Common ion salt

When the same considerations are applied to the propagation rate-constant, kp, one can find the reason for the nil-effect of a common-ion salt in terms of the apparent propagation rate-constant kp+A, defined in analogy with equation [13]. It must mean that ion-pair formation is negligible because either the KA is very small (highly polar solvent) or for... 

Several workers have attempted to use the common ion technique to depress [Pn+] and thus to achieve a monoeidic Pn+A system, as was done so successfully for anionic systems. However, because generally the solvents used for cationic polymerisations are much more polar, the KD of the chain-carriers and of the common-ion salts are considerably greater than in the anionic systems. Therefore the electro-chemical situation is likely to be complicated by triple ion formation and the effects of ionic strength on the KD and on the rate-constants, so that any results obtained by extrapolations to infinite ionic strength need to be scrutinised most carefully. 

Various groups of workers have attempted to determine the k, by the use of common-ion salts so as to repress the dissociation of the ion-pairs at the growing end. In this context the two types of initiator pose different, but related, problems. If the initiator is a protonic acid AH, as in most of the stopped-flow experiments, the greatest part of the AH does not participate in the initiation (see Section 4.3.2). In these systems, therefore, conjugate ions A2H can be formed from the added salt Bs+A" and therefore the effective [A ] < [Bs+A ]0. In the usual plots [Equation (23) below] of kp (defined in Section 2.5.3) versus [Bs+A ]01/2 this uncertainty will not affect the intercept k, at [Bs+A ]0 1/2 = 0, but it will make the slope, from which the kp can be found, smaller, so that the resulting kp will be an underestimate. 

Following the lead in the study of anionic polymerisations, several workers used common-ion salts to shift equilibrium (11) in attempts to determine kp and by means of... 

The plots of the data according to this equation were rectilinear for polymerisations at -1 °C, 10 °C, 20 °C and 30 °C, so that values of kp and kp+-KDm could be obtained. The authors then used the common-ion salt (n-Bu)4N+ CF3S03 and interpreted the kinetic results by the equation... 

The kp is a weighted mean of kp and kp and of course the [Pn5 ] determined as indicated above is a weighted mean of [Pn+] and [Pn+A ]. Additional evidence from experiments with common-ion salts, from the dependence of the DPD on the reaction conditions, etc., is required to resolve such ambiguities. Nevertheless, the order of magnitude of the k obtained and the manner in which it varies with reaction conditions, e.g., the dielectric constant of the medium, can give useful clues as to whether one is dealing with an enieidic or monoeidic system, and if monoeidic whether it is, for example, cationic or pseudocationic. 

For most of the systems reported in the literature, C/K is not known—very often, neither K nor C is known. For two-component initiator-coinitiator systems, C is usually taken to he the initiator concentration [YZ] when the coinitiator is in excess or the coinitiator concentration [I] when the initiator is in excess. C may be lower than [YZ] or [I] due to association that is, only a fraction of [YZ] or [I] may be active in polymerization. This may also he the case for one-component initiators such as triflic acid. It would be prudent to determine the actual value of C in any polymerization system—usually a difficult task and seldom achieved. Experimental difficulties have also limited our knowledge of K values, which are obtained most directly from conductivity measurements or, indirectly, from kinetic data. A comparison of polymerization in the absence and presence of a common ion salt (e.g., tetra-n-butylammonium triflate for the triflic acid initiated polymerization) is useful for ascertaining whether significant amounts of free ions are present in a reaction system. 

In addition to the choice of Lewis acid, added common ion salt, and temperature, the fast equilibrium between active and dormant species can be fostered by including additional nucleophiles (separate from the nucleophilic counterion) in the reaction system and by variations in solvent polarity. Nucleophiles act by further driving of the dynamic equilibrium toward the covalent species and/or decreasing the reactivity of ion pairs. Nucleophilic counterions and added nucleophiles work best in nonpolar solvents such as toluene and hexane. Their action in polar solvents is weaker because the polar solvents interact with the nucleophiles and nucleophilic counterions, as well as the ion pairs. Polar solvents such as methylene... 

Table I shows the tacticities of PMMA samples which were prepared in counterion/solvent systems which so far have been investigated kinetically and in which the contribution of free anions was suppressed by the addition of a common ion salt. It is seen, that the degree of isotacticity as well as the persistence ratio decrease with decreasing solvation, exhibiting the following order Na/DME < Cs/DME Na/THF < Cs/THF.

With 5-33.3 vol.% water/acetone mixtures, it is found136 that common-ion salts have no effect on the rate of hydrolysis of benzoyl chloride whereas the rate in 15% (but not 33.3%) water is increased by the addition of neutral salts such as lithium bromide or potassium nitrate. The increase in ionic strength on the addition of neutral salts is not the major reason for the increase in rate and nucleophilic catalysis via the more easily hydrolysed benzoyl bromide was postulated. 

Nevertheless, the situation is by no means as clear cut as it may seem, since addition of common ion salts R4N+BF4, to these systems reduces the value of rc in a given solvent Such salts would be expected to favour the formation of ion pairs from free ions, and it seems likely therefore that some contribution from free ions must also be accounted for in the original reactions. 

Three approaches have been used to decrease the molecular weight distributions. One approach suppresses dissociation of ion pairs to free ions by adding salts with common counteranions however, this may cause a special salt effect [274]. Addition of a common ion salt shifts the equilibrium between ions and ion pairs toward the latter by mass law. (In spite of speculation to the contrary [275], the common ions can not influence the equilibrium between covalent species and ion pairs.) The kinetics of association is also affected because ion pair formation is a bimolecular reaction whose rate increases with increasing anion concentration, This decreases the lifetime of free ions. In such systems, kdeact in Eq. (68) should be replaced with the product of /cdeacl and deactivator [D] = [A ], in which the deactivator is a counteranion (DP ./DP = 1 + [l]0 p/ ([D] deacl)). 

Thus, by lowering solvent polarity (CH2Cl2/CCl4 mixtures) or by the addition of a common ion salt (rtBtuNI), these researchers obtained poly(pMOS) of unimodal and relatively narrow MWDs, whose shape and position corresponded to those of the lower polymer population in the two-peaked distribution. Under these conditions the number-average molecular weights increase with conversion. Upon sequential addition of fresh monomer feeds, and block copolymers of pMOS and isobutyl vinyl ether (IBVE) are obtained [52], Similar results were also obtained for the polymerization of IBVE [53,54]. 

For data obtained in the presence of a common ion salt, e.g. Bu4N BF4, this relationship becomes... 

Taste acceptability is a particular issue with oral liquid dosage forms, lozenges, and chewable tablets. The problem may be overcome by the preparation of poorly soluble salts. Thus the bitterness of erythromycin and of bacitracin may be ameliorated by use of the estolate (lauryl sulfate) and zinc salts, respectively. Propoxaphene may be taste masked by forming the napsylate, the solubility of which may be further reduced and the taste improved by adding a common-ion salt such as sodium or calcium napsylate. 

Veiy similar conclusions have been drawn for the CFaSOsH/styrene system and for common ion salt effects on monomer reactivity ratios.In the case of initiation by trifluoroacetic acid, bimodal distributions are also obtained and similar conclusions reached. With this conclusion a relatively stable ester might be formed with the growing carbocation, but the experimental evidence indicates that this is not one of species responsible for the production of polymer. 

Further work by Schulz and his co-worker has shown the results for the polymethylmethacryl sodium system to be influenced by the bifunctional nature of the living polymer employed, with in particular a contribution from an intramolecular association of ion pairs. This has led to a careful re-evaluation of the system using one-ended living polymer in the presence of excess of common ion salt, Na+BPh4 , with, for example, a resulting estimate for / p( ) at —73 °C of 168 s" somewhat higher than previously reported. 

With isoprene in THF the situation is more complicated. While with Li as counterion the system is chemically stable up to 0 °C, with Na" considerable instability arises even at 40 °C. In the former situation reasonable kinetic behaviour is exhibited and u.v. absorption spectra show the presence of three types of anionic centre with absorption maxima at 287, 305, and - 335 nm. The first is assigned to a cis species, the second to the corresponding trans isomer, and the third to some irreversibly isomerized entity whose structure is still by no means certain. At low temperatures — —40 °C) in the absence of common ion salts, as with butadiene polymerizations, fast reactions occur owing to the presence of free anions, predominantly in the trans form, and addition of common ion species depresses the rate to very low values. Above —20 °C under ion pair conditions polymerization via the cis isomer of the active centre appears to be abundant, but, unlike butadiene, there is the added complication in the possibility of 3,4- and 1,2-terminal units arising (20) and (21). In THF the... 

Additional support for the zwitterion-carbene mechanism comes from the observation of the mass-law effect of added common ion salts In the presence of 0.22 M sodium bromide, the initial rate of the second-order reaction of 3-bromo-3-methyl-l-butyne is depressed by about 40%. The same concentration of either sodium perchlorate or sodium nitrate depresses the rate by only about 18%. 

Common ion salts are considered to suppress the ionic dissociation of covalent species and ion pairs to free ions, which are believed to result in nonHving polymerization [34]. In the light of recent results, which confirmed similar reactivity of free ions and ion pairs, this view may require revision. In addition to the common ion effect, addition of salts can also change the nucleophilicity of counterions by... 

Scheme 6. The effect of COj partial pressure on the solubility of saturated calcite solution with and without added common-ion salt. The multiphase thermodynamic model allows the determination of the solubUities of carbonates in different p(CO )s.

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