The Effect of Counterions

The stereoselective polymerization of various acrylates and methacrylates has been studied using initiators such as atkyllithium [Bywater, 1989 Pasquon et al., 1989 Quirk, 1995, 2002]. Table 8-12 illustrates the effects of counterion, solvent, and temperature on the stereochemistry of the anionic polymerization of methyl methacrylate (MMA). In polar solvents (pyridine and THF versus toluene), the counterion is removed from the vicinity of the propagating center and does not exert an influence on entry of the next monomer unit. The tendency is toward syndiotactic placement via chain end control. The extent of syndiotacticity... 

To evaluate the effect of counterion screening on the chemical potential of the chosen solute ions, we first multiply (S8.6-12) by the appropriate prefactor for ions of charge Q (= ze) in a medium of dielectric constant k ... 

We first describe the NMR parameters for the duplex to strand transition of the synthetic DNA poly(dA-dT) (18) with occasional reference to poly(dA-dU) (24) and poly(dA- brdU) and the corresponding synthetic RNA poly(A-U) (24). This is followed by a comparison of the NMR parameters of the synthetic DNA in the presence of 1 M Na ion and 1 M tetramethylammonium ion in an attempt to investigate the effect of counterion on the conformation and stability of DNA. We next outline structural and dynamical aspects of the complexes of poly(dA-dT) with the mutagen proflavine (25) and the anti-tumor agent daunomycin (26) which intercalate between base pairs and the peptide antibiotic netropsin (27) which binds in the groove of DNA. 

The effect of counterion on the synthetic DNA conformation has been probed by comparing the NMR parameters in high salt sodium and tetramethylammonium solutions. We observe a specific change at one of the two glycosidic torsion angles and at one of the two phosphodiester linkages for poly(dA-dT) on proceeding from Na+ to TMA+ solutions. 

Of course, these relationships do not take into account the variations in SO4 chemical shift due to the effects of counterions, pH, concentration and temperature. It has not been possible to adopt the IUPAC chemical shift scale exactly, because that would require measuring the chemical shifts of all secondary references reported in the literature with respect to the standard suggested by IUPAC. 

Mah et al. demonstrated the effect of counterions on the cationic polymerization of styrene [35-37]. The radiation-induced polymerization is much more sensitive to impurities than the catalytic polymerization, as the former involves the cationic species in a free ion state. Thus, one can expect, in the presence of stable anions, the promotion of the cationic polymerization because of the ion-pair formation between the dimer cation and the counterion. The effect was... 

The role of sodium ions is predicted to be small according to Schulze-Hardy s rule that is, the effectiveness of counterions in the interfacial phenomena is approximately proportional to the sixth power of the counterion charge. The formation of complexes between Na+-ions and anionic sites on the montmorillonite surface is included here for the sake of completeness, although such complexes are subject to reservations because they are unknown in aqueous solutions, and evidence against their physical reality has been presented (Janssen and Stein 1986). Indeed, as will be seen later, they are not prevalent. 

Editor s note When careful attention is paid to the effect of counterion concentration levels of the solution phase on the pH measured, the intrinsic pK, value of the hydroxy-lated silica, SifOH), repeated in the clay mineral phase can be expected to correspond to the literature value of 9.3 to 9.4 reported for silicic acid. 

We have observed rapid transport of polyacrylate (PA) and hyaluronate (HA) in dextran solution matrix, which will be the first extension to linear polyelectrolytes. The whole transport process did not follow the flow rcgime(linear in t) nor diffusion(linear in t ) but a combination of the two, when ionic strengths were not high enough. In the media of low ionic strengths, diffusion of linear polyelectrolytes is very rapid due to the effect of counterion diffusion. We sometimes observed structured flows under a situation that transport rate followed diffusion law. This behavior was more clearly observed on HA than PA. Effect of charges favored the rapid transport of both polyclcctrolytcs, since (a)... 

Boeckman and coworkers studied the reaction of bis(thmethylsilyl) ester (361) with aldehydes to form the silyl-substituted unsaturated ester (362 equation 86). The anion was formed with potassium or lithium diisopropylamide. Other metals, such as magnesium or aluminum, were introduce by treating the lithium anion with Lewis acids. The addition step produced a single diastereomer, en ling the effects of counterion and steric bulk on the elimination to be ascertained. Excellent selectivity for the ( )-isomer (362) may be obtained by using K or Li cations and a sterically hindered aldehyde. In studies directed toward the synthesis of substituted pseudomonic acid esters, the Peterson alkenation was utilized to form a mixture of (Z)- and ( )-alkene isomers, one example of which (365) is depicted in equation (87). In this example the conditions were optimized to form the highest degree of selectivity for the (Z)-alkene. 

Polymers incorportating viologen units in their main chains were also prepared (Figure 6 see Appendix for details of the procedure).46,47 The effects of counterions and additives on the photochromism of viologen ionene polymers... 

Also, for the counterion effect on the effective diameters it is found (18) that Do decreases with Increasing ionic radius of the counterions 1800 A for Na Jon (0.96 X), 1600 X for K ion (1.33 A), and 1500 A for Cs ion (1.69 A). Original Kc/Rq vs. c curves were reported elsewhere (16). We also pointed out by viscosity measurements (li) that for sulfonated ionomers, the effect of counterion (i.e., counterion binding) Increased in the order of Li < Na < K < Cs, since the solvated ion size decreased in the order of Li > Na > K > Cs. Light scattering results show this tendency more quant i tat ively. 

The partition behavior of compounds between aqueous phases and octanol is now a routine measurement or calculation used in life sciences research as a model for membrane transport, particularly across the blood-brain barrier. The effect of counterions on the partition of saccharin has been extensively studied to investigate permeation of, and solubility in, lipid membranes <89PHA66, 90PHA191). 

Buckley and coworkers117 have noted a correlation between the composition of the PPy, in terms of the ratio of counterion to monomer and the tensile strength of the films. These workers observed that the concentration of counterion intercalated with the polymer depends upon the potential applied during polymerization. Flowever, they noted that the change in mechanical properties could also be due to changes in the density of films,117 as also noted in a more recent study.116 Thus, the effect of counterion content and polymerization potential on the mechanical properties of the films remains unknown. 

Qian and Qui126 have observed that the morphology of PPy films depends upon the type of counterion used. For example, C104 , BF f, and PF6- yielded films with high concentrations of voids, whereas PPy/pTS films consisted of densely packed nodules. Other workers have noted a dependence of the density of PPy films on the polymerization conditions.115116117 Thus, the change in film morphology may explain the effect of counterion type and polymerization conditions on mechanical properties, as surveyed previously. 

An illustrative example of the effect of counterions on selectivity is the work by Brinchi et al, in which they demonstrated that reaction of sulphonate esters in the presence of equimolar amounts of bromide and hydroxyl ions took completely different paths depending on whether the reaction was performed in a micellar system based on cationic surfactant or in a homogeneous solution, see Fig. 5.5 [29]. When there is no surfactant present, attack by the hydroxyl ion dominates. In the micellar solutions, on the other hand,... 

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