Initiators, alkali lithiums

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 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. 

Alkali metal salts with nucleophilic anions are notably good initiators for chloral anionic polymerization (Fig. 26). The most studied initiator is lithium ferf-butoxide. When 0.2 mole % of lithium ferf-butoxide (based on chloral) was added to neat chloral monomer at 60°C the alkoxide (CH3)3C0CH(CCl3)0 Li was formed instantaneously, but no further addition of chloral occurred. This reaction was observed by an NMR study of the system and confirmed by the chemical reactions of the product alkoxide, which acted as the initiator. Tertiary amines such as pyridine and NR3 where R is an alkyl group have been found to be good initiators for chloral polymerization. They are slower initiators than lithium... 

A competing reaction in any Birch reduction is reaction of the alkali metal with the proton donor. The more acidic the proton donor, the more rapid IS the rate of this side reaction. Alcohols possess the optimum degree of acidity (pKa ca. 16-19) for use in Birch reductions and react sufficiently slowly with alkali metals in ammonia so that efficient reductions are possible with them. Eastham has studied the kinetics of reaction of ethanol with lithium and sodium in ammonia and found that the reaction is initially rapid, but it slows up markedly as the concentration of alkoxide ion in the mixture... 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

One of the most widely explored systems is derived from the interpolation of Li between the TiS2 layers in varying amounts to form nonstoichiometric phases with a general formula LivTiS2. Because the bonding between the layers is weak, this process is easily reversible. The open nature of the structure allows the Li atoms to move readily in and out of the crystals, and these compounds can act as convenient alkali metal reservoirs in batteries and other devices. A battery using lithium intercalated into TiS2 as the cathode was initially developed some 30 years ago. 

It became apparent that the mechanism of anionic polymerisation induced by the alkali metals should also involve initiation by electron transfer from the metal to the monomer. For example, in the Lithium initiated polymerisation of butadiene, we have,... 

Easily ionizable anthracene forms the cation-radical as a result of sorption within Li-ZSM-5. In case of other alkali cations, anthracene was sorbed within M-ZSM-5 as an intact molecule without ionization (Marquis et al. 2005). Among the counterbalancing alkali cations, only Li+ can induce sufficient polarization energy to initiate spontaneous ionization during the anthracene sorption. The lithium cation has the smallest ion radius and its distance to the oxygen net is the shortest. The ejected electron appears to be delocalized in a restricted space around Li+ ion and Al and Si atoms in the zeolite framework. The anthracene cation-radical appears to be in proximity to the space where the electron is delocalized. This opens a possibility for the anthracene cation-radical to be stabilized by the electron s negative field. In other words, a special driving force for one-electron transfer is formed, in case of Li-ZSM-5. 

The effect of the addition of alkali metal b-butoxides on the lithium morpholinide-initiated heterogeneous butadiene polymerization was also studied at 30°C in hexane. The data in Table III show that the 1,2 content of the polybutadiene is sensitive to the type of alkali metal used. For example, by changing metal alkoxides from Li to Na to K, the 1,2 content of polybutadiene changes from 46.0% to 58.2% to 55.4%, respectively, at an initiator concentration of about 10 mmoles. However, the 1,2 content of the polybutadienes is unaffected by the metal alkoxide concentration (1,2 content, 42-46% for LiOtBu, 55-58% for NaOtBu, 53-55% for KOtBu) when the alkoxide/ morpholinide ratio is greater than 1 1. 

The discovery that lithium and its alkyls produce a highly cis-1,4 polyisoprene in hydrocarbon solvents (103) has led to a renewed interest in metal and metal alkyl initiated polymerization. About the same time Szwarc (109) postulated an electron transfer mechanism for the initiation of polymerization by sodium naphthalene in ether solvents. This was extended to lithium metal catalysis by Tobolsky (80) and Overberger (83) and subsequently generalized to cover all alkali metal initiation, e" + M M (1) ... 

Owing to uncertainties in the analysis methods, only general trends seem to warrant discussion. With butadiene, the 1,2/1,4 ratio increases in the series Li to Cs. In tetrahydrofuran the polymer is even more rich in 1,2 structures and varies little with the alkali metal or its compounds. With isoprene, only lithium and its compounds give a highly cis-1,4 polymer in hydrocarbons. Increasing amounts of the 3,4 structure occur with the other alkali metals. The amount of 1,4 polymer and its internal distribution is in doubt. In ethers the 3,4 polymer is the major constituent with all initiators. The N. M. R. data suggest the microstructure... 

The use of alkali metals for anionic polymerization of diene monomers is primarily of historical interest. A patent disclosure issued in 1911 (16) detailed the use of metallic sodium to polymerize isoprene and other dienes. Independendy and simultaneously, the use of sodium metal to polymerize butadiene, isoprene, and 2,3-dimethyl-1,3-butadiene was described (17). Interest in alkali metal-initiated polymerization of 1,3-dienes culminated in the discovery (18) at Firestone Tire and Rubber Co. that polymerization of neat isoprene with lithium dispersion produced high t /s- 1,4-polyisoprene, similar in structure and properties to Hevea natural rubber (see Elastomers, synthetic-polyisoprene Rubber, natural). 

For iodides, the order of efficiency is tetrabutyl > tetraethyl > tetramethyl 56 A rise in the catalytic effect of initiator is observed for alkali salts and for ammonium salts with increasing diameter of the cation, hence with growing distance between charge centres. This is in agreement with an increase of the electropositivity of cations and the increasing ability of salts to dissociate 54), Despite of this, the remarkable efficiency of lithium salts in curing of epoxy resins 58) or in copolymerization reactions 411 was confirmed in some papers. 

These are usually reactions of anhydrous transition and B metal halides with dry alkali metal salts such as the sulphides, nitrides, phosphides, arsenides etc. to give exchange of anions. They tend to be very exothermic with higher valence halides and are frequently initiated by mild warming or grinding. Metathesis is described as a controlled explosion. Mixtures considered in the specific reference above include lithium nitride with tantalum pentachloride, titanium tetrachloride and vanadium tetrachloride, also barium nitride with manganese II iodide, the last reaction photographically illustrated. 

Analogous products were obtained from the reaction of silylene 85 with silyl lithium compounds, with alkali metal silylamides and alkali metal alkylamides, and sodium methoxide <2000CC1427, 2004JCD3288, 2005JCD2720, 2005CC5112>. In the case of the reaction with metallated silylamides a thermally initiated rearrangement (114 — 115) to give the new silylamide 115 took place (Scheme 11). 

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],... 

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