Styryl radical anion

The naphthalene radical-anion transfers an electron to a monomer such as styrene to form the styryl radical-anion which dimerizes to a dianion... 

The styryl radical-anion is shown as a resonance hybrid of the forms wherein the anion and radical centers are alternately on the a- and ])-carbon atoms. The styryl radical-anion dimerizes to form the dicarbanion (XX)... 

In a slightly more detailed study of dry styrene, Metz et al. (23) have reported similar observations. They tentatively identified a species with broad absorption maximum at 370 m/x, decaying with a first-order half-life of approximately 4 /xsec., extremely sensitive to water, sensitive to oxygen, insensitive to N20, and formed with an apparent radiolytic yield (measured immediately after a 0.2-/xsec. pulse) of G 0.15, as the styryl radical anion. 

The styryl radical anion may dimerize to a dianion that is capable of growing at both ends ... 

The solution becomes an intense green, and an electron spin resonance (ESR) measurement at this point confirms the presence of the naphthalene radical anion. Addition of styrene at this stage causes the color of the solution to change to red and the ESR signal disappears. The styryl radical anion formed initially is correctly viewed as a resonance hybrid (Eqs. 22.38 and 22.39). 

The styryl radical anion species is much more reactive than the naphthalene radical anion, and rapidly couples to form a dimeric dianion, the source of the red color and the reason for the disappearance of the ESR signal (Eq. 22.40). The dimeric dianion is a double-ended anionic propagating species useful for the initiation of a number of valuable homopolymerizations (Eq. 22.41). 

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. 

Thus the first step in the initiation reaction (Eq. (2.64)) involves a reversible electron transfer reaction from the alkali metal to the styrene monomer to form the styryl radical anion in a rapid subsequent reaction, two radical anions couple to form a di-anion which can grow a polymer chain at both ends. In the case of the soluble alkali metal aromatic complexes, the overall initiation reaction is extremely fast, due to the high concentrations of radical anion M) and... 

Thus the first step in the initiation reaction [Eq. (64)] involves a reversible electron transfer reaction from the alkali metal to the styrene monomer to form the styryl radical anion in a rapid subsequent reaction, two radical anions couple to form a di-anion which can grow a polymer chain at both ends. In the case of the soluble alkah metal aromatic complexes, the overall initiation reaction is extremely fast, due to the high concentrations of radical anion ( 10 M) and monomer (-1M), and so is the subsequent propagation reaction. However, in the case of the alkah metal initiators, the electron transfer step [Eq. (64)] is very much slower, due to the heterogeneous nature of the reaction, so that the buildup of radical anions is much slower. In fact, there is evidence [153] that, in such cases, a second electron transfer step can occur between the metal and the radical anion to form a di-anion, rather than coupling of the radical anions. In either case, the final result is a di-anion, i.e., a difunctional growing chain. 

The naphthalide radical anion is a powerful reducing agent. For example, styrene undergoes a one-electron reduction to form the styryl radical anion, which couples to form a dianion. The latter then propagates polymerization at both ends, growing chains in two directions simultaneously. 

In MeCN the radical anion derived from 9-anthryl styryl ketone, 20b, dimerizes ( dim = 10 s ), formmg a stable dimeric dianion, most likely via coupling in the... 

The Sn2 mechanism is ruled out for reaction between the tertiary halide, r-BuBr, and radical anions derived from the more easialy reduced compounds cinnamonitrile (9) ethyl cinnamate (12a), methyl styryl ketone (23a), and phenyl styryl ketone (20a). Reduction of the activated alkenes in the presence of an excess of r-BuBr leads to mixtures of products where a r-Bu group has been introduced in a- or j0-position or in the phenyl ring. For 9 and 12a small amounts of butylated hydrodimers were obtained in addition, and for the enone 23a formation of the unsaturated alcohol with introduction of the /-Bu group at C-1 was a major product [192]. In this case the mechanism is unambiguously reduction of the activated alkene followed by electron transfer to r-BuBr concerted with halide cleavage, in... 

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 ... 

The styryl radical ion in (XI) is a resonance hybrid of two forms having both anion and radical centers. It dimerizes by reacting at the radical ends to form a red-colored dicarbanion (XU) ... 

The initiation process is thus similar to alkali metal initiation described earlier [cf. Eq. (8.10)]. The dimerization of radical anions is highly probable because of their high concentrations, typically jlO -10" M, and the large rate constants (10 -10 L/mol-s) for radical coupling (Odian, 1991). Anionic propagation takes place by monomer addition at both carbanion ends of the styryl dianion ... 

Regarding anion radical transfer, low-molecular weight azo compounds were used as terminating agents in anionic polymerizations. An interesting example is the addition of a living polystyrene chain to one nitrile group of AIBN [71]. The terminal styryl anion is likely to form... 

If the styryl substituent retained its donor nature in the anion-radical state, an increase, not a decrease in the value of the nitrogen HFC constant (a(N)) would have been observed. Experiments show that fl(N) values for anion-radicals of nitrostilbenes decrease (not increase) in comparison with the fl(N) value for the anion-radical of nitrobenzene (Todres 1992). Both naked anion-radicals and anion-radicals involved in forming complexes with the potassinm cations obey such regularity. In the cases of potassinm complexes with THF as a solvent, a(N) = 0.980 mT for PhNOj anion-radical and fl(N) = 0.890 mT for PhCH=CHCgH4N02-4 anion-radical. In the presence of 18-crown-6-ether... 

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... 

FIGURE 3-33 Formation of a styryl anion radical and subsequent dimerization. 

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