Carboxonium centres

Reactions of carbocations with acetal bonds, aldehydes, ketones, etc. yield carboxonium ions. Various structures have been assigned to these ions. They most probably actually exist in various variants, mutually connected by equilibria. The proportion and structure of the predominant form depend on the structure of the original particles and on their neighbourhood. 

Immediately after the start of dioxolane polymerization, initiated by the triethyloxonium salt Et30 + B, the carboxonium centres present are of the form [127] 

At sufficiently high conversions, the more basic stabilizing diethyl ether is replaced by the less basic but more abundant polymeric chain, and the generated centres are of the form III [128] 

Eizner and Yeruzalimski [129] observed that the bond between O and the terminal methylene group in CH30=—-= h2 is of double bond character. 

The carboxonium centres are resonance-stabilized [130], and therefore much less reactive than carbenium centres. 

---

Analogous to carboxonium centres are the siloxonium centres (see Chap. 3, Sect. (3.2) [133, 134]... 

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 linear acetals units of polymer segments can also stabilise the open chain carboxonium ion 18 (thereby accelerating its formation), and the species formed (i.e. 19) can be regarded as the effective active centre, able only to propagate and unable to participate in hydride ion transfer. For steric reasons 1,3-dioxolane cannot itself stabilise carboxonium ions in this way. 

Even with reactions of a non-radical active centre, the generated polymer is not always inert. Carbanions react with —C=N and —COOR sub-stitutents, carboxonium ions produce less acid centres by reaction with an ether-type chain (see Chap. 4, Sect. 2.3), carbocations alkylate aromatic groups, etc. All these reactions affect propagation. Sometimes the physical effect of the generated insoluble polymer is combined with its ability to react chemically in a certain way. 

Under conditions where polymer does form it is still by no means clear what is the polymerization mechanism [153], nor is it entirely understood whether cyclic, as opposed to linear, macromolecules are formed. Yamashita et al. [148] have proposed that the active centre is a linear carboxonium ion, i.e. 

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... 

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. 

---