Activity of the counterion

Recently, the stiff-chain polyelectrolytes termed PPP-1 (Schemel) and PPP-2 (Scheme2) have been the subject of a number of investigations that are reviewed in this chapter. The central question to be discussed here is the correlation of the counterions with the highly charged macroion. These correlations can be detected directly by experiments that probe the activity of the counterions and their spatial distribution around the macroion. Due to the cylindrical symmetry and the well-defined conformation these polyelectrolytes present the most simple system for which the correlation of the counterions to the macroion can be treated by analytical approaches. As a consequence, a comparison of theoretical predictions with experimental results obtained in solution will provide a stringent test of our current model of polyelectrolytes. Moreover, the results obtained on PPP-1 and PPP-2 allow a refined discussion of the concept of counterion condensation introduced more than thirty years ago by Manning and Oosawa [22, 23]. In particular, we can compare the predictions of the Poisson-Boltzmann mean-field theory applied to the cylindrical cell model and the results of Molecular dynamics (MD) simulations of the cell model obtained within the restricted primitive model (RPM) of electrolytes very accurately with experimental data. This allows an estimate when and in which frame this simple theory is applicable, and in which directions the theory needs to be improved. 

One of the simplest ways to observe the electrostatic effect inherent in polyion systems is to measure the activity of the counterion of the polyelectrolyte solution in the absence of simple salt. Single ion activities of these sodium salt solutions, a a, measured electrochemically by use of a Na" ion selective glass electrode can be expressed by the foUowing equation ... 

Basically the reason is that in the former case there is one degree of freedom less because the activity of the counterion is fixed. 

Due to cation complexation and aftereffects relating to the increased solubility of inorganic salts in low-polarity organic media and enhanced activity of the counterion reactant (quasi-naked anion), the podands, just as the crown compounds and cryptands, are promising catalysts in the synthetic methodology of phase-transfer catalysis. 

The results are recorded in Figure 1 for monovalent and divalent counterions. The experimental results are compared to the free fractions of counterions calculated by the treatment of Oosawa (yo) [5] and to the activity of the counterions given by Manning (y ) [6] these parameters are given for infinite dilution by ... 

Surface active electrolytes produce charged micelles whose effective charge can be measured by electrophoretic mobility [117,156]. The net charge is lower than the degree of aggregation, however, since some of the counterions remain associated with the micelle, presumably as part of a Stem layer (see Section V-3) [157]. Combination of self-diffusion with electrophoretic mobility measurements indicates that a typical micelle of a univalent surfactant contains about 1(X) monomer units and carries a net charge of 50-70. Additional colloidal characterization techniques are applicable to micelles such as ultrafiltration [158]. 

The reaction medium plays a very important role in all ionic polymerizations. Likewise, the nature of the ionic partner to the active center-called the counterion or gegenion-has a large effect also. This is true because the nature of the counterion, the polarity of the solvent, and the possibility of specific solvent-ion interactions determines the average distance of separation between the ions in solution. It is not difficult to visualize a whole spectrum of possibilities, from completely separated ions to an ion pair of partially solvated ions to an ion pair of unsolvated ions. The distance between the centers of the ions is different in... 

Figure 20 shows the plot of the surface tension vs. the logarithm of the concentration (or-lg c-isotherms) of sodium alkanesulfonates C,0-C15 at 45°C. In accordance with the general behavior of surfactants, the interfacial activity increases with growing chain length. The critical micelle concentration (cM) is shifted to lower concentration values. The typical surface tension at cM is between 38 and 33 mN/m. The ammonium alkanesulfonates show similar behavior, though their solubility is much better. The impact of the counterions is twofold First, a more polarizable counterion lowers the cM value (Fig. 21), while the aggregation number of the micelles rises. Second, polarizable and hydrophobic counterions, such as n-propyl- or isopropylammonium and n-butylammonium ions, enhance the interfacial activity as well (Fig. 22). Hydrophilic counterions such as 2-hydroxyethylammonium have the opposite effect. Table 14 summarizes some data for the dodecane 1-sulfonates.

Jorgensen et al. [84] studied how solvent effects could influence the course of Diels-Alder reactions catalyzed by copper(II)-bisoxazoline. They assumed that the use of polar solvents (generally nitroalkanes) improved the activity and selectivity of the cationic copper-Lewis acid used in the hetero Diels-Alder reaction of alkylglyoxylates with dienes (Scheme 31, reaction 1). The explanation, close to that given by Evans regarding the crucial role of the counterion, is a stabilization of the dissociated ion, leading to a more defined complex conformation. They also used this reaction for the synthesis of a precursor for highly valuable sesquiterpene lactones with an enantiomeric excess superior to 99%. 

The most important factor determining the sensitivity of the conformation to the concentration of polyions is the change in ion activity or osmotic pressure with conformation. If the activity coeflScient of the counterions is sensitive to conformation then conformational change resulting from concentration changes of polyions becomes large. 

Fixing the location of the counterion midway between two identical electrophores has been achieved in the radical anion of the dibenzo[ 18] crown-6 derivative [46] (Mazur et al, 1980). When its radical anion exists as the ion pair with Na + or K+, the intramolecular electron transfer becomes detectable on the esr timescale. The activation energy determined for the electron transfer (1.4 kcal mol-1) clearly demonstrates that in this case a significant contribution to the activation barrier from ion pairing can be negated. 

A remarkable feature of iridium enantioselective hydrogenation is the promotion of the reaction by large non-coordinating anions [73]. This has been the subject of considerable activity (anticipated in an earlier study by Osborn and coworkers) on the effects of the counterion in Rh enantioselective hydrogenation [74]. The iridium chemistry was motivated by initial synthetic limitations. With PFg as counterion to the ligated Ir cation, the reaction ceases after a limited number of turnovers because of catalyst deactivation. The mechanism of... 

It was assumed that the triflate counterions of 269c are fully dissociated in solution, at least in the presence of the dicarbonyl substrate. However, an examination of the influence of the counterion revealed that SbF6 provides a much more active catalyst than the triflate counterpart (199). Whereas the triflate catalyst 269c requires 10 h for the reaction to proceed to completion at -78°C, the SbF6 catalyst 271c induces complete conversion to cycloadduct in 4 h under identical conditions, albeit with slightly eroded diastereoselectivity (96 4 vs 98 2) (200). Enantioselec-tivity is identical for the two catalysts (>98% ee, endo diastereomer). 

Larger changes in bond lengths, as expected, are observed for more localized carbocations. Most of the structures available are for stabilized systems, such as protonated carbonyl compounds [e.g. the protonated cyclopropyl ketones referred to on page 110 (Childs et al., 1990), and dioxacarbocations (Paulsen and Dammeyer, 1973, 1976 Paulsen and Schuttpelz, 1979 Childs et al., 1986, 1991). It is normal to see one of the atoms of the counterion (in most cases MXJ or MX ) packing in the position expected for addition to the activated C=OH(R)+ system, apparently just within the sum of the van der Waals radii for the neutral centres (Childs et al., 1986). This can happen without significant pyramida-lization, however (Childs et al., 1991), and on both sides of the planar carbon centre it tells us little new about reactivity. 

The last results described are a strong indication that any computer modeling of the activity of early transition metal catalysts for the polymerization of olefins probably requires the inclusion of the counterion in the simulations. 

Nevertheless, there is still much work to do in this field. The inclusion of solvent and/or counterions is just at the beginning, and solvent effects have been included with continuum models only. In the next years we will probably arrive to dynamically simulate the whole polymerization process in the presence of the counterion and of explicit solvent molecules. As for the experimental issues which have been not rationalized yet computationally, we remark that still it is not easy to model the relative activity of different catalysts, and even to predict if a certain catalyst will show any activity at all. Moreover, copolymerizations still represent an untackled problem. However, considering the pace at which the understanding of once obscure facts progressed it is not difficult to predict that also these challenges will be positively solved. 

The exact composition in terms of the relative amounts of linear, cyclic, and three-dimensional structures and molecular weight probably varies with the detailed method of preparation. Most workers favor a three-dimensional spherical cagelike structure as the structure responsible for MAO s coinitiator property. However, this may be an oversimplification, and more than one structure may be responsible for the observed activation of metallocenes by MAO. After activation of a metallocene initiator, MAO forms the basis of the counterion, (ClMAO) or (CH3MAO). MAO normally contains TMA in two forms free TMA and... 

The choice of the counterion has a significant influence on the activity of the catalyst, no methylester is produced by complex 19. Similar results were obtained for compounds 20 (980%) and 21 (0%), where the steric demand of the ligands R (= methyl) is significantly lower. Additional reactions under different conditions show that the yields can be improved. After 14 hours at 90 °C catalyst 20 yielded 3000% relative to palladiiun (TON 30) [55]. 

DPT calculations indicated that the mechanism most Hkely involves three steps electrophilic substitution, oxidation and reductive ehmination. The inactivity of the iodine complexes prompted us to investigate the counterion dependence. For the methyl-substituted complexes (Scheme 23, R = CH3) we synthesized the acetate (X = OCOCH3) 22 and the chloride complex (X = Cl) 23. The catalytic conversions are within experimental error identical to the results of the bromide complex 20. This indicates that the dissociation of a counterion is a necessary condition for the activity of the complex [59]. 

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. 

The decrease in the activation energy difference in the series EtAlCl2 to PF5 might be due to differences in the nucleophilicity of the counterions formed by the various coinitiators. The less nucleophilic counterion forms a looser ion pair with the cationic chain end, which in turn reduces the steric barrier to propagation without affecting isomerization. This is reflected in a decrease in the activation energy difference between the two processes and results in a decrease in the change of I with temperature. 

There are differences in isotherm shape, and for DTAB the behavior is not amenable to a simple explanation. Of particular interest are plots of the amount adsorbed against the mean ionic activity of the surface active agent (including the counterion of the added electrolyte). In the case of DTAB all the data, including others at various salt concentrations up to 0.5M, lie on one line which, after an initial steep rise, is linear to the c.m.c. This indicates that for other than the initial strong adsorption at low concentrations (possibly because of specific interactions with the surface) the adsorption follows the law of mass action. For SDS a similar result is obtained except that positive deviations from the straight line occur below a — 4 X 10 3M for the cases (salt concentration < O.lAf) when there is a point of inflection in the isotherm. These deviations may reflect specific interactions of the DS" with the surface when the ions are adsorbed in parallel orientation. 

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