Styrene naphthalene anion

The naphthalene anion-radical (which is colored greenish-blue) transfers an electron to a monomer such as styrene to form the styryl radical-anion (XIX)... 

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. ... 

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. 

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. 

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)... 

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). 

The propagation rate constant and the polymerization rate for anionic polymerization are dramatically affected by the nature of both the solvent and the counterion. Thus the data in Table 5-10 show the pronounced effect of solvent in the polymerization of styrene by sodium naphthalene (3 x 1CT3 M) at 25°C. The apparent propagation rate constant is increased by 2 and 3 orders of magnitude in tetrahydrofuran and 1,2-dimethoxyethane, respectively, compared to the rate constants in benzene and dioxane. The polymerization is much faster in the more polar solvents. That the dielectric constant is not a quantitative measure of solvating power is shown by the higher rate in 1,2-dimethoxyethane (DME) compared to tetrahydrofuran (THF). The faster rate in DME may be due to a specific solvation effect arising from the presence of two ether functions in the same molecule. 

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 ... 

Anionic polymerization Initiated by electron transfer (e.g., sodium-naphthalene and styrene In THF) usually produces two-ended living polymers. Such species belong to a class of compounds called bolaform electrolytes (27) In which two Ions or Ion pairs are linked together by a chain of atoms. Depending on chain length, counterion end solvent, Intramolecular Ionic Interactions can occur which in turn may affect the dissociation of the ion pairs Into free ions or the llgand-lon pair complex formation constants. 

Funt et al. followed this work and have extended it in detail. When naphthalene was added to the tetrahydrofuran solution of NaAl(CaH6)4 and NaB(C6H8)4 without monomer, they found the green sodium naphthalene complex produced by electrolysis (11). Upon the addition of styrene monomer, generation of the orange living polystyryl anions was spectrophotometrically observed. 

Smith (29) showed that the polymerization of styrene by sodium ketyls with excess sodium produced low yields of isotactic polystyrene. Smith also believed that sodium ketyls initiated the styrene polymerization in the same way as the anionic alfin catalyst. Das, Feld and Szwarc (30) proposed that the lithium naphthalene polymerization of styrene occured through an anionic propagating species arising from the dissociation of the alkyllithium into ion pairs. These could arise from the dimeric styryllithium as a dialkyllithium anion and a lithium cation... 

Complications resulting from kjk > 1 may be responsible for the observations of Lyssy (34) who found deviations in the molecular weights of anionically polymerized styrene from the theoretically expected value of Mtotaljl0 when the degree of polymerization was very low. In the discussion of his results he implies that the initiation is slow, and indeed, he was able to demonstrate the presence of naphthalene- radical-ions in the solution of living polystyrene. The work of Levy and Szwarc (35) leads to similar conclusions. 

A recent paper by Wenger (35a) deserves some comments. This careful worker proved again that mono-dispersed polystyrene can be produced through an anionic polymerization. Most unfortunately, however, he confused some issues and their clarification is therefore necessary. Wenger found, in agreement with Waack (Ph. D. Thesis, Syracuse, June 1959), that polymerization of styrene initiated by sodium naphthalene at —78° C produces polystyrene having a broad molecular distribution. In our opinion this results from an incomplete solution of sodium naphthalene in tetra-hydrofuran at —78° C, whereas Wenger assumes that this indicates the unfavorable position of the equilibrium... 

Anionic polymerization techniques and naphthalene chemistry were used by Teyssie et al. to prepare A2B miktoarm stars, where A is poly(ethylene oxide) (PEO) and B is PS, PI, poly(a-methyl styrene) or poly(tert-butyl styrene) [25]. The reaction sequence is shown in Scheme 7. 

In a solvent such as methyl tetrahydrofuran (MTHF), anion and cation states of an unsaturated hydrocarbon are typically stabilized by 1-1.5 eV. For example, the ground state anions of butadiene, benzene, naphthalene, and styrene, which are unstable by 1.1 eV or less in the gas phase, are all stable in MTHF glass at 77 K making possible their study by optical spectroscopy. On the other hand, the anions of ethylene and 1,4 cyclohexadiene which are unstable by about 1.8 eV in the gas phase have not been prepared in MTHF glass. Presumably, the use of a more polar solvent would allow the preparation of these anions in a glass. 

The energy levels of the anion states of naphthalene and styrene as determined from the gas phase studies are compared to those obtained from optical absorption measurements on the anions in organic matrices as well as with theoretical predictions. It is demonstrated that the effect of solvation on the energy of an anion state depends on its lifetime, with larger shifts being observed for the shorter lived anions. 

Figure 10.6. Rate of anionic polymerization of styrene initiated by sodium naphthalene in 3-methyl tetrahydrofuran at 20°C. Left linear variation of rate with (C°)l/2 right inverse linear variation of rate with concentration of Na+ in presence of added sodium tetraphenyl borate. (Data from Schnitt and Schulz [79].)...

Examples of intramolecular trapping of carbonyl ylide dipoles by alkenes have now been reported.These include, for example, the conversion of the oxirane (172) into the tetrahydrofuran (173). Carbonyl ylides have also been prepared by irradiation of 2,3-bis-(p-methoxyphenyl)oxirane in the presence of dicyanoanthracene as electron-transfer sensitizer direct or triplet-sensitized irradiation, however, leads mainly to rearrangement via carbon-oxygen bond cleavage. In contrast, cyclohexene oxide and styrene oxide, on naphthalene-sensitized irradiation in alcohols, undergo solvolysis via oxide anion-radical intermediates. ... 

In an earlier report Mazzocchi and his coworkers reported that the photo-reaction of A) methylnaphthalimide (325) with phenyIcyclopropane involved the production of a radical cation/radical anion pair. The product from the reaction was the cyclic ether (326). - A study of the mechanism of this reaction using suitably deuteriated compounds has demonstrated that the reaction is not concerted and takes place via the biradical (327). - Other systems related to these have been studied. In the present paper the photoreactivity of the naphthalimide (328) with alkenes in methanol was examined. Thus, with 1-methylstyrene cycloaddition occurs to the naphthalene moiety to afford the adducts (329) and (330). The mechanism proposed for the addition involves an electron transfer process whereby the radical cation of the styrene is trapped by methanol as the radical (331). This adds to the radical anion (332) ultimately to afford the observed products. Several examples of the reaction were descr ibed. 

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). 

Problem 8.10 The kinetics of sodium naphthalene initiated anionic polymerization of styrene was studied in a less polar solvent dioxane at 35°C. Using an apparatus which permitted quick mixing of the reaction components in absence of air and moisture, aliquot samples were withdrawn periodically and deactivated quickly with ethyl bromide. From the residual monomer and the average degree of polymerization of the polymer formed the following data were obtained... 

ABA triblock copolymers cannot be produced by monofiinctional initiation when the A anion is not sufficiently basic to initiate polymerization of B monomers. In such cases the bifunctional initiators like alkali metal complexes of polycyclic aromatic compounds (e.g., naphthalene and biphenyl) can be used to produce ABA triblock copolymers. In these cases polymerization would be started with monomer B to produce a polymeric dianion which would initiate polymerization of the A monomer that is added later. The process is illustrated below for the commercially important styrene-butadiene-styrene (SBS) triblock copolymer ... 

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