One Electron Transfer Initiation

Alkali metals initiate anionic polymerizations of olefins in either homogeneous or heterogeneous conditions. This depends upon the metal and upon the solvent. For instance, potassium is soluble in ethers, like dimethoxyethane or tetrahydrofuran, and the initiation conditions are homogeneous. On the other hand, sodium dispersions are insoluble in hydrocarbons and the initiations are heterogeneous. Liquid ammonia is a solvent for many alkali metals, though for some, like sodium, it can be a reactant and form metal amides, provided that traces of iron are present. Initiation reactions in many metal solutions take place by an electron transfer from the metal to the monomer to form anion radicals. The resultant anion radicals may then undergo propagation reactions. These propagations can proceed anionically, or by a free-radical mechanism, or by both simultaneously. If the radicals are unstable the anion-radicals dimerize and the propagation proceeds by an anionic mechanism at both ends of the chain. When liquid ammonia is employed as the solvent, as stated earlier, the particular mechanism of initiation will depend upon the metal used. Lithium metal forms solutions in liquid ammonia and initiates polymerization of monomers like methacrylonitrile by an electron transfer [169]:

These radical-anions couple and chain growth takes places from both ends of the chains:

Dimerization of radical-ions depends not only upon the radical’s stability but also upon the p-energy changes that accompany the reaction [170, 171].

Potassium metal in ammonia, however, initiates polymerizations of monomers like methacry lonitrile or styrene differently. These reactions include additions of amide ions to the olefins and formations of amine groups at the end of the chains [172]:

Solutions of potassium metal in ethers, however, form ion radicals through additions of electrons to the monomers. It should be noted, that fresh solutions of potassium metal in various ethers like tetrahydrofuran or dimethoxyethane are blue in color. This blue color is attributed to a presence of spin-coupled electron pairs (e2). The initiation of styrene polymerization that takes place between O and -78°C is, therefore, pictured as both, reactions of the spin-coupled pair with the monomers and also additions of individual electrons [169]:

When conditions are heterogeneous and sodium metal dispersions are used in polymerizations of dienes, the initiation mechanism is also by an electron transfer. It is believed [135] that initially a 1,2 anion-radical forms. This is followed by a coupling reaction:

The coupling process:

a different kind of coupling is also possible:

The following initiation mechanism was postulated [173] for heterogeneous conditions. At first, the highly polarized monomers adsorb strongly to the metal surfaces. Electron transfer takes place. The adsorbed molecules are assumed to be still sufficiently mobile to be able to rotate after adsorption. The rotation allows the radical-anions that form to couple. The concentration of the anionic charges that develop on the methine carbons creates strong enough attractive forces to remove the metal cations from the surface. Solvents that strongly solvate the cations, like tetrahydrofuran, enhance the process. To fit the above mechanism [173], the monomers must be capable of adsorbing strongly to the metal surfaces and the solvents must also be strongly solvating. When the reactions are carried out without solvents or in inert solvents like heptane, the free-energy gain due to solvation is largely lost. This increases the probability that in such instances the propagations occur on the surface of the metals until the oligomers grow to a certain length. When sufficient length is reached to create a favorable free-energy change, desorption occurs and the molecules pass into solution [173]. Electron transfer initiations can also result from reactions of alkali metals with aromatic hydrocarbons or with aromatic ketones that result in formations of radical-ions:

The ion-radicals transfer both the electron and the ion charge to the monomers in the process of initiation, as was shown by Szwarc and coworkers [174–180]:

The styrene monomer-radical-ion is unstable and tends to dimerize:

Metal ketyls form in reactions of alkali metals with aromatic ketones in some polar solvents, like dioxane or tetrahydrofuran. These ketyls exist in equilibrium mixtures of monomeric anion-radicals and dimeric dianions [136]. Originally there was some controversy about the mechanism of initiation of monomers like acrylonitrile or methyl methacrylate by sodium benzophenone. The following mechanism was derived from spectral evidence. The initiation is by transfer of the electron and the ion charge [181–183]:

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