Oxonium centres

Oxonium centres are formed by the reaction of a cation with the oxygen of an ether-type monomer. They are even more stable than carboxonium centres. For example the reactivity of an oxonium ion is not sufficient for the separation of a hydride ion from the monomer [128], The behaviour of the centre strongly depends on the stability of the counter- ion (see Chap. 6, Sect. 2.1). 

The structure of oxonium centres is described by the general formula... 

Which illustrates only one of the possible (limiting) forms actually only this form, in the shape given above or in connection with a counter-ion, represents an oxonium centre. In reality they exist in several forms (as esters, ion pairs, free ions) connected by equilibria. 

Of higher efficiency is the change of an oxonium centre to a secondary amine, which can be transformed to a carbanion by alkyllithium [240]... 

During the polymerization of isobutyl vinyl ether, macromolecules grow about 10 times more rapidly than the chains of poly(tetramethylene oxide) (PTHF) from tetrahydrofuran (THF) (at the same temperature and with the same initiator, PhjC SbCI ). Therefore it is possible, even by relatively rough methods, to record the change in lenght of PTHF macromolecules during the reaction, whereas with poly(isobutyl-vinyl-ether) this is not possible, even by sensitive and rapid methods. Nevertheless, both chain types grow by stepwise monomer addition to carboxonium or oxonium centres, respectively. 

A special case of the internal stabilization of a cationic chain end is the intramolecular solvation of the cationic centre. This can proceed with the assistance of suitable substituents at the polymeric backbone which possess donor ability (for instance methoxy groups 109)). This stabilization can lead to an increase in molecular weight and to a decrease in non-uniformity of the products. The two effects named above were obtained during the transition from vinyl ethers U0) to the cis-l,2-dimethoxy ethylene (DME)1U). An intramolecular stabilization is discussed for the case of vinyl ether polymerization by assuming a six-membered cyclic oxonium ion 2) as well as for the case of cationic polymerization of oxygen heterocycles112). Contrary to normal vinyl ethers, DME can form 5- and 7-membe red cyclic intermediates beside 6-membered ringsIl2). 

The propagation for cyclic formals also involves a solvated oxonium ion and a highly polar monomer [10, 12]. Since under optimum conditions the polymers are essentially free from any kind of end-group (except very small amounts of -OH) Plesch and Westermann [36, 37] concluded that they must be formed by a ring-expansion mechanism, involving a 4-centred transition state ... 

In the propagation by the Jaacks mechanism the transition state (XIV) would resemble closely that in the polymerisation of cyclic ethers, such as THF for this the transition state is (XV) which is evidently very similar to (XIV). It is reasonable to suppose that the AS corresponding to (XIV) would have a value close to that for (XV) however, the AS1 for (XV) shown in Table 2 is much less negative than that for the polymerisation of DXL by (VII). It is evident therefore that whatever may be the growth mechanism when initiation is by organic cations, it is very unlikely to involve the microcyclic tert.-oxonium ion growth centre of Jaacks. Thus all the evidence taken together indicates that in the... 

The polymerization of vinyl ethers follows much the same mechanism, using the oxonium ion as an intermediate instead of the tertiary carbocation. Termination might again be by loss of a proton or by picking up a nucleophile at the oxonium ion centre. 

The rigid CDA group depresses the reactivity of the glycosidic centre owing to the increase in strain consequent to flattening the pyranose ring as it forms an oxonium ion intermediate. This torsional deactivation has important conse-... 

The precipitating polyformaldehyde deactivated the centres by formation of the oxonium complex... 

Even carboxylate ions can serve as active centres of / -propiolactone polymerization [316]. Cationic polymerization is characterized by the formation of an oxonium transition salt generated by the reaction of an active centre with an exo- or endocyclic oxygen atom. The reaction mode depends on the kind of initiator and monomer [317]... 

Propagation according to scheme (168a) is typical of carbocation initiators both modes were observed with acylium (RC+=0) initiators. During chain growth from y -propiolactone, the concentracion of acylium ions decreases until oxonium ions become the active centres. In e-caprolactone polymerization, both types of centre continue to operate. 

Furthermore, in order to account for the fast transfer to polymer which can occur within this system, Penczek [154] has suggested yet another possibility for the structure of the active centre, a tertiary cyclic or linear oxonium ion, i.e. 

The nature of the counter-ion X and the temperature at which the polymerization is carried out are important. For example, in a study of THF polymerizations at 30°C initiated by equivalent amounts of triethyl-oxonium salts with different counter-ions, Dreyfuss and Dreyfuss [82] observed differences in both conversion and rates of viscosity change with time of polymerization. In the cases of BF4 and SbCl, the final viscosities and conversions that were attained were lower than when SbFg or PFg counter-ions were used. The viscosity of BF4 polymers remained constant after constant conversion was reached, so termination can be inferred. The viscosity of SbClj polymers continued to decrease even after constant conversion was attained. With the SbClg counter-ion, both termination and transfer occurred. In a comparison of rates of THF polymerization at 0°C initiated by Et3 0 BF4 and Et3 0 AlCl4, Saegusa and Matsumoto [83] confirmed the termination inferred by Dreyfuss and Dreyfuss [82] and even earlier by Vofsi and Tobolsky [86] in a study with only the BF4 counter-ion at 0°C. Saegusa and Matsumoto applied the [P ] method [53] for determining active centres described in Section 3.1.1. The termination reaction was less obvious at 0°C than at 30°C nevertheless, it was clearly evident. Further they found that termination... 

In 1971 Lyudvig et al. [97] reported an investigation of the polymerization of 1,3-dioxolane using precision purification of reactants and Et3 0 SbCl6 as initiator. Under these conditions polymerization with different initial monomer concentrations are reported to take place without any induction period. Lyudvig et al. report that the polymerization is first order with respect to both monomer and initiator concentration. Polymerization of 1,3-dioxolane is a reversible process. The final kinetic equation takes a form similar to eqn. (6). A UV spectroscopic method was used to investigate the nature of the active centre in the polymerization. Very briefly, what these workers found was that the maximum they observed for the polymerizing mixture was different from that which could be attributed to a simple cyclic oxonium ion. Hence they propose that the active centre has the polymeric tertiary oxonium ion structure... 

Trioxane is the cyclic trimer of formaldehyde and it can be polymerized to yield polyoxymethylene having the same structure as polyformaldehyde. Polymerization has been carried out with or without catalyst in the liquid, solid, and sublimed states. All polymerizations appear to proceed by a cationic mechanism and the usual type of cationic initiators are effective [122,138—141]. The structure of the cationic chain ends is not clear and two types of props ating centres have been proposed [142], namely, tertiary oxonium ions and carbenium ions. Their propagation reactions are... 

The nature of the active sites is open to discussion. Toby et al. chose to follow a possible earlier suggestion and used the addition of formaldehyde to a neutral polymer chain as the propagation mechanism. Because of a slight inhibition of the polymerization by oxygen, a radical mechanism was not completely ruled out. Looking at the formaldehyde polymerization as a whole and accepting some of the observations of Toby et al., it must be concluded that their formaldehyde polymerization was a cationic polymerization. Active centres, or active sites were actually oxonium... 

The two stypes of stabilization are not equivalent the cation and the bromonium ion are different molecules with different shapes, while the two representations of the oxonium ion are just that—they aren t different molecules. This stabilization of an adjacent cationic centre by a heteroatom... 

On the other hand, 13C NMR spectroscopy has extensively been used to study the structure of oxonium, carboxonium and oxycarbenium ions and diprotonated carboxylic acids,144-146 since it allows the direct monitoring of the cationic centre and since the chemical shifts and coupling constants can be correlated with the geometry and hybridization of the cation. This technique has also been used by Olah et al. to provide... 

---