Termination in anionic polymerization

Termination reactions in anionic polymerization, particularly with nonpolar monomers and in nonpolar solvent, are not common. If carbanion quenching impurities are absent, many polymerization reactions may not terminate after the complete disappearance of the monomer. Styryl anion (one of the most stable ones), for instance, can persist for a long time, such as weeks, after the monomer is consumed. Addition of more monomer results in a continuation of the reaction and a further increase in the molecular weight. The anionic living polymers retain their activities for considerably longer periods of time than do the cationic living ones.  

The termination steps in anionic polymerizations can result from deliberate introductions of carbanion quenchers, such as water or acids, or from impurities. Terminations, however, can take place in some instances through chain transferring a proton from another molecule like a solvent or a monomer, or even from a molecule of another polymer. In some solvents, like liquid ammonia, transfer to solvent is extensive, as in styrene polymerization by amide ions.  

In addition, in some polymerizations termination might occur from the following reactions  

Some irreversible reaction of the active center with a molecule of a monomer or a solvent. 

It was observed, for instance, that hydrogen transfer from a monomer to the growing chain can be a way of termination in polymerizations of polar monomers, like acrylonitrile  

---

The most common mechanism of termination in anionic polymerization involves reactions with solvents or with impurities. For... 

It was suggested that the terminations in anionic polymerizations of acrylonitrile proceed by proton transfer from the monomer. This, however, depends upon catalyst concentrations. At low concentrations the terminations can apparently occur by a cyclization reaction instead CH2—CH CH2— CH... 

Regarding anion radical transfer, low-molecular weight azo compounds were used as terminating agents in anionic polymerizations. An interesting example is the addition of a living polystyrene chain to one nitrile group of AIBN [71]. The terminal styryl anion is likely to form... 

In anionic polymerization, as in carbonium ion polymerization, termination does not involve bimolecular reaction between two growing chains. Neither can recombination of ions lead to termination, since a carbon-metal bond is highly polar, in the case of alkali metals frequently completely ionized, and in every case very reactive. The termination step leading to the formation of a terminal C=C double bond is not too probable. This reaction involves the formation of a metal hydride, and this does not contribute greatly to the driving force. Consequently, such a termination is observed at higher temperatures only and it is probably more common in coordination polymerization where the metals involved are less electropositive. 

The preparation of ABA triblock polymers requires use of a telechelie bisthiol prepared by termination of anionic polymerization initiated by a difunctional initiator. The relative yields of homopolymer, di- and triblock obtained in these experiments depend critically on conversion.273... 

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. 

Sequential addition of monomer works well in anionic polymerization for producing well-defined block copolymers [Morton, 1983 Morton and Fetters, 1977 Quirk, 1998 Rempp et al., 1988]. An AB diblock copolymer is produced by polymerization of monomer A to completion using an initiator such as butyllithium. Monomer B is then added to the living polyA carbanions. When B has reacted completely a terminating agent such as water or... 

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 ... 

C olvents have different effects on polymerization processes. In radical polymerizations, their viscosity influences the diffusion-controlled bimolecular reactions of two radicals, such as the recombination of the initiator radicals (efficiency) or the deactivation of the radical chain ends (termination reaction). These phenomena are treated in the first section. In anionic polymerization processes, the different polarities of the solvents cause a more or less strong solvation of the counter ion. Depending on this effect, the carbanion exists in three different forms with very different propagation constants. These effects are treated in the second section. The final section shows that the kinetics of the... 

In ionic polymerization a hydride (H-) transfer or a proton transfer are the analogues of the hydrogen atom transfer in radical polymerization. A hydride (H-) ion transfer is observed in many isomerizations and dimerizations of hydrocarbons which proceed via carbonium-ion mechanism. A similar process is responsible for chain transfer ip some carbonium-ion polymerizations. The transfer of negative ions like Cl- is also common, e.g. triphenyl methyl chloride is an efficient transfer agent in such a polymerization. Transfer of a proton is, on the other hand, a very common mode of termination of anionic polymerization. Indeed, this mode of termination was discussed previously in connection with branching reactions, and it was postulated in the earliest studies of anionic poly-... 

Various modes of termination of anionic polymerization can be visualized. The growing chain end could split out a hydride ion to leave a residual double bond. This is, however, a high activation energy process and has not as yet been reported in the cases where alkali metal cations are present. It is important in systems involving Al—C bonds, however (73). A second possibility is termination through isomerization of the carbanion to an inactive anion. Proton transfer from solvent, polymer, or monomer would also cause termination of the growing chain. Lastly, the carbanion could undergo an irreversible reaction with solvent or monomer. The latter three types have been shown or postulated as termination or transfer reactions. 

In anionic polymerization, the counter-ions are usually simple, derived mainly from a single element, as a whole they are therefore highly stable. On of the most important termination mechanisms is absent, and the polymeriza-... 

A special kind of termination in ionic polymerizations is the mutual combination of anionic and cationic living chains (see Chap. 5, Sect. 5.8). When the two polymers consist of different monomers, block copolymers are formed. The two macroions can also consist of the same monomer. 

Polymers with even narrower mass distributions, e.g. with PDI values close to 1, arise in living polymerization systems, in which no chain termination processes can occur at all, such that all chains remain bound to the metal centre from which they have started to grow at the same time. Living polymerizations, which offer useful opportunities, e.g. with regard to the production of block copolymers by exchange of one monomer for another, occur in anionic polymerizations of styrenes or butadienes such as are induced by simple lithium alkyls. For a-olefin polymerization catalysts of the type discussed above, living polymerizations are rare. These more elaborate catalysts can thus release a newly formed polymer chain within a time interval of typically less than one... 

Termination. As in anionic polymerization, termination by coupling or disproportionation cannot occur, leaving chain transfers as the most likely mechanisms. 

There are several similarities between classical anionic polymerization and GTP of methacylates. Indeed, side reactions observed in anionic polymerization are also observed in GTP. Brittain and Dicker showed that the intramolecular cyclization of the end-group with the ester function of the antipenultimate unit is the major termination reaction in... 

One main difference between anionic polymerization and GTP has to be found in the amount of enolates active in polymerization. In anionic polymerization, all the chains are end-capped by an enolate, which is the case for only a small part of the chains in GTP consistent with the very good control of GTP even at room temperature. In this respect, Brittain and Dicker showed that prop/ term is by far higher in GTP (250) than in classical anionic polymerization ( prop/ term = 8) . In line with slow termination compared to propagation in GTP, Bandermann and coworkers found that the amount of the nucleophilic catalyst is essential to the polymerization control. Indeed, as far as the tris(piperidino)sulfonium bifluoride-mediated GTP of MMA in THF is concerned, the polydispersity index increases with the amount of catalyst . 

The choice of counterions (anions) in the cationic polymerization of heterocyclic monomers can be almost as wide as in anionic polymerization, but only for the most nucleophilic monomers (i. e. cyclic amines). Unfortunately, in the polymerization of cyclic ethers, this choice is much more restricted. Thus, the small anions like F or OH cannot be used because, due to their high nucleophilicity and ability to form covalent bonds, they give rise to fast termination. In order to suppress or even to eliminate termination by collapse within an ton pair (cf. Sect. 5.1.), it is necessary to use complexed anions having large ionic radii. These are shown below (rctyst)-... 

---