Anionic polymerization alkali metal alkyls

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]. 

Shima, Smid and Szwarc (56) studied the effect of the methyl substitution in the polymerization of butadiene, isoprene and dimethyl-butadiene. They showed that the electron-donating methyl group decreased the rate of polymerization catalysed by polystyrylsodium. This same electron releasing effect of the methyl is seen, since the 3.4-structure, not 1.2-structure, is produced predominantly from isoprene. This results from the anionic propagation mechanism of the alkali metal alkyl catalysed polymerization of dienes which produced 1.2 and 3.4-structures. 

Note Added in Proof Coordinated anionic polymerization of dienes by alkali metal alkyl catalysts and the effect of cation solvation on mechnism have been discussed in a similar manner by Medvedev and Gantmakher (231a). 

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

Anionic Polymerization. Complementing the diversity in microstructure inherent to anionic chemistry, living anionic polymerization based on alkali metal alkyl initiating systems can also afford a wide range of macrostructural possibilities for polybutadiene products. 

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. 

A wide variety of typical anionic initiators is described for the polymerization of PMVK. Grignard reagents are used as well as complexes formed of alkylaluminum or alkylzinc compounds with alkali metal alkyls (so called -ate complexes ). Alkali metal initiators and alkoxides are also described. Some examples are given in Table 5. 

With regard to anionic polymerization of conjugated dienes, alkali metal alkyls and free alkali metals are the most commonly used initiators. The resultant polymers generally have much higher contents of 3,4-units compared to the polymers prepared by free radical polymerization. When an alkali metal alkyl (M R") is the initiator, the initiation reaction may be represented as ... 

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... 

Enolate Initiators. In principle, ester enolate anions should represent the ideal initiators for anionic polymerization of alkyl methacrylates. Although general procedures have been developed for the preparation of a variety of alkali metal enolate salts, many of these compounds are unstable except at low temperatures (67,102,103). Useful initiating systems for acrylate polymerization have been prepared from complexes of ester enolates with alkali metal alkoxides (104,105). 

The nature of the initiating agents, the rapidity of the reaction, and the reluctance of butadiene and styrene to polymerize with Na in ammonia prompted Beaman to propose an anionic chain mechanism, in contrast to the stepwise reaction that had been suggested by Ziegler for the alkali-metal or alkali-alkyl initiated polymerization of butadiene. It was later shown that sodamide in liquid ammonia (56) would also polymerize styrene to a polymer of molecular weight 3000 and an anionic mechanism was suggested involving initiation by amide ion. 

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... 

The character of the counterion and the solvent both affect the microstruclure of polymers made anionically from dienes. In general, the proportion of 1,4 chains is highest for Li and decreases with decreasing clecironegativity and increasing size of the alkali metals in the order Li > Na > K > Rb > Cs. A very high (>90%) 1,4 content is achieved only with lithium alkyl or lithium metal initiation in hydrocarbon solvents. The properties of polymers of conjugated diolefins tend to be like those of thermoplastics if the monomer enchainment is 1,2 or 3,4 [reactions (4-3) and (4-4)]. Elastomeric behavior is realized from 1,4 polymerization and particularly if the polymer structure is cis about ihe residual double bond. 

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... 

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