Carbanionic end

The dimer behaves simultaneously as a radical and as a carban-ion, and thus the radical end might grow by a radical mechanism, anionic polymerization proceeding from the carbanion end. This behavior is particularly interesting when two monomers are present in the system, one polymerizable by a radical but not by an anionic mechanism, the other behaving in the opposite sense. In such a hypothetical case the resulting product would be a block polymer, -A—A. . . A—B—B. . . B-. 

In the same way, the addition of monomer yields the radical ions which possess the most stable carbanion ends and the most stable radical ends, e.g.,... 

That this reaction occurs is shown by electron spin resonance measurements, which indicate the complete disappearance of radicals in the system immediately after the addition of monomer. The dimerization occurs to form the styryl dicarbanion instead of - CH2CH4>CH4>CH2 -, since the former is much more stable. The styryl dianions so-formed are colored red (the same as styryl monocarbanions formed via initiators such as n-butyl-lithium). Anionic propagation occurs at both carbanion ends of the styryl dianion... 

It appears that the highest coupling efficiency is obtained for a living polystyrene with a a methylstyrene carbanion end-group and in presence of potassium as counter-ion. 

In this paper we describe the reaction between carbanionic ends of oligomers or polymers and elemental compounds such as oxygen and sulfur we observe two sets of reactions coupling and functionalization. 

The question of greater interest is How can the solvent influence the state and the reactivity of the carbanionic end group ... 

All these results permit us to suggest a complete scheme for the enthalpy and entropy of the transition between the three states of the carbanionic end group in ethereal solvents (Figure 15). 

The mechanism for the formation of the alkene is open to discussion especially as there is no agreement on the source of the stereoselectivity.5 We suggest that the carbanion end of the ylid... 

When all the monomer is depleted the carbanion ends remain active provided extreme care is taken to avoid adventitious water, carbon dioxide, or oxygen. One consequence of this conclusion is that all polymer chains should grow to the same length and therefore the polymer should have equal weight average and number average molecular... 

A polymer with an active carbanionic end will initiate the polymerization of a different monomer if the initial macroion is more nucleophilic than the anion formed from the second monomer in the particular solvent/counterion environment. Thus alpha-methylstyryl anion (9-1) will initiate the polymerization of methyl methacrylate (Fig. 1-4) but the poly(methyl methacrylate) carbanion will not initiate polymerization of alpha-methylstyrene. 

Initiation by electron transfer is based on the ability of the alkali metals to supply electrons to the double bonds. This yields an anion radical and a positively charged, alkali-metal counterion. Initiation may be effected (a) by direct attack of the monomer on the alkali metal, or (b) by attack on the metal through an intermediate compound such as naphthalene. Both result in bifunctional initiation, that is, formation of species with two carbanionic ends. 

This initiation process is thus similar to alkali metal initiation in (a). That this reaction occurs is shown by electron spin resonance measurements, which indicate the complete disappearance of radicals in the system immediately after the addition of monomer. The monomer in these systems often has a lower electron affinity than the polycyclic hydrocarbon, but dimerization of the monomeric radical anion [Eq. (8.15)] drives the equilibrium of reaction (8.14) to the right. Dimerization of radical centers is highly favored by their high concentrations, typically 10 -10 M and the large rate con-stants (10 -10 L/mol-s) for radical coupling. (Note that the dimerization occurs to form the styryl dicarbanion instead of CH2CH0CH0CH2 , since the former is much more stable.) The styryl dianions are colored red (the same as styryl monocarbanions formed via initiators such as n-butyllithium). Anionic propagation occurs at both carbanion ends of the styryl dianion ... 

The initiation process is thus similar to alkali metal initiation described earlier [cf. Eq. (8.10)]. The dimerization of radical anions is highly probable because of their high concentrations, typically jlO -10" M, and the large rate constants (10 -10 L/mol-s) for radical coupling (Odian, 1991). Anionic propagation takes place by monomer addition at both carbanion ends of the styryl dianion ... 

In the present work, we shall confine our attention to one particular facet of this area, namely the formation of homo-polymers. The easy generation of the intermediate containing cationic center suggests it as an initiator for cationic polymerization. However, the presence of the carbanionic end would probably interfere. 

Provided inert solvents and pure reactants are used, most monomers under appropriate conditions will give rise to systems in which active carbanion end-groups are always present. The indefinite activity of the growing chains has led to the rather inappropriate term, living polymers for these materials. 

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