Tetraalkylammonium counterions

Anionic polymerization of (meth)acrylates with hindered ester functions can most likely be conducted at room temperature and above to remove the heat of polymerization with low boiling solvents. Polymerization of the important methyl and ethyl (meth)acrylate members of the family, however, are still plagued by chain termination at higher temperatures. The phosphorus based counterions have a stability advantage over tetraalkylammonium counterions which undergo Hoffman elimination. 

The production of the 2-pyrrolidone anion by direct cathodic electrolysis of 2-pyrrolidone has been referred to [33,34] and examples of its use as an ex situ reagent presented (Scheme 2). Despite the early promise of this conveniently prepared strong base [pXa(2-pyrrolidone) = 24.1], it is only recently that its advantages have been exploited more widely. The 2-pyrrolidone anion is formed as an ex situ reagent by direct cathodic electrolysis at —2.5 V (VS.SCE) and a further example of its use is given in Scheme 8, in which reaction the tetraalkylammonium counterion was essential in preventing defluorination [35]. 

Effective Sn2 substitution reactions are also greatly effected by anion nucleophilicity. An anion will be less nucleophilic when it is effectively solvated and when it is restricted by its counterion. This can be circumvented by selection of a tetraalkylammonium counterion, addition of a crown ether, and use of a solvent that effectively solvate cations. 

Fig. 4, Number average aggregation number (n) of AOT with different tetraalkylammonium counterion vs. surfactant concentration in benzene at 25 C. ...

Anomalous titration curves for PLL in pure water are observed as illustrated by Figure 1 which shows the effect of various counterions on the potentiometric behaviour of PLL [12, 14]. A conformational change between a very compact form (stable at low a values) and an extended coil-like form clearly appears. Stabilization of the compact conformation is found to be more pronounced as the length of apolar chains of tetraalkylammonium counterion chains increases. The transition between the two forms, which is initiated at a critical oc of 0.15-0.20 (in the presence of Na" or K ), is shifted to higher values of a in the presence of tetraalkylammonium counterions attaining an of about 0.7 with bulky (and hydrophobic) N-tetrabutylammonium ions (Bu4N ). 

The differences in pK values in the case of tetraalkylammonium ions as counterions can he explained by the size of the CH ) N ion in comparison with the ion and delocalization of the positive charge on the... 

When a tetraalkylammonium cation is used as a counterion in solvents of high polarity, such as AN or DME, the alkyl groups of the cation hinder the mutual approach of species with different charges. Ion pairs with the potassium cation are stable. This follows from a comparison of the polarographic behavior of the three isomeric dinitrobenzenes in the same solvent (DMF) using tetraethylammonium or potassium perchlorate as the carrier electrolyte (Todres 1970). The halfwave potentials corresponding to the conversion of p- and m-dinitrobenzenes into anion-radicals are independent of whether tetraethylammonium or potassium counterions are employed. The anion-radical is formed from o-dinitrobenzene at a potential that is less negative by almost 100 mV when... 

Usually, addition of an appropriate counterion, commonly an alkah metal, ammonium, or tetraalkylammonium permits the isolation of the polyanion. Lithium and sodium salts are generally more water-soluhle than those of the larger cations. In contrast, salts of alkylammonium and similar cations are insoluble in water but can be recrystaUized from several nonaqueous solvents. 

Abnormal relationships between the reactivities of the free ion and ion pair for the Na+ and K+ counterions were also obtained for the other model systems 174). Unfortunately, similar data for tetraalkylammonium cations are absent at present. 

Tetraalkylammonium and Other Bulky Counterions for Anionic Polymerization... 

The presence of counterions bearing hydrocarbon-miscible tails, such as the trichloroacetate anion just mentioned, or tetraalkylammonium cations, instead of the usual inorganic ions, may be an aid in solubilizing proteins in nonaqueous solvents. 

---