Centres of anionic polymerization

These centres are formed by the addition of monomer to a suitable anion. They are almost always simpler than their cationic reverse part. The counter ion is usually a metal cation able to interact with the electrons of the growing end of the macromolecule, and to bind in its ligand sphere monomer or solvent molecules or parts of the polymer chain. This changes the properties of the whole centre. Therefore, by selection of the metal, the stability of the centre, the tendency of the centres to aggregation, the position of the equilibrium between the contact and solvent-separated ion pairs and free ions, and the stereoselectivity of the centre [the ability to produce polymers with an ordered structure (tacticity, see Chap. 5, Sect. 4.1)] are predetermined. The chemical reactions of the metal cations are, however, very limited. Most solvents and potential impurities are of nucleophilic character. They readily solvate the cation, leaving the anion relatively free. The determination 

There are rate exceptions, for example R4N+, R4P+ may act as counter-ions instead of the metal. 

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The active centre of anionic polymerization is formed by the addition of an anion to a... 

Non-stationary polymerization are complicated from the kinetic point of view. The changing concentrations of active centres, of monomer and possibly even of further components produce conditions unsuitable for an analysis of the process. Even technical and technological difficulties occur. Nevertheless, these have to be solved as most known coordination and cationic, and a considerable number of anionic, polymerizations are non-stationary. Information on the polymerization mechanisms of the more conventional monomers are summarized in Table 3. 

Busson and van Beylen [205] studied the role of the cation and of the carbanionic part of the active centre during anionic polymerization in non polar media. They were interested in the problem of complex formation between the cation and the monomer double bond [206] and they therefore measured the reaction of various 1,1-diphenylethylenes with Li+, K+ and Cs+ salts of living polystyrene in benzene and cyclohexane at 297 K. Diphenylethy-lene derivatives were selected for two reasons. 

The initiation of methyl methacrylate polymerization by alkyl- and aryl-lithium compounds is a very complex process which has not been completely elucidated so far. Even less information is available on the efects of or-ganometals not containing lithium. Recently, the interest of some authors has centred on the application of Grignard reagents for the initiation of anionic polymerizations, especially of MMA [171, 172],... 

The most important representatives of anionic polymerization centres are formed from vinyl and diene monomers. The trivial schematic representation of a carbanion... 

The above-mentioned techniques have had an important effect on elucidation of kinetics and mechanism of anionic polymerization. They depend, naturally, on the fact that there is essentially no termination in the systems, and that the concentration of active centres is relatively large. Measurements made after all monomer has been consumed apply to a... 

The high speed of anionic polymerization is due to the fact that both reacting species are chemically activated and hence highly reactive the lactam anion represents an activated monomer with increased nucleo-philicity and the terminal N-acylated lactam unit represents an activated end group (growth centre) with increased acylating ability. 

Due to the relatively fast side reactions consuming both initiator and growth centres, the evaluation of the kinetics of anionic polymerization becomes very difficult. We are dealing with a system of varying concentration of both active species which, according to schemes (45), (51) and (52), can be not only consumed but also regenerated in the complicated set of side reactions. Hence, the key problem of the anionic lactam polymerization consists in the determination of the instantaneous concentrations of lactam anions and growth centres. 

The particular features of anionic polymerization that made the polymer chains living were discussed above. The main requirement for a living polymerization is the absence of any process for spontaneous termination so that the degree of polymerization is controlled by the ratio of monomer to initiator concentrations. The molar-mass of the polymer therefore increases linearly with monomer conversion. On exhaustion of the monomer, the initiation centres remain, so chains may be re-initiated by addition of further monomer. Termination or chain transfer is controlled by the delibemte addition of a reagent to remove the living end. The resulting polymers will also have very narrow molar-mass distributions since rapid initiation ensures that all chains are initiated at the same time. 

The high reactivity and low concentration of carbanionic active centres makes anionic polymerization very susceptible to inhibition by trace quantities of reactive impurities (e.g. H2O, CO2, O2). Thus all reactants and solvents must be rigorously purified and the reactions must be carried out under inert conditions in scrupulously-clean sealed apparatus. High vacuum techniques are often used for this purpose. 

In the anion of 17 two Cp Mo03 moieties are bound via an oxygen atom to a (o-tolyl)2Bi+-fragment forming a 10 electron four-coordinate bismuth centre. As anticipated coordination of the second Cp MoOs unit saturates the Lewis acidity of the Bi centre thereby avoiding the formation of a polymeric structure as in 16. Even though 17 is a monomeric complex the... 

A review is given on the kinetics of the anionic polymerization of methyl methacrylate and tert.-butyl methacrylate in tetrahydrofuran and 1,2-dimethoxy-ethane, including major results of the author s laboratory. The Arrhenius plots for the propagation reaction+are linear and independent of the counterion (i.e. Na, Cs). The results are discussed assuming the active centre to be a contact ion pair with an enolate-like anion the counterion thus exhibiting little influence on the reactivity of the carbanion. 

Monomer reactivity is a broad concept, and it can not always be limited only to reactions of the double or triple bonds of a vinyl or acetylene group. Weakly polar monomers, such as styrene or butadiene, react almost exclusively by their double bonds. The anionic polymerization of polar monomers, such as a, /S-unsaturated esters and nitriles, is accompanied by many side reactions. A fairly large amount of oligomers and side products are formed, and these may affect the active centres, thus indirectly modifying propagation. 

The pH dependence of the polymerization rate of acrylic acid [54] in the presence of various neutralizing agents does not exhibit a course. Between pH 2 and 6, the rate drops abruptly afterwards it grows, depending on the degree of neutralization and the kind of neutralizing agent. The rate increase is connected with the presence of a cation which restricts the repulsion between the anions of the active centre and the monomer... 

The use of the silyl anion derivative (CH30)n (CH3)3 Si Na+ [100] in the role of an active centre is as yet only possibility. The usefulness of theoretical studies of some centres of this kind, for example of those formed during phos-phazene polymerization (see Chap. 2, Sect. 1.1), should increase with time. 

In polymerizations of heterocycles, the reactivity of anionic centres is considerably affected by cation solvation with the polymer [104]. Schematically... 

The interpretation of the mechanism of anionic lactam polymerization based on the conventional scheme (ionic active centre with approaching monomer) could not exhaustively explain all the observed effects. Agreement could only be obtained when the acido-basic properties of lactams and polyamides had been respected. The equilibrium... 

Solvation of the active centre considerably affects even the ratio of 1,2 and 1,4 addition in diene propagation. Solvent can profoundly change the mode of addition in anionic polymerization [91]. 

The formation of aggregates with low to zero polymerization activity is quite general in ionic polymerizations. This statement can be further documented by the observation of centre aggregation during anionic polymerization of oxirane [100-103]... 

Formally similar reaction sequences occur in anionic polymerization. Here, a H2C=CHR double bond reacts with a strongly nucleophilic anion X to form a new carbon-centred anion XCH2-CHR. Continuation of this process leads to the formation of polymer chains, especially again for those vinyl derivatives... 

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