Larger counterion

The use of larger counterion (such as K" ) and polar solvents (such as DME) favors an open transition state (PT = phenyltetrazolyl) ... 

With larger counterions (K) and polar solvents, an open transition state becomes possible. 

A larger counterion, such as R4N is alleged to suppress backbiting or the reverse reaction and reduce the concentration of cyclosiloxane byproducts (4), However, more convincing evidence will have to be presented to show that these data were indeed taken at equilibrium. 

The regiochemical outcome of the reaction largely depends on the counterion and reaction media and has been extensively reviewed. In general, smaller counterions, such as sodium and lithium, favor ortho-substitution of the carboxy group, while larger counterions, like potassium, preferentially lead to / ara-substitution. ... 

A comparison of the observed propagation rate constants for styrene polymerization with different alkali metal counterions is shown in Table 2. Poly(styryl)sodium was presumably associated into dimers since kinetic orders of one-half were observed for the rate dependence on the active chain-end concentration. Poly(styryl)potassium exhibits intermediate behavior dependence on chain-end concentration was one-half order at higher concentrations, but first order at low concentrations. Poly(styryl)rubidium and poly(styryl)cesium exhibit first-order dependencies on chain-end concentrations which is consistent with unassociated chain ends in cyclohexane. The counterion dependence is K+ > Rb+ > Cs+ Li+ in cyclohexane and K+ > Na+ > Li+ in benzene. The interpretation of these results is complicated by the fact that the complex observed rate constants ( obs) reflect both the fact that the dissociation constant for the dimers increases with increasing cation size (no association for rubidium and cesium) and also the fact that the requisite energy associated with charge separation in the transition state would be less for the larger counterions. 

The data in Table 1 also illustrate the effect of the size of the counterion on the values of the propagation and termination rate constant (150,151). This table illustrates that the tetrakis pentafluorophenyl borate anion, which is 14 times more voluminous than the hexafluoroantimonate anion, results in a higher propagation rate constant and lower termination rate constant for the photopolymerization. These are the expected trends since the larger counterion will result in a more weakly associated ion pair. 

For larger counterions, like Cs, dissociation into free anions and cations was observed at very low concentrations. However, the dissociation constants are much lower than those of polystyryl ion pairs and the contribution of free anions to polymerization is negligible in most cases, except for [Na, 222], that is, the sodium ion encaged by cryptand 222, which has a large interionic distance (see Figure 3). ... 

The heteroaryl sulfone 18 is deprotonated to give the carbanion 21. Addition of canbanion to the carbonyl compound gives adduet 22, which then undergoes a series of transformations resulting in the expulsion of sulphur dioxide and the potassium derivative of PT alcohol 25 with concomitant formation of the alkene 20. The use of larger counterion (such as K ) and polar solvents (such as DME) favors an open transitin state 26 (PT = phenyltetrazolyl) ... 

This asymptotic behavior is a consequence of the different distribution of counterions on the middle and at the extremes of the chain. Therefore, as the size of the chain increases, end-effects become less important. Polyelectrolytes with a greater linear charge density achieve a larger 0p " value at a larger chain size j ateau End effects are more important in this case, due to the larger counterion condensation in these systems. The plateau values are 0 o.78 0.63 0.43... 

The Counterion The counterion incorporated into the polymer during synthesis influences both the chemical and electrochemical properties of the material. For example with polypyrroles the incorporation of simple anions such as chloride produces an anion exchange material (12). However, incorporation of larger counterions such as dodecylsulfate (13) or polyvinylsulfate (14,15) produce cation exchange materials. 

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