Lithium-metal reactions Living polymers

The initiation step is normally fast in polar solvents and an initiator-free living polymer of low molecular weight can be produced for study of the propagation reaction. The propagation step may proceed at both ends of the polymer chain (initiation by alkali metals, sodium naphthalene, or sodium biphenyl) or at a single chain end (initiation by lithium alkyls or cumyl salts of the alkali metals). The concentration of active centres is either twice the number of polymer chains present or equal to their number respectively. In either case the rates are normalized to the concentration of bound alkali metal present, described variously as concentration of active centres, living ends or sometimes polystyryllithium, potassium, etc. Much of the elucidation of reaction mechanism has occurred with styrene as monomer which will now be used to illustrate the principles involved. The solvents commonly used are dioxane (D = 2.25), oxepane (D = 5.06), tetrahydropyran D = 5.61), 2-methyl-tetrahydrofuran (D = 6.24), tetrahydrofuran (D = 7.39) or dimethoxy-ethane D = 7.20) where D denotes the dielectric constant at 25°C. 

Hyperbranched polymers have also been prepared via living anionic polymerization. The reaction of poly(4-methylstyrene)-fo-polystyrene lithium with a small amount of divinylbenzene, afforded a star-block copolymer with 4-methylstyrene units in the periphery [200]. The methyl groups were subsequently metalated with s-butyllithium/tetramethylethylenediamine. The produced anions initiated the polymerization of a-methylstyrene (Scheme 109). From the radius of gyration to hydrodynamic radius ratio (0.96-1.1) it was concluded that the second generation polymers behaved like soft spheres. 

Further support for the proposed mechanism is provided by the results of experiments involving phenylbromide instead of ethylbromide (Jj6). The polarizable TT electrons of this aryl compound allow it to effectively compete with styrene for the sites on the lithium surface and thus the Wurtz coupling reaction becomes dominant. Similar results were obtained with ethyltosylate. Although the reaction of tosylate with living polystyrene is rapid and quantitative, yielding ethyl capped polymers, its reaction with the monomer and metallic lithium produces only 10% of the ethyl capped polymers, the remainder being evolved as butane. Again, the aromatic nature of tosylate allows it to compete with styrene for the lithium sites. 

Polymeric organolithium compounds exhibit limited stability in ether solvents similar to alkyllithium compounds. Living carbanionic polymers react with ether solvents such as THF in a pseudo-tirst-order decay process and the rate decreases in the order Li > Na > K. For example, a 10 M solution of poly(styryl)lithium in THF at 25 °C exhibited a rate of decay of a few percent per minute, but poly(styryl)cesium was found to be exceptionally stable [96], Metalation and decomposition reactions can also occur in the presence of amines such as TMEDA. 

Typical examples of initiators for cycHc carbonates-as shown for DTC-are alkali metal organic compounds such as sec-butyUithium (seoBuLi), sodium- and potassium naphthalene, and Hthium-, sodium- and potassium alkoxides or polymeric living vinyl or diene polymers with alkah metal counterions, as weU as polymeric alcoholates. The use of these macroinitiators enables the identification of side reactions, as will be shown exemplarily for polystyrene lithium (PS li ) [25]. Besides the initiation reaction of PS"Li, which represents a site transformation of... 

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