Living carbanion

Anionic polymerization of ethylene oxide by living carbanions of polystyrene was first carried out by Szwarc295. A limited number of methods have been reported in the preparation of A-B and A-B-A copolymers in which B was polystyrene and A was poly(oxyethylene)296-298. The actual procedure was to allow ethylene oxide to polymerize in a vacuum system at 70 °C with the polystyrene anion initiated with cumyl potassium in THF299. The yields of pure block copolymers are usually limited to about 80% because homopolymers are formed300. ... 

Interest in anionic polymerizations arises in part from the reactivity of the living carbanionic sites4 7) Access can be provided to polymers with a functional chain end. Such species are difficult to obtain by other methods. Polycondensations yield ro-functional polymers but they provide neither accurate molecular weight control nor low polydispersity. Recently Kennedy51) developed the inifer technique which is based upon selective transfer to fit vinylic polymers obtained cationically with functions at chain end. Also some cationic ring-opening polymerizations52) without spontaneous termination can yield re-functional polymers upon induced deactivation. Anionic polymerization remains however the most versatile and widely used method to synthesize tailor made re-functional macromolecules. 

According to the competition theory (p. 39) for short-lived carbanions. According to thermodynamic control for long- w... 

We will refer to carbanions which give elimination faster than rotation, being responsible for the clean retention or inversion for both isomers, as short-lived carbanions . They may be short-lived if the C—Xbond breaking has very slightly progressed even at (90) and in conformations on the way to (91) concurrently with the bond formation. The simultaneous development of partial double-bond character between Ca and Cp creates a barrier for rotation. The conformer which requires the least rotation is formed, and the properly situated X leaves from this conformation. An alternative explanation, based on high population for conformations (91c) and (92t), seems much less likely. 

Formation of both isomers (100 and 101) from the reaction of alkoxide ions with cis-(99) and fnms-2,3-dichlorohexafluoro-2-butene (102) (Park and Cook, 1965) should result from a genuine addition-elimination since elimination-addition is impossible, and there was no isomerization of the starting olefin. MeO-, EtO- and i-PrO- ions gave 95 + 2% retained cis ether (100) from the cis isomer (99), and 70 + 2% trans ether (101) from trans-( 102). The preferred retention points to short-lived carbanions. If CF8 interactions are the larger ones, as suggested by the models, the competition theory predicts predominant inversion for (99)... 

Since cis-Cl, cis-Cl, trans-Cl and trans-Cl are formed by 0°, 60°, 120° and 180° rotations, respectively, if (107) is symmetrical with respect to the chlorine atoms, the carbanion is long-lived and the same cis/trans ratio is expected from both isomers. The explanation of the higher retention for the trans isomer by using (107) and (108) is equivalent to discussion in terms of short-lived carbanions. 

Retention and isomerization via short-lived carbanions requires that each act of retention or inversion would be accompanied by exchange, and no trans-Cl is expected. Dissection of the exchange rate into retention and inversion contributions by assuming kinetic control (last column of Table 6) shows high inversion for the cis isomer and similar contributions of retention and inversion for the trans isomer. Since both the preferred-retention mechanism and the competition theory predict more retention for both isomers, this analysis is inconsistent with the results. 

Although analysis in terms of short-lived carbanions fits the stereochemistry discussed up to now, the results should not be taken as indicative of short-lived ions. Thermodynamic control via long-lived carbanions will give the same results, as was actually suggested by Truce and coworkers. 

Routes (i) and (ii) differ only in the life-time of the intermediate, although the intermediate of route (i) might only be a transition state. We will see that the stereochemistry of the product and the element effect can give information on this question. Most of the evidence points to a short-lived carbanionic intermediate, but in some examples an a,j8-adduct seems essential. Since even the direct substitution is in itself an addition-elimination process involving the nucleophile and the leaving group, and since differentiation between the routes of Scheme 2 is not always possible, we will designate all routes of Scheme 2 as addition-elimination . 

The reaction of an unsaturated compound with an antagonist function located at the end of a polymer chain is still the most commonly used method to synthesize macromonomers. We have already mentioned some processes that can be used to introduce into the chain end of a macromolecule a functional group, e.g. by deactivation of living carbanionic sites and transfer reactions of various kinds in cationic polymerization. We have also described some methods used to link an active terminal double bond to the chain end originally bearing hydroxy groups. 

Methods of obtaining block copolymers by radical processes have been developed rather lately about other processes, and especially ionic methods. This may be due to the nature of the radical, which is an intermediate with a very short lifetime, and a very high non-selective reactivity. These characteristics do not favor a well-controlled architecture as in the case of living carbanions appearing in anionic polymerization. However, the recent development of new... 

In general it is very difficult to make any distinction between a concerted process and the involvement of a short-lived carbanion. For clarity a number of alkene rearrangements in the following text are written as two-step processes, but it should be emphasised that they could involve concerted mechanisms. Products arising from substitution with rearrangement are frequently encountered in reactions of cyclic fluoroalkenes and in fluoride-ion-induced rearrangements (Subsection 6, below). 

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. 

In the termination step, the living carbanionic polymer can be deactivated by proton abstraction from protic solvent, by which process the end of the polymer is saturated. 

Tonhauser C, Frey H (2010) A road less traveled to functional polymers epoxide termination in living carbanionic polymer synthesis. Macromol Rapid Commun 31 1938-1947... 

It is claimed that many types of reactions can be performed more easily with multiple polymers. To avoid undesirable side reactions two reacting species can be used with each attached to a different polymer. Such polymer-bound reactants can coexist in the same reaction vessel without interacting. An example is the preparation of benzoyl acetonitrile by Patchomik and coworkers. Molecules of triphenylmethane lithium, attached to polystyrene supports, were combined with also immobilized o-nitrophenol. The < -nitrophenols were prereacted with benzoyl chloride. The two species were combined and acetonitrile molecules containing acidic hydrogens were introduced into the reaction mixture. This resulted in hydrogens being abstracted from the introduced molecules and formation of short-lived carbanions ... 

In 1997 Khauss et al. presented an elegant synthesis to star polymers with a hyperbranched core and (hyp>er)branched materials by slow addition of a suitable termimer (or a mixture of monomer and termination agent) to living, carbanionic polymer chains ( convergent anionic polymerization ).By this technique, a variety of different macromolecular architectures can be synthesized with moderate polydispersities. 

In contrast to many other functionalization reactions, termination of living carbanionic polymers with ethylene oxide (Eq. 19) is relatively... 

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