Sodium naphthalene initiator, anionic

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apparatus which permitted quick mixing of the reaction components in absence of air and moisture, aliquot samples were withdrawn periodically and deactivated quickly with ethyl bromide.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... 

When the polymerizations of cyclic sulfides are carried out with anionic initiators, many side reactions can occur. On the other hand, common anionic initiators, like KOH, yield optically active polymers from optically active propylene sulfide. An example of a side reaction is formation of propylene and sodium sulfide in sodium naphthalene initiated polymerizations. 

Polytetrafluoroethene (Teflon) is etched, giving an active surface which can then be bonded strongly to an epoxy resin. With a source of acidic hydrogen such as water or an alcohol, sodium naphthalene affords 1,4-dihydronaphthalene. Polymers with functional groups at each end of the chain ( telechelic polymers) can be made by polymerization of styrene with sodium naphthalene. The anionic radical which is produced in the initiation step dimerizes to a dianion containing two propagating centres. 

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). 

An interesting initiator is the naphthalene radical-anion formed by electron-transfer from sodium to naphthalene... 

The precipitated silica (J. Crosfield Sons) was heated in vacuo at 120° for 24h. before use. Two grades of surface areas 186 and 227 m g l (BET,N2), were used during this project. Random copolymers, poly(methyl methacrylates) and polystyrene PS I were prepared by radical polymerization block polymers and the other polystyrenes were made by anionic polymerization with either sodium naphthalene or sodium a methylstyrene tetramer as initiator. The polymer compositions and molecular weights are given in Table I. 

Some alkali metal complexes are also capable of initiating anionic polymerisation. For example, sodium and naphthalene in tetrahydrofuran is a homogeneous solution initiates polymerisation as follows ... 

Szwarc and coworker have studied the interesting and useful polymerizations initiated by aromatic radical-anions such as sodium naphthalene [Szwarc, 1968, 1974, 1983]. Initiation proceeds by the prior formation of the active initiator, the naphthalene radical-anion (XVIII)... 

The reaction involves the transfer of an electron from the alkali metal to naphthalene. The radical nature of the anion-radical has been established from electron spin resonance spectroscopy and the carbanion nature by their reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. 5-65 depends on the electron affinity of the hydrocarbon and the donor properties of the solvent. Biphenyl is less useful than naphthalene since its equilibrium is far less toward the anion-radical than for naphthalene. Anthracene is also less useful even though it easily forms the anion-radical. The anthracene anion-radical is too stable to initiate polymerization. Polar solvents are needed to stabilize the anion-radical, primarily via solvation of the cation. Sodium naphthalene is formed quantitatively in tetrahy-drofuran (THF), but dilution with hydrocarbons results in precipitation of sodium and regeneration of naphthalene. For the less electropositive alkaline-earth metals, an even more polar solent than THF [e.g., hexamethylphosphoramide (HMPA)] is needed. 

The radical-anion proceeds to propagate in the same manner as discussed above for initiation by sodium naphthalene. (Polymerizations in liquid ammonia are very different from those in organic solvents in that free ions probably constitute the major portion of propagating species.)... 

Isocyanates are polymerized through the carbon-nitrogen double bond to 1-nylons by anionic initiators such as metal alkyls, sodium naphthalene, and sodium cyanide [Bur and Fetters,... 

The anionic polymerization of epoxides such as ethylene and propylene oxides can be initiated by metal hydroxides, alkoxides, oxides, and amides as well as metal alkyls and aryls, including radical-anion species such as sodium naphthalene [Boileau, 1989 Dreyfuss and Drefyfuss, 1976 Inoue and Aida, 1984 Ishii and Sakai, 1969]. Thus the polymerization of ethylene oxide by M+A involves initiation... 

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 ... 

A variation of the sequential anionic polymerization is the use of dianions as initiator, like sodium naphthalene. One starts with the polymerization of monomer A. Then monomer B is fed to the reaction mixture which adds immediately to the living anions at each end of block A and thus leads to a triblock copolymer with an A-middle block and two B-outer blocks. This triblock copolymer is still alive and repetition of the above procedure results in a multiblock copolymer (see Example 3-49). 

Eor comparison, polystyrene and poly(4-vinylpyridine) are prepared by anionic polymerization with sodium naphthalene as initiator. Poly(4-vinylpyridine) precipitates from THE the mixture is poured into 200 ml of diethyl ether and the polymer filtered off.The polymer is then reprecipitated from pyridine solution into a ten-fold amount of diethyl ether and dried in vacuum. 

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. 

Polymer Preparation. Poly-para-methylstyrene (P-p-MS) was prepared by anionic polymerization in benzene at 50"C initiated by n-butyllithium (9) or in THF at 25°C initiated by sodium naphthalene (10). Polymerizations in benzene allowed preparation of more monodisperse materials than those prepared in THF since the propagation rate is slower relative to the initiation rate in the nonpolar solvent (11). Two different molecular weight materials were chlorinated (P-p-MS 1 and P-p-MS2). 

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 ... 

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... 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, arc efficient difunctionai initiators, However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization. 

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... 

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].)...

Scheme 14 Polymerization of polystyrene initiated with the sodium naphthalene anion.

The initiation of ethylene oxide polymerization by sodium naphthalene involves direct addition of the monomer to the radical anion and reduction of the adduct by sodium naphthalene producing a dianion. Suggest two tests to support this mechanism. 

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