Two-ended living anion

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. 

A bimolecular process was reported by two independent groups, i.e., Hocker [19] and Rempp [20] in 1980. Macrocyclic polymers have been successfully prepared by the coupling reaction of a two-ended living polystyryl anion with a difunctional electrophile such as a, a -dibromo-p-xylcnc under high dilution to yield cyclic and linear mixtures. The cyclic polymer was isolated by a fractional precipitation. This bimolecular end-to-end reaction process has been used for synthesizing cyclic polystyrene by many researchers [21-25]. 

Well developed is the anionic polymerization for the preparation of olefin/di-olefin - block copolymers using the techniques of living polymerization (see Sect. 3.2.1.2). One route makes use of the different reactivities of the two monomers in anionic polymerization with butyllithium as initiator. Thus, when butyl-lithium is added to a mixture of butadiene and styrene, the butadiene is first polymerized almost completely. After its consumption stryrene adds on to the living chain ends, which can be recognized by a color change from almost colorless to yellow to brown (depending on the initiator concentration). Thus, after the styrene has been used up and the chains are finally terminated, one obtains a two-block copolymer of butadiene and styrene ... 

A variation of the sequential anionic polymerization is the use of dianions as initiator, like sodium naphthalene. One starts with the polymerization of monomer A. Then monomer B is fed to the reaction mixture which adds immediately to the living anions at each end of block A and thus leads to a triblock copolymer with an A-middle block and two B-outer blocks. This triblock copolymer is still alive and repetition of the above procedure results in a multiblock copolymer (see Example 3-49). 

Another way of synthesizing block copolyers it to have two polymers which possess mutually reacting chain ends. A picturesque example is the mutual deactivation of living" cationic polytetrahydrofuran and of "living" anionic polystyrene.12... 

Living anionic polymerization has been used to place a central polystyrene chain between two dendrimers [124]. Prior to the coupling reaction at -78 °C the polystyrylpotassium reactivity is reduced by end-capping with diphenyleth-ylene (Scheme 14e). 

Recent developments have also been reviewed for the synthesis of telechelic (functional groups at both ends) and semitelechelic (functional group at one end) polymers via anionic methods (54). The use of two basic procedures is reported 1. termination of living anionic chains with suitable electrophiles, and 2. the use of functionally substituted anionic initiators. Two of these latter initiators are acetals that give good molecular weight control and monodispersity ... 

The kinetics of copolymerization provides a partial explanation for the copolymerization behavior of styrenes with dienes. One useful aspect of living anionic copolymerizations is that stable carbanionic chain ends can be generated and the rates of their crossover reactions with other monomers measured independently of the copolymerization reaction. Two of the four rate constants involved in copolymerization correspond at least superficially to the two homopolymerization reactions of butadiene and styrene, for example, and k, respectively. The other... 

It is amply evident that statistical copolymerization, both free radical and ionic, cannot produce an ideal network because of the unequal reactive ability of the comonomers in their competition for interaction with the active functional end of the growing polymer chain. However, using the so-caUed living anionic polymerization, it is possible to eliminate the competition between the comonomers by separating the stages of the formation of chain precursors and the formation of network per se, that is, chain crosslinking. Such an approach may be realized in two subsequent stages via anionic stepwise block polymerization of first styrene and then DVB. 

CRPs produce polymers with predetermined molecular weights, well-defined architectures, low polydispersities, and functional end groups. CRPs are similar to living anionic and cationic polymerizations in that they produce polymers with a narrow molecular-weight disttihution however, they have greater tolerance for functional groups such as alcohols, esters, amides, and carboxylates. ATRP and RAFT polymeiization ° are two of the most widely used CRP techniques. 

Hirao et al. have further developed the above functionahzation reactions using DPE derivatives in an excellent procedure referred to as the chain-multi-functionalization of living anionic polymers [176]. For this purpose, a new DPE derivative, l,l-bis(3-terFbutyldimethylsilyloxymethylphenyl)ethylene (12). has been synthesized. This DPE is designed in such a way that the tert-butyldimethylsilyl ether acts as a protected functionality in a reaction with living anionic polymers, and is quantitatively transformed into a benzyl bromide (BnBr) or even chloride and iodide functions [176, 183]. As illustrated in Scheme 5.18,12 was first reacted with PSLi to introduce two silyl ether functionalities at the chain-end, followed by treatment with Me3SiCl/LiBr to transform into two BnBr fimctions as a result, a well-defined chain-end-(BnBr)2-functionalized PS was obtained. The same functionalized DPE-derived anion was then separately synthesized, and reacted with the above chain-end-(BnBr)2-functionalized PS. The four silyl ether functionalities thus introduced were transformed into four BnBr functions by the same treatment with MesSiCl/LiBr, and this resulted in a chain-end-(BnBr)4-functionalized PS. As the coupling and transformation reactions proceeded both cleanly and quantitatively, the same reaction sequence could be repeated four more times to successfiiUy introduce 8,16, 32, and 64 BnBr functions at the chain-ends (Scheme 5.19 Table 5.3) [184]. Furthermore, the same reaction sequence could be carried out with a-(BnBr)2-functionalized PMMA to afford a series of weU-defined chain-end-BnBr-multi-functionalized PMMAs with up to 16 BnBr functions [185]. 

A variety of three-arm ABC star-branched polymers can also be synthesized via an addition reaction of a living anionic polymer to a chain-end-DPE-functionalized polymer, followed by the polymerization of an additional monomer (Scheme 5.26) [238, 242-244]. A four-arm ABCD star composed of PS, PaMS, PtBMA, and P2VP, could be synthesized by a similar methodology using l,4-bis(l-phenylethenyl)benzene [245]. Since 2000, two more four-arm ABCD stars have been synthesized using a new method that combines the above-described SiCl and BnX methodologies [246, 247]. 

A tadpole polybutadiene with two linear PS chains has been prepared by Quirck and Ma living anionic poly(styryl) lithium chains were first coupled by reacting the living ends with the double diphenylethylene. Then the resulting polymer bearing two reactive catbanionic centers located in the middle of the chain was used as a bifimcdonal macroinitiator to initiate the anionic polymerization of butadiene. The two... 

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