Anionic chains, termination

The addition of the anion takes place at the unsubstituted carbon atom, which, in this case, carries a partial positive charge. Since the growing chain end is a genuine anion, chain termination can occur by addition of a reactive cation. As in cationic polymerization, combination of two growing ends is not possible. Chain transfer with electrophiles can also occur. 

The manufacture of siHcone polymers via anionic polymerization is widely used in the siHcone industry. The anionic polymerization of cycHc siloxanes can be conducted in a single-batch reactor or in a continuously stirred reactor (94,95). The viscosity of the polymer and type of end groups are easily controUed by the amount of added water or triorganosUyl chain-terminating groups. 

If the nucleophilicity of the anion is decreased, then an increase of its stability proceeds the excessive olefine can compete with the anion as a donor for the carbenium ion, and therefore the formation of chain molecules can be induced. The increase of stability named above is made possible by specific interactions with the solvent as well as complex formations with a suitable acceptor 112). Especially suitable acceptors are Lewis acids. These acids have a double function during cationic polymerizations in an environment which is not entirely water-free. They react with the remaining water to build a complex acid, which due to its increased acidity can form the important first monomer cation by protonation of the monomer. The Lewis acids stabilize the strong nucleophilic anion OH by forming the complex anion (MtXn(OH))- so that the chain propagation dominates rather than the chain termination. 

If anions, built during the initiation by means of HY/MtXn from Lewis acids, form a covalent species, a chain termination is the result according to ... 

Scheme 10 Polypeptide chain termination by reaction with NCA anions...

Another innovative approach to controlling amine-initiated NCA polymerizations was reported in 2003 by Schlaad and coworkers [20]. Their strategy was to avoid formation of NCA anions, which cause significant chain termination after rearranging to isocyanocarboxylates [11, 12], through use of primary amine hydrochloride salts as initiators. The reactivity of amine hydrochlorides with NCAs was first explored by the group of Knobler, who found that they could react... 

The majority of commercial polystyrene molecules consist of a backbone of carbon atoms with phenyl groups attached to half of the carbon atoms, as shown in Fig. 21.1. Free radical initiator residues terminate each end of the chain. Minor variants include chains terminated by anionic or cationic initiator residues. All commercial polystyrene products are atactic that is, the placement of the phenyl groups on either side of the chain is essentially random, as illustrated in Fig. 21.2. 

The active species in anionic chain polymerisations are anionic growing chain ends. The main characteristic of such a process is the almost total absence of termination and transfer reactions. For this reason, anionic polymerisation is often called "living polymerisation". 

Polymers having one or two carboxyl endgroups are prepared by anionic polymerisation and subsequent chain termination with C02+ and further reaction with mineral acids. 

Functionalization of these reactive anionic chain ends can be achieved by a variety of methods all based on the general concepts of carbanion chemistry. For example, reaction with C02 or succinnic anhydride leads to the carboxy terminated derivatives [10], while hydroxy-terminated polymers can be easily obtained by reaction with ethylene oxide (Scheme 3) [11]. In select functionalization reactions, such as alkylation with p-vinyl benzyl chloride, the nucleophilicity of the carbanionic species may be necessary and this can be achieved by reaction of the chain end with 1,1-diphenylethene followed by functionalization [12,13]. 

Spin traps can act as one-electron oxidizers. This property is even more pronounced in the interactions of traps with anion-radicals. Traps can block the ion-radical pathway. In other words, they inhibit the whole reaction, including the ion-radical step. This can be explained by both the oxidation of substrate anion-radical and chain termination due to oxidation of product anion-radical. An illustrative example is the inhibition of nucleophilic substitution of 2-chloroquinoxaline by the radical trap bis(tcrt-butyl)nitrone (Carver et al. 1982). 

A reaction of chain transfer to alcohol or other proton-donor compounds is the most common process among the chain termination reactions in the anionic polymerization of the epoxy compounds in proton media, including the action of TA ... 

The second important chain termination reaction characteristic of the catalysis of anionic polymerization of epoxy compounds by the TA consists in the abstraction of the hydrogen atom from the p-carbon atom in the tetraalkylammonium cation by the growing alkoxy anion (P-elimination reaction)I58 164). 

Butyllithium initiation of methylmethacrylate has been studied by Korotkov (55) and by Wiles and Bywater (118). Korotkov s scheme involves four reactions 1) attack of butyllithium on the vinyl double bond to produce an active centre, 2) attack of butyllithium at the ester group of the monomer to give inactive products, 3) chain propagation, and 4) chain termination by attack of the polymer anion on the monomer ester function. On the basis of this reaction scheme an expression could be derived for the rate of monomer consumption which is unfortunately too complex for use directly and requires drastic simplification. The final expression derived is therefore only valid for low conversions and slow termination, and if propagation is rapid compared to initiation. The mechanism does not explain the initial rapid uptake of monomer observed, nor the period of anomalous propagation often observed with this initiator. The assumption that kv > kt is hardly likely to be true even after allowance is made for the fact that the concentration of active species is much smaller than that of the added initiator. Butyllithium disappears almost instantaneously but propagation proceeds over periods from tens to hundreds of minutes. The rate constants finally derived therefore cannot be taken seriously (the estimated A is 2 x 105 that of k ) nor can the mechanism be regarded as confirmed. 

In systems where chain termination is absent, anionic polymerizations provide in principle an excellent method for the preparation of... 

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