Oxonium ions, secondary equilibria

A protonic acid derived from a suitable or desired anion would seem to be an ideal initiator, especially if the desired end product is a poly(tetramethylene oxide) glycol. There are, however, a number of drawbacks. The protonated THF, ie, the secondary oxonium ion, is less reactive than the propagating tertiary oxonium ion. This results in a slow initiation process. Also, in the case of several of the readily available acids, eg, CF3S03H, FS03H, HC104, and H2S04, there is an ion—ester equilibrium with the counterion, which further reduces the concentration of the much more reactive ionic species. The reaction is illustrated for CF3SO 3 counterion as follows ... 

It has also been shown, that the interaction of one molecule of acid (HBr) with one molecule of a weak organic base (e.g. THF) in chlorinated hydrocarbons leads pre-(tominantly to the formatfon of a hydrogen bonded complex. A second molecule of the tese is needed in order to convert this complex into the secondary oxonium ion. However, the equilibrium constant of the ionization reaction is stfll low ... 

Macrocyclic structure/Ki, supposedly growing by ring-expansion, requires that a proton would exclusively constitute a component of the secondary oxonium ion both during propagation and after establMunent of the pofymer-monomer equilibrium ... 

The induction periods observed in the polyrnwization of 1,3-dioxolane initiated by acids (e. g. perchloric acid trifluoromethaneailfonfc acid is apparently due to the equilibrium shifted toward much less reactive secondary oxonium ions at the early stages (short chains) of polymerization. 

Since this review was written a wealth of kinetic data on THF poly-menzations initiated by esters of trifluoromethyl sulphonic acid, fluoro-sulphonic acid and other super acids has appeared [164—168]. It has correctly been pointed out [167, 169] that secondary oxonium ions should not be observable separately under the experimental conditions of Pruckmayr and Wu [96] and that some of their NMR assignments are wrong. Instead, it has been verified that the observed spectrum corresponds to a very solvent sensitive equilibrium between a macro-ester and a macro-ion as proposed earlier by Smith and Hubin [98], viz. 

In the polymerization of DXL, using the phosphine end-capping method (the principle of the method is outlined in Scheme 10), it was shown that secondary oxonium ions exist in equilibrium with tertiary ones and the position of equilibrium is shifted toward tertiary oxonium ions with increasing chain length. ... 

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