Anionic chain polymerization electron transfer

Anionic chain polymerization can be initiated by metal alkoxides, aryls and alkyls and electron-transfer from sodium naphthalene. Alkyllithiums are among the most useful, being employed commercially in the polymerization of 1,3-butadiene, isoprene, and styrene. Initiation involves addition of alkyl anion to monomer... 

The first step in the polymerization is the electron transfer from sodium to dichlorosilane and the formation of the corresponding radical anion. The latter upon elimination of the chloride anion is transformed to the silyl radical. To fit the chain growth mechanism, the reactivities of the macromolecular radicals must be higher than the reactivities of the monomeric radicals. The latter after electron transfer and elimination of chloride anion could be transformed to the reactive silylenes. Thus, in principle, two or more mechanisms of chain growth are possible ... 

Anionic polymerization Initiated by electron transfer (e.g., sodium-naphthalene and styrene In THF) usually produces two-ended living polymers. Such species belong to a class of compounds called bolaform electrolytes (27) In which two Ions or Ion pairs are linked together by a chain of atoms. Depending on chain length, counterion end solvent, Intramolecular Ionic Interactions can occur which in turn may affect the dissociation of the ion pairs Into free ions or the llgand-lon pair complex formation constants. 

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

Apart from the relevance to the radiation-induced polymerizations, the pulse radiolysis of the solutions of styrene and a-methylstyrene in MTHF or tetrahy-drofuran (THF) has provided useful information about anionic polymerization in general [33]. Anionic polymerizations initiated by alkali-metal reduction or electron transfer reactions involve the initial formation of radical anions followed by their dimerization, giving rise to two centers for chain growth by monomer addition [34]. In the pulse radiolysis of styrene or a-methylstyrene (MS), however, the rapid recombination reaction of the anion with a counterion necessarily formed during the radiolysis makes it difficult to observe the dimerization process directly. Langan et al. used the solutions containing either sodium or lithium tetrahydridoaluminiumate (NAH or LAH) in which the anions formed stable ion-pairs with the alkali-metal cations whereby the radical anions produced by pulse radiolysis could be prevented from rapid recombination reaction [33],... 

A PET in intramolecular CPs between pyridinium ions and bromide, chloride or thiocyanate ions for polymerization initiation is described, too [137-139]. As expected, an equilibrium exists among free ions, ion pairs, and CT, which is shifted to the free ions in polar solvents and to the complex in a less polar solvent That complex serves as the photosensitive species for the polymerization (see Scheme 10). The photodecomposition of the CT yields radicals of the former anion and N-alkylpyridinyl radicals. Probably, the photopolymerization is initiated only by X- radicals, whereas latter radicals terminate the chain reaction. By addition of tetrachloromethane, the polymerization rate is increased owing to an electron transfer between the nucleophilic pyridinyl radical and CC14 (indirect PET). As a result, the terminating radicals are scavenged and electrophilic -CQ3 radicals are produced. 

Sonochemical homopolymerization of dichlorosilanes in the presence of sodium is successful at ambient temperatures in nonpolar aromatic solvents (toluene or xylenes) only for monomers with a-aryl substituents. Dialky 1-dichlorosilanes do not react with dispersed sodium under these conditions, but they can be copolymerized with phenylmethyldichlorosilane. Copolymers with a 30-45% content of dialkylsilanes were formed from equimolar mixtures of the corresponding comonomers. Copolymerization might indicate anionic intermediates. A chloroterminated chain end in the polymerization of phenylmethyldichlorosilane can participate in a two-electron-transfer process with sodium (or rather two subsequent steps separated by a low-energy barrier). The resulting silyl anion can react with both dichlorosilanes. The presence of a phenyl group in either a or P position in chloroterminated polysilane allows reductive coupling, in contrast to peralkyl species, which do not allow the reaction. Therefore, dialkyl monomers can copolymerize, but they cannot homopolymerize under sonochemical conditions. 

The macromolecular silyl chloride reacts with sodium in a two-electron-transfer reaction to form macromolecular silyl anion. The two-electron-trans-fer process consists of two (or three) discrete steps formation of radical anion, precipitation of sodium chloride and generation of the macromolecular silyl radical (whose presence was proved by trapping experiments), and the very rapid second electron transfer, that is, reduction to the macromolecular silyl anion. Some preliminary kinetic results indicate that the monomer is consumed with an internal first-order-reaction rate. This result supports the theory that a monomer participates in the rate-limiting step. Thus, the slowest step should be a nucleophilic displacement at a monomer by macromolecular silyl anion. This anion will react faster with the more electrophilic dichlorosilane than with a macromolecular silyl chloride. Therefore, polymerization would resemble a chain growth process with a slow initiation step and a rapid multistep propagation (the first and rate-limiting step is the reaction of an anion with degree of polymerization n[DP ] to form macromolecular silyl chloride [DP +J, and the chloride is reduced subsequently to the anion). 

In case the anion is the polymeric species (anionic polymerization) a carbanion or an alkoxide anion forms the active chain end. Initiation is achieved by direct attack of organometaUic compounds, or by electron transfer from alkali metals, alkali metal complexes, or ionizing radiation. In case the cation is the polymeric species (cationic polymerization) a... 

Electrically conducting polymers are quite different systems to the above elec-troinitiated chain polymerizations since they are formed by an unusual step-growth mechanism involving stoichiometric transfer of electrons. The polymers are obtained directly in a conductive polycationic form in which charge-compensating counter anions from the electrolyte system are intercalated into the polymer matrix [173], Exact mechanistic details remain the subject of discussion, but Scheme 4, which shows polypyrrole formation is plausible. Polythiophene is similar where S replaces NH in the ring. 

An alternative method of initiation is through the use of the radical anion produced from the reaction of sodium (or lithium) with naphthalene. Such radical anions react with styrene by electron transfer to form styrene radical anions these dimerize to produce a dianion, which initiates polymerization as outlined in Scheme 14. One particular feature of this method is that polymerization proceeds outwards from the centre. Subsequent reaction of the living chains ends with another suitable monomer system produces a triblock copolymer. This is the principle by which styrene-butadiene-styrene triblock copolymers (formed when butadiene is polymerized in the same way. and styrene is added as second monomer) are produced commercially. This material behaves as a thermoplastic elastomer, since the rigid styrene blocks form cross-links at room temperature on heating these rigid styrene portions soften, allowins the material to be remoulded. ... 

Chain polymerization involves three processes chain initiation, chain propagation, and chain termination. (A fourth process, chain transfer, may also be involved, but it may be regarded as a combination of chain termination and chain initiation.) Chain initiation occurs by an attack on the monomer molecule by a free radical, a cation, or an anion accordingly, the chain polymerization processes are called free-radical polymerization, cationic polymerization, or anionic polymerization. A free radical is a reactive substance having an unpaired electron and is usually formed by the decomposition of a relatively unstable material called an initiator. Benzoyl peroxide is a common free-radical initiator and can produce free radicals by thermal decomposition as... 

Both the 2,2-diphenyl vinyl and the l-methoxy-l,l-diphenylethyl chain ends are potential endgroups for the anionic polymerization of a variety of monomers by metalation. Our earlier results indicate that quantitative metalation of the 2,2-diphenylvinyl endgroups with alkyllithium cannot be achieved, most likely because of steric hindrance. However, as described recently, the ether cleavage of 1-methoxy-l,l-diphenyl-3,3,5,5-tetramethylhexane or electron transfer to 3,3,5,5-tetra-methyl-l,l-diphenylhex-l-ene by K/Na alloy, Cs or Li led to quantitative metalation resulting in nearly quantitative initiation of the polymerization of methacrylic monomers. Both precursors led to identical (macro)initiators verified by H NMR. These compounds can be considered as models of PIB chain ends formed by LCCP of IB and subsequent end-capping with DPE. The present study deals with the application of this method to the synthesis of different AB and ABA block copolymers by the combination of LCCP and living anionic polymerization. 

In principle, the appearance of electron donor groups near the double bond of the monomer leads to a cationic mechanism, while positive groups that withdraw electrons mostly lead to the anionic process. An increase of temperature usually leads to a decrease in rate of reaction or length of the chain. Polymerization always occurs in solution, wherein the solvent acts as separator of the ion-pair, and quite often as a transfer reagent. As already mentioned, most monomers undergo polymerization via free radicals, mainly when conjugated double bonds are present or substitute groups that withdraw electrons. 

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