Carbenium-oxonium ion equilibria

Table 5 Enthalpy of Carbenium-Oxonium Ion Equilibria in solution, CH2CI2, 25°C, AsF6", SbF6, PF6- Counterions...

Plesch and collaborators have recently introduced this technique for the study of carbenium and oxonium ions and of Br nsted acid dissociation equilibria ° The ultimate aim of this work is of course to gain a better understanding of the mechanism of initiation in cationic polymerisation. For the moment however it is difficult to assess the real potential of polarography in that context and we feel that, althou it certainly possesses a hi sensitivity, the conditions required for meaningful measurements are still too distant from those employed in an actual polymerisation, in particular the high ionic strength of the medium. Its use is therefore still doubtful and one can only hope that more research will bridge the gap. 

Equilibria like (62) are strongly shifted to the side of oxonium ions e. g. the calculated heat of formation of the triethyloxonium ion from ethyl cation and diethyl ether is as high as 128 kcal mole" in the gas phase. This would give no chance for a single carbenium ion to exist at the usual polymerization conditions of the majority of heterocycles cyclic sulfides and amines are even stronger nucleophiles than cyclic ethers (cf., however, Ihe discussion on cyclic acetals below). 

Propagation. The structure of the growing species (tertiary oxonlum ions) in ring-opening polymerization of several monomers was already characterized by NMR spectroscopy (Table III). Carbenium-oxonium equilibria were also evidenced and measured. 

---