Initiation by alkali metals

The equilibrium between alkali metals and many acceptors is often shifted to the far right and, because of this, experimental studies of the equilibrium constant are very difficult. Such studies are only possible under certain conditions, including knowledge of the qualities of the metal, of the acceptor and of the solvent (see, for example, ref. 183). 

The polymerization of dienes by sodium metal has been known since the beginning of this century [184]. Ziegler et al. [185] postulated dianion formation from butadiene 

Vinyl monomers with an aromatic group in the molecule are also efficient acceptors. Styrene and its derivatives undergo several transformations under the effect of alkali metals 

The relatively slow reaction [40) is the reason why a polymer with various chain lengths is produced. New centres continue to be generated during the course of the polymerization with less and less monomer available. This difficulty is circumvented by experimental separation of initiation from propagation. A solution of oligomeric diions is added to the system prepared for polymerization. This technique requires special measures to prevent 

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If coordination were not important, the polymerization process would presumably resemble those initiated by alkali metal alkyls. The latter are very effective initiators for the polymerization of such monomers as styrene, butadiene, etc., and it is generally considered that propagation proceeds through intermediates of the type (43)... 

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 ... 

The anionic polymerization of cyclic siloxanes can be initiated by alkali metal hydroxides, alkyls, and alkoxides, silanolates such as potassium trimethylsilanoate, (CH3)3SiOK, and other bases. Both initiation... 

Although the polymerization of olefins by heat and acid catalysts has been known since about 1840, polymerization by alkali metals and their alkyls was discovered more recently. Initiation by alkali metals was described by Matthews and Strange (65), Harries (42), and... 

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. 

Apart from the relevance to the radiation-induced polymerizations, the pulse radiolysis of the solutions of styrene and a-methylstyrene in MTHF or tetrahy-drofuran (THF) has provided useful information about anionic polymerization in general [33]. Anionic polymerizations initiated by alkali-metal reduction or electron transfer reactions involve the initial formation of radical anions followed by their dimerization, giving rise to two centers for chain growth by monomer addition [34]. In the pulse radiolysis of styrene or a-methylstyrene (MS), however, the rapid recombination reaction of the anion with a counterion necessarily formed during the radiolysis makes it difficult to observe the dimerization process directly. Langan et al. used the solutions containing either sodium or lithium tetrahydridoaluminiumate (NAH or LAH) in which the anions formed stable ion-pairs with the alkali-metal cations whereby the radical anions produced by pulse radiolysis could be prevented from rapid recombination reaction [33],... 

The classical examples are the polymerization of styrene in NH3, initiated by KNH2 [176], and the polymerization of octamethylcyclotetrasiloxane initiated by alkali metal hydroxides [177]. At the present time, simple strong mineral bases are no longer used for the initiation of anionic polymerizations. 

In initiation by alkali metal, an electron is transferred from the metal to the monomer (the metal is usually charged as a solid or colloid). Sodium-initiated butadiene polymerization provides an example [74,75] ... 

Rates of reaction (Rp) in ring opening polymerizations initiated by alkali metals are typically proportional to a fractional order of the initiator and close to first order in cyclosiloxane ... 

The initiation step is normally fast in polar solvents and an initiator-free living polymer of low molecular weight can be produced for study of the propagation reaction. The propagation step may proceed at both ends of the polymer chain (initiation by alkali metals, sodium naphthalene, or sodium biphenyl) or at a single chain end (initiation by lithium alkyls or cumyl salts of the alkali metals). The concentration of active centres is either twice the number of polymer chains present or equal to their number respectively. In either case the rates are normalized to the concentration of bound alkali metal present, described variously as concentration of active centres, living ends or sometimes polystyryllithium, potassium, etc. Much of the elucidation of reaction mechanism has occurred with styrene as monomer which will now be used to illustrate the principles involved. The solvents commonly used are dioxane (D = 2.25), oxepane (D = 5.06), tetrahydropyran D = 5.61), 2-methyl-tetrahydrofuran (D = 6.24), tetrahydrofuran (D = 7.39) or dimethoxy-ethane D = 7.20) where D denotes the dielectric constant at 25°C. 

The modem concept of linear polymers was introduced in 1920 by Staudinger, who fully recognized the idea of chain addition reaction yielding long molecules composed of monomeric units linked by covalent bonds. He was also the first to understand the anionic character of formaldehyde polymerization initiated by bases such as sodium methoxide4. In fact, studies of this reaction led him to the notion of linear macromolecules. Polymerization of ethylene oxide initiated by alkali metals and reported as early as 18785 could also be interpreted in these terms. 

Exchange Reactions In epoxide polymerizations initiated by alkali metal alkox-ides and hydroxides, protonic substances such as water or alcohol are often added to dissolve the initiator and produce a homogeneous system. However, these substances can contribute to an exchange reaction with the propagating chain. In the presence of alcohol (ROH), for example, the exchange reaction can be represented by R-fOCH2CH2-) rO-Na+ + ROH ... 

An anionic mechanism is proposed for those polymerizations initiated by alkali metal organometallic species, where there is good reason to assume that the... 

An anionic mechanism is proposed for those polymerizations initiated by alkali metal organometallic species, where there is good reason to assume that the metal is strongly electropositive relative to the carbon (or other) atom at the tip of the growing chain [21,143-151]. However, analogous to the discussion of the active species in cationic polymerization, a multiplicity of active species may be involved as propagating species in anionic polymerization as shown below [150]. In contrast to cationic polymerization, however, there is experimental evidence for the involvement of many of these species under certain experimental conditions [145,147,148]. 

Kurcok R, Matuszowicz A., Jedlinski Z., Kricheldorf H.R., Dubois R, Jerome R., Substituent effect in anionic polymerization of b-lactones initiated by alkali metal alkoxides, Macromol. Rapid Commun., 16,1995, 513-519. 

Anionic Polymerization. Anionic polymerizations (Eqs. 8-9) are industrially employed for the production of thermoplastics, most notably that of formaldehyde and -caprolactam and block copolymers (e.g., thermoplastic elastomers of the styrene-butadiene-styrene type). Initiation by alkali metals is achieved by adding across the double bond in anionic polymerization. 

The mechanism of the anionic polymerization of st5T enes and 1,3-dienes initiated by alkali metals has been described in detail (27). Initiation is a heterogeneous process occurring on the surface of the metal (Mt) by reversible transfer of an electron to adsorbed monomer (M), as shown in the following scheme ... 

Soum and Fontanille report that di-s-butyl magnesium generates living polymer from 2-vinylpyridine without the involvement of the side-reactions that afflict the polymerization initiated by alkali metal alkyls the resulting polymer has an isotacticity index of 0.9. Arai et al. have synthesized styrene-butadiene-4-vinylpyridine triblock copolymers. Hogen-Esch et a/. have continued their study of the stereochemistry of the anionic polymerization of 2-vinylpyridine in THF solution. Oligomers were synthesized by addition of alkali salts of 2-ethylpyridine to 2-vinylpyridine termination was effected by reaction with methyl iodide. Highly isotactic products were obtained with U and Na as counterions but with K or Rb there was no stereoselection. Epimerization resulted in the expected statistical mixtures of stereoisomers and it was concluded that stereoselection is kinetically controlled. 

Hexamethylguandinium chlorides have been found to exhibit a pronounced acceleratory effect on the polymerization of both caprolactam and 2-pyrrolidone for systems initiated by alkali metal salts of these lactams. A series of papers have investigated the effect of reaction variables on the polymerization of 2-pyr-... 

Anionic Epoxide Polymerization Initiated by Alkali Metal Derivatives... 

In contrast to the living character of the polymerization of EO, the anionic polymerization of substituted epoxides like POx initiated by alkali metal derivatives is subject to transfer reactions to the monomer. Indeed, the highly basic alkoxide propagating species can pull out a proton of the monomer substituent, leading to chain termination and the formation of a new growing chain bearing a terminal double bond, as indicated in Scheme 12 in the case of POx. 

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