Styrene radical anion

This mechanism is based on initiation by electron-transfer which leads to a styrene radical anion, which couples rapidly due to its high concentration, and forms a dimeric styrene dianion that is capable of further propagation by anionic attack on styrene monomer. 

Photoaddition of amines has also been reported and is often the result of initial electron-transfer from the amino-group. Photoadducts of triethylamine with styrene and a-methylstyrene have been obtained carbon dioxide reacts with the styrene radical anion intermediates to give carboxylic acids. In the photoreaction of arenecarbonitriles with aliphatic amines, both aminyl and... 

Since the required polymer is a functionalized polystyrene, the most sensible approach would be to co-polymerize styrene and some 3,4-dihydroxystyrene, perhaps protected as an acetal or a silyl derivative. A proportion of the benzene rings in the polystyrene would have the correct functionalization and the crown ether could be built on to them by passing a large excess of a suitable reagent, such as one of those we discussed in Problem 2 of this chapter or in the main text (p. 1456), deprotecting as required. A potassium salt would be used as a base in the final cyclization to take advantage of complexation by the crown ether. The various methods of polymerizing styrene (radical, anionic, etc.) are described in the chapter (pp. 1459-62). 

Styrene radical-anion Formed by one-electron transfer... 

An alternative method of initiation is through the use of the radical anion produced from the reaction of sodium (or lithium) with naphthalene. Such radical anions react with styrene by electron transfer to form styrene radical anions these dimerize to produce a dianion, which initiates polymerization as outlined in Scheme 14. One particular feature of this method is that polymerization proceeds outwards from the centre. Subsequent reaction of the living chains ends with another suitable monomer system produces a triblock copolymer. This is the principle by which styrene-butadiene-styrene triblock copolymers (formed when butadiene is polymerized in the same way. and styrene is added as second monomer) are produced commercially. This material behaves as a thermoplastic elastomer, since the rigid styrene blocks form cross-links at room temperature on heating these rigid styrene portions soften, allowins the material to be remoulded. ... 

The difference spectrum results from the absorbance of the intermediates and bleaching of the dimers. Hence, the spectrum of the intermediates is constructed by adding the known spectrum of the photolyzed dimers to the observed difference spectrum. Such a procedure is illustrated in Fig. 3. The resulting spectrum of the intermediates closely resembles that of ot-methyl styrene radical-anions reported by three independent groups (4), who used pulse radiolysis in their studies, it follows that the photolysis leads to direct or indirect photo-dissociation of the dimeric-dianions into radical-anions of ot-methyl styrene, ot, i.e.,... 

Reactions 7 and 8 are somewhat oversimplified presentations. The transfer of a second electron from the naphthalene to the styrene is also possible and produces a monostyrene di-anion rather than the distyrene di-anion shown in-Reaction 8. In addition, the possibility of coupling of naphthalene and styrene radical-anions exists. However, the only species ever isolated (e.g., by hydrolysis to 1, A-dipheny1 butane) from this type of initiation process has been related to the distyrene species of Reaction 8. 

This result can probably be ascribed to the fact that the styrene radical anions are the most reactive (least stable) of the species. 

Thus the localization energy <5 at the terminal carbon in the styrene radical anion is given [see equations (7.106) and (7.107)] by... 

These green radical ions react with styrene to produce the red styrem radical anions ... 

Sodium naphthalene [25398-08-7J and other aromatic radical anions react with monomers such as styrene by reversible electron transfer to form the corresponding monomer radical anions. Although the equihbtium (eq. 10)... 

Monomers which can be polymerized with aromatic radical anions include styrenes, dienes, epoxides, and cyclosiloxanes. Aromatic radical anions... 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique abiUty of lithium to provide high 1,4-polydiene microstmcture is lost in polar media (1,33,34,57,63,64). Consequentiy, a significant research challenge has been to discover a hydrocarbon-soluble dilithium initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

There are two problems in the manufacture of PS removal of the heat of polymeriza tion (ca 700 kj /kg (300 Btu/lb)) of styrene polymerized and the simultaneous handling of a partially converted polymer symp with a viscosity of ca 10 mPa(=cP). The latter problem strongly aggravates the former. A wide variety of solutions to these problems have been reported for the four mechanisms described earlier, ie, free radical, anionic, cationic, and Ziegler, several processes can be used. Table 6 summarizes the processes which have been used to implement each mechanism for Hquid-phase systems. Free-radical polymerization of styrenic systems, primarily in solution, is of principal commercial interest. Details of suspension processes, which are declining in importance, are available (208,209), as are descriptions of emulsion processes (210) and summaries of the historical development of styrene polymerization processes (208,211,212). 

Polymerization Reactions. The polymerization of butadiene with itself and with other monomers represents its largest commercial use. The commercially most important polymers are styrene—butadiene mbber (SBR), polybutadiene (BR), styrene—butadiene latex (SBL), acrylonittile—butadiene—styrene polymer (ABS), and nittile mbber (NR). The reaction mechanisms are free-radical, anionic, cationic, or coordinate, depending on the nature of the initiators or catalysts (194—196). 

As the final example in this section, a Li-mediated carboaddition/carbocycliza-tion process will be described. Thus, Cohen and coworkers observed a 5-e%o-trig-cy-clization by reaction of the lithium compound 2-349 and a-methyl styrene 2-350 to give 2-352 via 2-351 (Scheme 2.82). Quenching of 2-352 with methanol then led to the final product 2-353 [189]. In this process, 2-349 is obtained by a reductive lithia-tion of the corresponding phenyl thioether 2-348 with the radical anion lithium 1-(dimethylamino)naphthalenide (LDMAN) (2-354). Instead of the homoallylic substance 2-348, bishomoallylthioesters can also be used to provide substituted six-membered ring compounds. 

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

An electron donating substituent such as phenyl and methoxy will polarise electron density on the radical-anion of an alkene in favour of more positive charge density on the carbon atom bearing this substituent with more free electron density on the other carbon atom. This promotes dimer formation by linkage through atoms with free electron density. Styrene is oxidised at a graphite anode in methanol... 

Radical-anions derived from styrene derivatives are nucleophilic in character. Electrochemical reduction of a-methylstyrene 5 gives the radical-anion intermediate, which in the absence of other electrophiles is sufficiently nucleophilic to attack dimethylfonnamide or acetonitrile [19], The radical-anion from styrene 6 under-... 

Electron-transfer initiation from other radical-anions, such as those formed by reaction of sodium with nonenolizable ketones, azomthines, nitriles, azo and azoxy compounds, has also been studied. In addition to radical-anions, initiation by electron transfer has been observed when one uses certain alkali metals in liquid ammonia. Polymerizations initiated by alkali metals in liquid ammonia proceed by two different mechanisms. In some systems, such as the polymerizations of styrene and methacrylonitrile by potassium, the initiation is due to amide ion formed in the system [Overberger et al., I960]. Such polymerizations are analogous to those initiated by alkali amides. Polymerization in other systems cannot be due to amide ion. Thus, polymerization of methacrylonitrile by lithium in liquid ammonia proceeds at a much faster rate than that initiated by lithium amide in liquid ammonia [Overberger et al., 1959]. The mechanism of polymerization is considered to involve the formation of a solvated electron ... 

Another way to initiate anionic polymerization is by electron transfer. The reaction of sodium with naphthalene gives sodium naphthalene (sodium dihydro-naphthylide) in which the sodium has not replaced a hydrogen atom, but has transferred an electron to the electronic levels of the naphthalene this electron can be transferred to styrene or a-methylstyrene, forming a radical anion ... 

Such radical anions combine very quickly forming a dianion which can then add styrene at both ends ... 

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). 

Styrene is an interesting monomer, as it is polymerizable in any one of the radical, anionic and cationic mechanisms, depending on the catalyst used. Its radiation-incuded polymerization, however, had long been recognized to be radical polymerization, until it was suggested that cationic polymerization was also possible in alkylhalide solutions at low temperature, as mentioned in the previous chapter (2, 3, 4). 

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. 

The use of alkali melals for anionic polymerization of diene monomers is primarily of historical interest. The electron-transfer mechanism of the anionic polymerization of styrenes and 1,3-diencs initiated by alkali metals has been described in detail the dimerization of radical anion intermediates is the important step. 

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