarboxylic acid, gives coupling in basic alcoholic medium but not in DMF [289]. 

I, vinylruthenocene, 66, vinylosmocene, and the T)5-(vinylcyclopentadienyl)metal carbonyl monomers in radical-initiated polymerizations summarized in Scheme 1.1 no longer exists for anionically initiated addition polymerizations. Styrene is readily initiated by such anionic species as BuLi and Na1 Naphth. Living anionic styrene homopolymerizations and block copolymerizations have been extensively commercialized for many years (e.g., Kraton thermoplastic elastomers). However, the exceptionally electron-rich vinyl metal-containing monomers 1, 8-18, 24-30, and 66 were never successfully initiated by anionic systems in our laboratory despite many attempts. In these systems, the a-carbocations are very stable, but the a-carbanions are quite unstable. Thus, the addition of an anion to tbe vinyl function of these monomers is unfavorable. 

Can any kind of initiator produce the cyclopolymer from St-C3 St Since the monomer is a kind of styrene derivative, so it could be polymerized by a variety of initiators cationic, radical, anionic, and coordination catalysts, and the question could be answered by the results of the polymerization. 

The formation of protons was detected experimentally during the peroxidation of lipids of bilayer lipid membranes.68,69 It was shown by direct measurements of the aqueous solution pH that X-ray irradiation of the aqueous solution on one side of the bilayer leads to ejection of protons into the solution. As a result, a transmembrane potential difference is generated on BLM. Since after X-ray irradiation peroxide anions are formed in the solution, peroxidation of lipids can be initiated by the anion radical O. 70... 

A mechanism which explains the simultaneous formation of 12 and 13 is shown in Figure 9. Intermolecular oxidation of a-complex 14 by nitrobenzene generates 12 and initially nitrobenzene radical anion, 15, which can decompose by a variety of pathways including disproportionation to give nitrosobenzene (9). The... 

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