Transfer to counteranion

Propagation by reaction of M3+ and M + is not possible at [M]o < [M]c. Therefore, the rate-limiting step in dimerization may be transfer to counteranion. The true rate constants of propagation for a-methylstyrene may therefore be even higher than reported with either counteranion. This is supported by the much higher rates of monomer consumption in p-isopro-... 

The intercept of a plot of [N] vs. [M]0 yields the number of chains formed by initiation ([AT] ), or more precisely, the sum of chains formed by initiation and intramolecular chain transfer to counteranion. The rate constant of initiation kt is calculated from the intercept, and k, and ktrm are obtained from the slope after solving the integral value graphically. 

The elementary reactions of carbocationic polymerizations can be separated into three types. Deactivation of carbenium ions with anions and transfer to counteranion are ion-ion reactions, propagation and transfer to monomer are ion-dipole reactions, and ionization is a dipole-dipole reaction [274]. Ion-ion and dipole-dipole reactions with polar transition states experience the strongest solvent effects. Carbocationic propagation is an ion-dipole reaction in which a growing carbenium ion adds electro-philically to an alkene it should be weakly accelerated in less polar solvents because the charge is more dispersed in the transition state than in the ground state [276]. However, a model addition reaction of bis(p-methoxyphenyl)carbenium ions to 2-methyl- 1-pentene is two times faster in nitroethane (e = 28) than in methylene chloride (e = 9) at - 30° C [193]. However, this is a minor effect which corresponds to only ddG = 2 kJ morit may also be influenced by specific solvation, polarizability, etc. [276,277]. 

Importantly, also, isobutylene polymerizations initiated by 7-radiation in bulk are characterized by AEjjv = —6-6 kcal/mole in the range from +29° to —78 °C52. Since counteranion is absent in these systems, termination by counteranion must also be absent and molecular weight control can only occur by transfer to monomer. 

It follows then that for the systems in which molecular weights are controlled by transfer to monomer (AEfjy = —6.6 1.0 kcal/mole), an increase in counteranion nucleophilicity increases PIB molecular weights. For systems in which molecular weight control is by a combination of termination and transfer to monomer,... 

Transfer to monomers is proposed to take place by a concerted mechanism with counteranion assistance13 ... 

In sum, a relation of counteranion nucleophilicity and the molecular weight in isobutylene polymerization is discovered, according to which an increase in G nucleophility leads to an increase in the rate of termination but a decrease in the rate of chain transfer to monomer. Thus, an increase in G6 nucleophilicity leads to increased termination and hence decreased molecular weight for systems in which termination is molecular weight governing. Similarly, it leads to a decrease in rate of transfer and hence to an increase in molecular weights for systems in which chain transfer controls molecular weight. The nucleophilicity of G is determined by the... 

Since the photoreduction of viologen is induced by electron abstraction (oxidation) by the excited bipyridinium cation from the counteranion, it is difficult to relate the photoelectron transfer to the dark redox potential. However, the photosensitivity for the same skeleton is almost proportional to the first dark reduction potential, as mentioned in the previous section. The degree of solvation... 

The absorbances at both 500 and 610nm, which appeared first, became larger with increasing number of carbons, reached a maximum of Cj n, and then became smaller with further increases in the carbon number. Presumably this is due to crystallization (orientation) of the long alkyl chains. As shown in Figure 9.6, the dication and counteranion moieties in an anisotropic phase are situated next to each other at the site, and the distance between both ions is large compared with that in an isotropic (disordered) phase. Thus the electron transfer from counteranion to dication hardly takes place via the exciplex mechanism. 

As discussed in Chapter 2 and later parts of this section, the reactivities of carbenium ions and ion pairs are similar, and independent of the structure of the counteranion. Although the rate constants of a-methylsty-rene polymerizations initiated by triflic acid are significantly different than those initiated by sulfuric acid, the reactions were performed at low monomer concentrations ([M]0 < [M]c), which thermodynamically prevent polymerization [214], Monomer is converted to linear unsaturated dimers. However, dimerization requires /3-proton elimination. Thus, the faster apparent rate of polymerization with sulfuric acid may be due to faster /3-proton elimination by transfer to the more basic sulfate counteranion according to Eq. (51). 

Figure 8 Effect of parameter b = (fcr/fcp)/[I]o on deviation from ideal behavior for unimolecular transfer (e.g., to counteranion) (from Ref. 16 ).

The relative rates of chain transfer to monomer (briefly transfer), termination and propagation, determine molecular weights and ultimate conversions in most cationic olefin polymerizations. The corresponding model reactions are proton elimination, alkylation by the counteranion and formation of Cfg and higher fractions. Thus, a quantitative analysis of [Cfa]. [C13 or C14] and [Cj ] could give clues as to the relative rates of these competing reactions. The following equations further illustrate the concept ... 

Intramolecular heterolysis involves proton transfer to a cis ligand L (e. g. H or Cl) or to the counteranion of a cationic complex. This can occur via the intermediacy of a so-called cis-interaction, which essentially is a hydrogen-bonding like interaction of H2 with a cis ligand, such as a hydride, that has a partial negative charge (S-) [2, 5aj. 

The above-mentioned results demonstrate that the voltammetric maximum due to the adsorption of the transferring ion at one of two LM/W interfaces is requisite for the oscillation of A I w W2 or /W1.W2. However, the oscillation was not always realized with systems which gave maximum currents. The recovery of the concentration of the objective ion in the interfacial region of one phase from which the objective ion transfers to another is necessary in order to get the oscillation. If the ion pair which has been accumulated at the interface is desorbed to the phase to which the ion transfers, the oscillation cannot be realized. Taking into account this circumstance, the fact that the oscillation was not observed when SO in Eq. (11) was replaced by Br can be explained as follows. When TPA+ transfers from W2 to LM, TPA+ is adsorbed on the interface as an ion pair irrespective that a counteranion is SO or Br. The hydrophobicity of ion pair adsorbed at the interface depends on the nature of the counterion, and the more hydro-phobic ion pair is desorbed more easily into LM. In the presence case, TPA+Br is desorbed to LM rather than to W2, but (TPA+)2SO is desorbed to W2 rather than to LM, which means the concentration of TPA+ in the interfacial region of W2 cannot be recovered when Br is adopted as a counteranion, but the concentration can be recovered... 

AuBr(NHC)] (158), while with [PdCl2(MeCN)2] the palladium complex [PdCl2(NHC)2] (159) was obtained. Interestingly, with PFe as counteranion, both NHCs were transferred to [AuCl(SMe2)] yielding the biscarbene complex (161). 

AEfjv = —4.6 1.0 kcal/mole) an increase in nucleophilicity leads to an increase in PIB molecular weight, e.g. Et2AlX series, indicating that transfer is more affected than termination by a change in counteranion nucleophilicity. 

In the absence of counteranions, as in irradiation induced polymerization, the transition state for transfer is least favorable, which leads to highest molecular weights. Kennedy35 has proposed a possibility of four membered transition state of transfer, which can take place in the absence of counteranion assistance. 

---