Counteranions

Thus in the presence of an excess of NH4+, which suppresses this forward reaction, and counteranions such as NOa" and C104, which have little tendency to coordinate, complexes such as [Hg(NH3)4] +, [Hg(L-L)2] + and even [Hg(L-L)3] + (L-L = en, bipy, phen) can be prepared. But, in the absence of such precautions, amino, or imino-compounds are likely to be formed, often together. Because of this variety of simultaneous reactions and their dependence on the precise conditions, many reactions between Hg and amines, although first performed by alchemists in the Middle Ages, remained obscure until the application of X-ray crystallography and, still more recently, spectroscopic techniques such as nmr, infrared and Raman. 

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

Finally, it is recommended to refer to polymerization systems in terms of four components initiator/coinitiator/monomer/solvent, eg. r-BuCl/Et2 AICI/1-C4H8 /MeCl. In cases where the identity of monomer or solvent is obvious, initiator/coinitiator sufficiently define the system. Hie symbol for counteranion, Ge, is independent... 

Since neither r-BuI34 nor I21 s are initiators in conjunction with R3A1 or Et2AlCl, the use of Et2 All becomes important from the point of view of introducing -I in the counteranion. Recently Italian workers23 also used this approach and studied isobutylene polymerization with I2/Et2AU initiator system. 

MeBr is a strong poison only with Et2 All coinitiator. Since Et2 All forms the least nucleophilic counterion, Et2AHXe, it is expected to produce a relatively free carbenium ion, facilitating bromonium ion formation by interaction with MeBr solvent. With more nucleophilic counteranions, like Me3 AlXe or Et2 AlXf (X = Cl, Br), bromonium ion formation is more difficult and poisoning is modest. Evidently, the less stable bromonium ions form only with weakly nucleophilic counterions. MeCl is the weakest poison or may be inert, since chloronium ions are highly unstable. 

A relation between A jjv and molecular weight controlling mechanisms was discovered and the effect of initiator system, solvent and temperature on a mv was explained. The present work has led to an understanding of the effect of counteranion on PIB molecular weight. These studies provide better insight into the detailed mechanism of isobutylene polymerization, in particular into the initiation and the molecular weight controlling events. 

VI. Effect of Counteranion, Solvent and Temperature on Molecular Weight... 

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. 

Effect of Counteranion on Molecular Weights In Isobutylene Polymerization... 

The above order with Et2AlX counteranions as well as that developed on the basis of data in Fig. 17 can be understood by the following considerations. The molecular weight of PIB is determined by the nucleophilicity of counteranion and... 

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,... 

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. 

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

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. 

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... 

Both the CTC and the bromonium tribromide species have their A-max at 272 nm, but with very different e. Furthermore, at the employed Br2 concentrations the counteranion of the bromonium ion is partitioned between Br3 and Br5 , the latter having its A-max batochromically shifted at 310 nm. The equilibrium constant between the tribromide and the pentabromide species of Scheme 4 is 22.3 M l, that is nearly coincident with that found for the tetrabutylammonium tribromide-pentabromide equilibrium (ref. 22). 

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