The Polyacid Case

A mechanism consisting of consecutive Dieckman cyclizations, possibly through an anhydride intermediate even in the polyacid case appears in accord with the observations. 

In the polyacid case, there are actually two sorts of acid equilibrium constants ... 

The concept of predominance area also applies to the polyacids case. Let us consider that of a diacid H2a- The ionization equilibria are... 

In this section, we will only study the polyacid case. 

Ionization of the carboxyl groups is accompanied by binding of the cations. But if counterions are site-bound the charge on the carboxyl groups is neutralized and chain contraction results. A special case is that of the polyacid which adopts a contracted form because the close association of hydrogen ions with carboxyl groups results in a neutral chain. 

It is well known that lyophilic sols are coagulated by the removal of a stabilizing hydration region. In this case, conversion of a sol to a gel occurs when bound cations destroy the hydration regions about the polyanion, and solvated ion-pairs are converted into contact ion-pairs. Desolvation depends on the degree of ionization, a, of the polyacid, and the nature of the cation. Ba ions form contact ion-pairs and precipitate PAA when a is low (0-25), whereas the strongly hydrated Mg + ion disrupts the hydration region only when a > 0-60. 

In solutions, the counterions of poly electrolytes are HjO (for the polyacids) and OH (for the polybases), cations such as K+ and Na, or anions such as Cl (for the polysalts). The addition of polyvalent counterions (such as Ca, Mg, Cu, AT ) produces ionic cross-linking interfering with solubility The polyelectrolyte precipitates and may be redissolved upon addition of a strong acid (such as HCl). This can be regarded as a special case of ion exchange. 

A comparable addition pattern with as many as five attached groups allowing further functionalization is represented in pentaaryl-fullerenes like 25-27 (Troshina et al., 2007 Zhong et al., 2006). Whereas the free acids are virtually insoluble in pure water, the use of basic water in the case of 25 (Zhong et al., 2006) and conversion of the polyacids 26 and 27 to the corresponding potassium salts leads to stable aqueous solutions with high fullerene concentrations. 

Spontaneous polymerization of 4-vinyl pyridine in the presence of polyacids was one of the earliest cases of template polymerization studied. Vinyl pyridine polymerizes without an additional initiator in the presence of both low molecular weight acids and polyacids such as poly(acrylic acid), poly(methacrylic acid), polyCvinyl phosphonic acid), or poly(styrene sulfonic acid). The polyacids, in comparison with low molecular weight acids, support much higher initial rates of polymerization and lead to different kinetic equations. The authors suggested that the reaction was initiated by zwitterions. The chain reaction mechanism includes anion addition to activated double bonds of quaternary salt molecules of 4-vinylpyridine, then propagation in the activated center, and termination of the growing center by protonization. The proposed structure of the product, obtained in the case of poly(acrylic acid), used as a template is ... 

The influence exerted by the matrix on the direction of the elementary growth steps of the daughter chains was observed for the matrix polymerization of 4PV on polyacids when the daughter P4VP had ionene structure 81), and for the matrix polycondensation of urea and formaldehyde in water, with PAA being present 88,89). In the latter case, the daughter chains of PFU contained the structures... 

In fact, studies of water uptake with a direct comparison of water uptake in com-pomers and conventional composite resins do not show particularly large differences between the two different types of composite material. For example, when the polyacid-modified composite resin brands Dyract and Compoglass were compared with the conventional composite resin Pekafill , there were only minor differences in equilibrium water uptake in both pure water and in 0.9% saline solution (Table 4.1) [18]. Pekafill showed lowest equilibrium water uptakes in both storage media, but only by a very small amount, and one that was not statistically significant in the case of pure water. 

As with glass-ionomer cements, flnoride release from polyacid-modified composite resins is snstained for long periods of time [23] and is enhanced by placing the polyacid-modified composite resin in acidic storage media [25,36]. This property has been snggested to be beneficial in the case of resin-modified glass-ionomers [37], since it wonld lead to enhanced release of protective fluoride ion under the very conditions that promote dental caries. A similar argnment can be advanced for polyacid-modified composite resins, and it may be that this ability to release extra fluoride under conditions of low pH is beneficial clinically. 

The size of the cooperative unit may be determined from the pH dependence of the transition parameter, 6. For the special case that the whole polyacid molecule encompasses the cooperative unit, i.e. that the whole polyacid molecule must be in one conformation or the other, the number of repeat units, iF, in a cooperative unit is given by the relation ... 

The titration curves of polyacids which undergo a conformational transition in aqueous solution greatly differ from those of polyacids without transition. A discontinuous change of pK vs the degree of ionization (a) is observed in the former case which can be related to a transition from a rather compact state to a mean extended conformation in the latter case, a gradual increase in the electric potential is observed in accordance with the absence of any special conformation. The cases of PM A and PAA clearly show such features although in our opinion PMA is a limiting case. 

Contrary to the acid-amine complexes the characteristic complex composition in this case corresponds to the ratio ICOOH-group 1 salt molecule. Tliis difference seems to be explained by the change of the proton affinity when one changes from neutral nitrogen atom (in the tertiary amine) 7963.7 kJ/mol (7P) to a chlorine anion (1395.3 kJ/mol (20)/. The latter being the stronger proton acceptor is able to distort intramolecular H-bonds in the polyacid, which may be responsible for the formation of 2 1 PAA-amine characteristic complex. 

If the acidities of the polyacid are too close to each other, it is simply impossible to see them separately. With a triacid in this case, only the sum of the three acidities can be titrated. This is the case with citric acid (pKa =2.94, pKai =4.14, pK = 5.82) (see Fig. 9.10). 

The existence of a discontinuity in the concentration dependence of the reduced viscosity has already been reported for many polyelectrolytes of different natures which indicates that the Fuoss relation is no longer valid at high dilution. Maxima have been reported for both weak and strong polyelectrolytes and many explanations have been proposed e.g. absorption of carbon dioxyde [7], hydrolysis of ionized groups [8], purity of the solvent [9], effect of rate of shear [10]. If the presence of carbon dioxyde can lead to a decrease of viscosity in the case of a weak polyacid, it can be reasonably excluded for the precise case of PVP (pH 4). Owing to the relatively low molecular weight of PVP (M 8 x 10 ), the effect of rate of shear can also... 

Solutions of polyelectrolytes contain polyions and the free (individual) counterions. The dissociation of a polyacid or its salt yields polyanions, and that of a polybase or its salt yields polycations, in addition to the simple counterions. The polyampholytes are amphoteric their dissociation yields polyions that have anionic and cationic functions in the same ion and often are called zwitterions (as in the case of amino acids having HjN and COO groups in the same molecule). Such an amphoter will behave as a base toward a stronger acid and as an acid toward a stronger base its solution properties (particularly its effective charge) will be pH dependent, and an isoelectric point (pH value) exists where anionic and cationic dissociation is balanced so that the polyion s charges add up to zero net charge (and solubility is minimal). 

By definition, in a solution all ions belong to the same phase, even though counterions may cluster more or less diffusely around the macroions. When significant amounts of a simple 1 1 electrolyte (such as KCl) are added to a polyelectrolyte solution, dissociation of the polyelectrolyte macromolecule is repressed in an extreme case the polyelectrolyte may be salted out. An undissociated polyacid may be precipitated by generous addition of a simple acid such as HCl. 

The effect on complex formation of structure defects, in our case the acrylate groups, is clearly shown in Figure 3. There is an exponential relation between degree of complexation 0, and acrylate ratio p = exp(-A.p). The acrylate ratio p is the total concentration of GGG over the total concentration of PAA, so p is equal to a plus the quantity of acrylate groups due to polyacid dissociation. The constant A is characteristic to the system and always higher than zero. The higher the complexation power of polybase the lower the A value (Figure 3). 

Polyelectrolytes (most notably ionic cellulose derivatives and crosslinked polyacid powders) are also commonly used as matrices, binders and excipients in oral controlled release compositions. In these applications, the polyelectrolytes provide hydrophilicity and pH sensitivity to tablet dosage forms. Acidic polyelectrolytes dissociate and swell (or dissolve) at high pH values whereas basic polyelectrolytes (for instance, polyamines) become protonated and swell at low pH. In either case, swelling results in increased permeability [290], thereby allowing an incorporated drug to be released. 

In vivo results correspond quite well to those of the in vitro experiments although the effect of a basic additive seems to be greater in vivo. It is possible to increase the dissolution rate of PVM-MA esters in tear fluid by adding disodium phosphate or possibly other basic salts to the matrices. With basic additives it may be possible to modify drug release and polymer dissolution also in the case of other polyacids. 

Despite all the methods available fdiQpmeasurement, there are cases where the compounds are too insoluble or too unstable to measure. In the case of polyacids and bases, assigning experimental pKa values to particular sites might be challenging. In some cases, only an estimatidrispfaiiiie is needed, such as in the early discovery stages. Forthese reasons, a method ftbfafpBtikrjation may be very useful. 

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