Poly alkyl lithium initiated

Many random copolymers have found commercial use as elastomers and plastics. For example, SBR (62), poly(butadiene- (9-styrene) [9003-55-8] has become the largest volume synthetic mbber. It can be prepared ia emulsion by use of free-radical initiators, such as K2S20g or Fe /ROOH (eq. 18), or in solution by use of alkyl lithium initiators. Emulsion SBR copolymers are produced under trade names by such companies as American Synthetic Rubber (ASPC), Armtek, B. F. Goodrich (Ameripool), and Goodyear (PHoflex) solution SBR is manufactured by Firestone (Stereon). The total U.S. production of SBR in 1990 was 581,000 t (63). 

The preparations by anionic mechanism of A——A type block copolymers of styrene and butadiene can be carried out with the styrene being polymerized first. Use of alkyl lithium initiators in hydrocarbon solvents is usually a good choice, if one seeks to form the greatest amount of c/s-1,4 microstructure [346]. This is discussed in Chap. 4. It is more difficult, however, to form block copolymers from methyl methacrylate and styrene, because living methyl methacrylate polymers fail to initiate polymerizations of styrene [347]. The poly(methyl methacrylate) anions may not be sufficiently basic to initiate styrene polymerizations [345]. 

Subsequent work by D. Braun and collaborators dealt with the effect of solvent polarity and countercation on the tacticity of poly(methyl methacrylate) (PMMA) initiated with various organo alkali metal compounds. By using proton NMR analysis they concluded that polar solvents and alkyl lithium initiators favored syndiotactic configurate on( ) while non-polar solvents favored isotactic piacement( )." ... 

A wide variety of anionic initiators [352] can also affect the polymerization of vinyl ketones. Crystalline poly(alkyl vinyl ketones) were prepared by precipitation polymerization using metallic lithium or alkyl lithium catalysts. Thomas [342] reported that lithium dust initiation at — 25°C produced two types of poly(isopropyl vinyl ketone). The ether-soluble crystalline fraction was unstable. The highest crystalline samples melted to a... 

The difficulties encountered in the early studies of anionic polymerization of methyl methacrylate arose from the unfortunate choice of experimental conditions the use of hydrocarbon solvents and of lithium alkyl initiators. The latter are strong bases. Even at —60 °C they not only initiate the conventional vinyl poly-addition, but attack also the ester group of the monomer yielding a vinyl ketone1, a very reactive monomer, and alkoxide 23). Such a process is described by the scheme. 

The reactions are commonly initiated with AIBN at ca. 80 °C in a solvent such as benzene or toluene, but alkyl halides have also been reduced at -60 °C with tributyltin hydride under sonication conditions.133-134 Some reductions of halides have been carried out by generating the tin hydride in situ from a molar equivalent of the reducing agent [lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane)] and a catalytic amount of organotin hydride, halide, or oxide. Reduction of halides has also been carried out under aqueous conditions, using 4,4 -azobis(4-cyanovaleric acid) (ACVA) as a water-soluble radical initiator.60... 

Solvents influence the rate of free-radical homopolymerization of acrylic acid and its copolymerization with other monomers. Hydrogen-bonding solvents slow down the reaction rates. Due to the electron-withdrawing nature of the ester groups, acrylic and methacrylic ester polymerize by anionic but not by cationic mechanisms. Lithium alkyls are very effective initiators of a-methyl methacrylate polymerization yielding stereospecific polymers.Isotactic poly(methyl methacrylate) forms in hydrocarbon solvents. Block copolymers of isotactic and syndiotactic poly(methyl methacrylate) form in solvents of medium polarity. Syndiotactic polymers form in polar solvents, like ethylene glycol dimethyl ether, or pyridine. This solvent influence is related to Lewis basicity in the following order ... 

The structure of the synthetic poly(isoprenes) is exceptionally dependent on the polymerization process (Table 25-3). Commercially, poly(isoprene) is produced by polymerization in hydrocarbons with lithium alkyls. When aliphatic solvents are used, a gel content of 20%-35% is obtained, and this is practically independent of the initiator concentration and conversion. It is caused, presumably, by the occasional occurrence of 3,4 addition on the catalyst surface. Conversely, only small amounts of gel are formed in aromatic solvents, since such solvents form stable complexes with the lithium alkyl catalysts. 

The melt temperature of nylon-1 pol5uners depends on the legth of the side chain. The longer the side chain, the lower the melting point of the polsrmer. Steric requirements for the homopolymerization of monoisocyanates are severe. No homopolymers are obtained from isopropyl, cyclohexyl, o-methox5tphenyl, and a-naphthyl isocyanate. All alkyl nylon-1 polymers, with the exception of poly(allyl isocyanate), have a low order of crystallinity as shown by X-ray diffraction. However, the polymerization of re-butyl isocyanate and phenyl isocyanate in various solvents at — 78°C, initiated by lithium or sodium alkyls, produces crystalline pol5uners (28). The mechanism of the sodium cyanide catalyzed polymerization in DMF involves intermediates having a spiro structure (29). 

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