Ammonia liquid, anionic polymerization

One of the most interesting processes in electrically initiated polymerization was an initiation with the solvated electron proposed by Laurin and Parravano (22), who studied electro-anionic polymerization of 4-vinylpyridine in liquid ammonia solution of alkali metal salts in the temperature range — 33 to — 78° C. Rapid and efficient polymerization occurred and conversions of monomer to polymers formed exclusively at the cathode in the form of an orange-red, porous, solid deposit, suggesting the formation of a pile of living polymers. 

Cyano and alkoxycarbonyl groups are favorable in this respect and propeneni-trile and methyl 2-methylpropenoate can be polymerized with sodium amide in liquid ammonia. Ethenylbenzene and 2-methyl-1,3-butadiene undergo anionic polymerization under the influence of organolithium and organosodium compounds, such as butyllithium and phenylsodium. 

The work of Wooding and Higginson (18) provides further evidence for the fast reduction of styrene by a solution of potassium in liquid ammonia, a reduction that proceeds more readily than the electron transfer process. These workers found that the anionic polymerization of... 

Higginson, W. C. E., and N. S. Wooding Anionic polymerization. Part I. The polymerization of styrene in liquid ammonia solution catalysed by potassium amide. J. Chem. Soc. (London), p. 760. 

Using the fact that the rate of anionic polymerization of a-methyl-styrene, initiated by potassium amide in liquid ammonia, is smaller than the rate of hydrogen exchange, it was possible to establish (Shatenshtem et al., 1962b) that hydrogen-deuterium exchange occurs... 

The first quantitative kinetic study of an anionic polymerization was afforded by Higginson and Wooding (27), who studied the polymerization of styrene using potassium amide in liquid ammonia at — 33°. The rate expression was found to be... 

Liquid ammonia can be used as a solvent in the anionic polymerization of styrene initiated chemically (15). Moreover, the basic nature of ammonia and amines needs no further justification. These data appear to lead to the tentative conclusion that the growing species in these systems is the positively charged carbonium ion, which may terminate with ammonia (and amines) either by a proton transfer mechanism or an addition reaction to form a nonpolymerizing ammonium ion (8)—viz.,... 

Hydrodimerization of AN by Na(Hg) may also take place in liquid ammonia [45] in the presence of a suitable proton donor the dimerization is faster than the amination. If water is used as proton donor, ADN is formed in 80% yield, but when NH4CI is used, the yield is raised to nearly 100%. The ratio of PN to ADN depends on the ratio between NH4CI and AN when the ratio is smaller than unity, some polymer is formed because there is not enough proton donor to terminate the anionic polymerization. 

Polymerization of styrene in liquid ammonia at low temperatures catalyzed by potassium metal represents a good example of a base-initiated anionic polymerization. Styrene, being relatively nonpolar, requires the strongly basic amide ion for effective anionic polymerization. The catalyst is made, in situ, usually before the styrene is added, by the addition of small pieces of potassium metal to liquid ammonia kept at dry ice (solid carbon dioxide) temperatures (Eq. 22.30). 

Potassium ions are also present in this solution, but because of the high dielectric constant and high solvating capability of liquid ammonia the propagating species formed here is truly a free ion, not more than very loosely associated with the cation. The less polar the solvent used, the greater would be the degree of association between the propagating anion and the cation present. Thus, the polarity of the solvent chosen can affect the rate of formation and sometimes also the stereochemistry of the anionic polymerization product. 

The polymerization rate for an anionic polymerization where termination occurs simultaneously with propagation follows in a manner similar to free-radical polymerizations. An example is the potassium amide initiated polymerization in liquid ammonia. This is one of the few anionic systems in which all active centers behave kinetically as free ions. In this system, initiation involves dissociation followed by addition of amide ion to monomer ... 

Amide-initiated polymerizations in liquid ammonia are about the only anionic polymerizations studied that proceed with termination. 

Most of the early mechanistic investigations of anionic polymerization were concerned with reactions taking place in liquid ammonia. The system liquid ammonia-alkali metals will be dicussed first, followed by a review of heterogeneous reactions taking place on alkali or alkali-earth surfaces. Thereafter homogeneous electron-transfer processes and the addition of negative ions to monomer will be discussed. Finally some esoteric reactions, such as initiation by Lewis bases, charge-transfer complex initiation, etc. will be briefly reviewed. 

The solubility of alkali metals in most of the solvents used in anionic polymerization is exceedingly low. Liquid ammonia and hexamethylphosphoric-triamide are exceptional in this respect relatively high concentrations of the metals can be attained in these media. Several distinct species co-exist in alkali metal solutions. A minute but constant concentration of unionized atoms, Met, is maintained when a solvent is kept in contact with solid alkali metal. These in turn are in equilibrium with the products of their ionization ... 

An aprotic solvent is used in order to prevent a proton from fixing on a carbanion C- with transfer of ionization to a solvent molecule. Thus, for this reason, liquid ammonia which had been used for the first anionic polymerizations, is no longer chosen as a solvent. 

Available kinetic data for the polymerization of styrene by potassium amide in liquid ammonia support the following steps in the mechanism of anionic polymerization. 

One of the first anionic reactions to be studied was the polymerization of styrene in liquid ammonia (Higginson and Wooding, 1952). This type of anionic polymerization consists of a two-step initiation process ... 

Bases were used as polymerization catalyst since early days of polymerizalion studies, but showed limited usefulness due to the encountered low extent of polymerization. Reaction conditions are adjusted to enhance living anionic polymerization as applied to styrene in liquid ammonia through initiation by K NH2. Addition of the NH2 to olelinic part of the monomer generates the intermediate carbanion which propagates until termination is attained by abstraction of proton by the anion from the ammonia. If this type of termination occurred fast, then a limited polymerization process would be attained as shown in Scheme 3.8. 

The termination steps in anionic polymerizations can result from deliberate introductions of carbanion quenchers, such as water or acids, or from impurities. Terminations, however, can take place in some instances through chain transferring a proton from another molecule like a solvent or a monomer, or even from a molecule of another polymer. In some solvents, like liquid ammonia, transfer to solvent is extensive, as in styrene polymerization by amide ions. ... 

The polymerization of styrene in liquid ammonia, initiated by potassium amide, was one of the first anionic polymerizations to be studied in detail. In this polymerization chain transfer to ammonia terminates the growth of polymer chains and living polystyrene is not formed. The reaction is of minor importance nowadays but will be briefly considered since it provides a further example of kinetics analysis. 

A t5rpical example of anionic polymerization, running with participation of free carbanions, is the poljmierization of styrene initiated by potassium amide in liquid ammonia. The mechanism of this reaction presents as follows ... 

The polymerization of styrene in liquid ammonia, initiated by potassium amide was one of the first anionic polymerization reactions studied in detail. It is of particular interest because propagation takes place through free ions rather than by ion-pairs because of the high dielectric constant of liquid ammonia. The reaction takes place at 240 K (-33°C). Initiation consists of two processes. The first is the breakdown of initiator into ions... 

The polymerization of S initiated by potassium amide in liquid ammonia yields polymers with one primary amine group per chain (Scheme 34)." Initiators with blocked amino groups such as p-lithio-N,iV-bis(trimethylsilyl)aniline (54) have been used to initiate anionic polymerizations. Difunctionality was then achieved by coupling two growing chains with dimethyldichlorosilane, and deprotecting the primary amine end groups by acid hydrolysis. 

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