Polymerization styrene, alkali-metal alkyl

If coordination were not important, the polymerization process would presumably resemble those initiated by alkali metal alkyls. The latter are very effective initiators for the polymerization of such monomers as styrene, butadiene, etc., and it is generally considered that propagation proceeds through intermediates of the type (43)... 

In initiation by an alkali-metal alkyl, the alkyl links up with the monomer to form a carbanion, leaving the metal as a cation to compensate the negative charge. An example is the initiation of styrene polymerization by butyl lithium [70-72] ... 

A major interest in narrow distribution polymers is for research and molecular weight calibrations in gel permeation chromatography. Narrow-molecular-weight polystyrenes are made by initiation with alkali metal alkyls that are particularly effective in this application but that only polymerize conjugated monomers like styrene or butadiene. 

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. 

The simplest structures, which can be either block or triblock copolymers, can be obtained by sequential polymerization of styrene and ethylene oxide. If styrene is first polymerized with an alkali metal alkyl initiator, butyllithium or amylsodium ((42, 170, 171),... 

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. 

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