Anionic polymerization metal amides

A variety of basic (nucleophilic) initiators have been used to initiate anionic polymerization [Bywater, 1975, 1976, 1985 Fontanille, 1989 Hsieh and Quirk, 1996 Morton, 1983 Morton and Fetters, 1977 Quirk, 1995, 1998, 2002 Richards, 1979 Szwarc, 1983 Young et al., 1984]. These include covalent or ionic metal amides such as NaNFU and LiN(C2H5)2, alkoxides, hydroxides, cyanides, phosphines, amines, and organometallic compounds such as n-C4H9Li and <)>MgBr. Initiation involves the addition to monomer of a nucleophile (base), either a neutral (B ) or negative (B ) species. 

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... 

Strong bases such as alkali metals, metal hydrides, metal amides, metal alkoxides, and organometallic compounds initiate the polymerization of a lactam by forming the lactam anion XXXIV [Hashimoto, 2000 Sebenda, 1989 Sekiguchi, 1984], for example, for e-caprolactam with a metal... 

For the anionic polymerization of methacrylonitrile (MAN), many initiators have been developed, which include alkali-metal alkyls such as butyllithium [42], triphenylmethylsodium [43], phenylisopropylpotassium [43], the disodium salt of living a-methylstyrene tetramer [44], alkali-metal amides [45], alkoxides [46], and hydroxide [47], alkali metal in liquid NH3 [48], quaternary ammonium hydroxide [49], and a silyl ketene acetal coupled with nucleophilic or Lewis acidic catalysts [50]. However, only a single example of the synthesis of PMAN with narrow molecular-weight distribution can be cited, and the reported number-average molecular weights were much higher than those calculated from the stoichiometry of the butyllithium initiator [42]. 

Methacrylonitrile can be polymerized almost instantaneously at —75° in liquid ammonia with lithium metal as initiator (83, 84). It was suggested that initiation occurs by a rapid electron transfer to monomer followed by a fast anionic reaction. Lithium amide produced in the reaction itself is not the initiator for it is a comparatively slow initiator of polymerizations at the temperature used. The polymer ions apparently abstract a proton from ammonia to form lithium amide which then reacts with nitrile groups on the polymer to produce cyclic structures. It is believed that this reaction is slow compared to the polymerization process. 

Anionic Polymerization. Anionic polymerization is limited to nonpolar monomers with carbon-to-carbon double bonds. It takes place in the presence of catalysts capable of generating carbanions—e.g., alkali metals, metal hydrides, metal alkyls, amides, and Grignard reagents. The... 

Initiation. The three most common kinds of initiators for anionic polymerization are alkali-metal alkyls, metal amides, and elementary alkali metals. 

Much faster reaction can be achieved with strong bases (/07). The chain is started by an N-acylimide which may be N-caproyl-caprolactam produced in a slow reaction from the monomer during an induction period or an N-acylamid produced by action of a cocatalyst like for example carboxylic acid chlorides or anhydrides on caprolactam. The cocatalyst action speeds up the reaction such, that fast polymerization below the melting point of the polymer becomes possible. The strong base, such as alkali metal, metal hydride, metal amid, or oiganometallic compound, activates the monomer by lactam anion formation ... 

Since most of the synthetically useful enolate anions described in the previous section are prepared by the reactions of enolizable substrates with alkali metal amide bases, it is appropriate to note a few structures of these amide bases. The common bases in synthetic organic chemistry include LDA and LHMDS. The structures of both of these bases are known as the THF solvates.Both of these compounds form bis-solvated dimers corresponding to structure (201). The diethyl ether solvate of LHMDS also forms a bis-solvated dimer (202).Sodium hexamethyldisilazide crystallizes as an unaggregated monomer from benzene solution.Two different cryst line forms of KHMDS are known as the polymeric dioxane solvate (203), ° and the unsolvated dimer (204). ... 

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). 

This reaction can be initiated by strong bases (metal hydrides, alkali metals, metal amides, metal alkoxides, and organometallic compounds), protonic acids, or by water. Water is often used as the initiator (cf. Problem 10.7) for industrial polymerization of lactams and the process is referred to as hydrolytic polymerization. Anionic initiation with bases is preferred when polymerization is to be carried out in the mold itself for converting monomer directly into a molded object. Cationic initiation with acids, however, is not so useful because both the extents of conversion and polymer molecular weights are significantly lower (Odian, 1991). 

Anionic Polymerization. Polymerization of olefins by metallic sodium, sodium ethoxide, and aqueous alkalies has been known for some time. The mechanism of this polymerization is still open to question though the present evi( ence indicates that the mechanism is similar to that of cationic polymerization, except that a polymeric anion is formed and the counterion is a cation. For example, potassium in liquid anunonia forms potassium amide, the amide ion attacks the monomer, forming a n ative ion. This native ion attacks another monomer, and so on, until termination occurs. 

M+B may be a metal amide, alkoxide, alkyl, aryl, and hydroxide depending on the nature of the monomer. The effectiveness of the catalyst in the initiation process depends on its basicity and the acidity of the monomer. For example, in the anionic polymerization of styrene, the ability to initiate reaction decreases in the order O-CHj-NH 2>NHj OH . Indeed, OH will not initiate anionic polymerization of styrene. Where the anion is polyvalent, such as tra(sodium ethoxy) amine NfCHjCHjCk Na+)j, an equivalent number of growing chains (in this case, three) can be initiated simultaneously. 

The kinetic picture of anionic chain polymerization also depends mostly upon the specific reaction. For those that are initiated by metal amides in liquid anunonia, the rate of initiation can be shown to be as follows ... 

Polyisocyanates are obtained by anionic polymerization using initiators such as NaCN, metal alkoxide, metal naphthalenide, metal amides, and organolithiums and have the structure of... 

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