The carbenium ion - oxonium ion exchange processes related to vinyl and ring-opening polymerization

It has previously been proposed (Ref. 1) that in the cationic vinyl polymerizations, proceeding with termination due to the collapse of ion pairs, addition of bases increases “livingness”, because of the fast convertion of the otherwise dead (within the time of polymerization) covalent species into the onium ions; these, in turn, fast convert into carbenium ions, the actually propagating species. Equilibria between carbenium ions (CH₃OCH₂⁺A⁻ has been used as a model) and their onium counterparts ((CH₃)₂O taken as a model base) as well as between covalent species (CH₃OCH₂OSO₂CF₃) and the corresponding oxonium ion (with a (CH₃)₂O ligand) have been studied by dynamic ¹H and ¹⁹F NMR. Total ionization of methoxymethyl triflate (CH₃OCH₂OSO₂CF₃) has been shown to increase indeed from 10⁴ (-10°C) to 10⁶ (-70°C) times when 1,0 mol·L⁻¹ of (CH₃)₂O is added. Although this model system better describes polymerization of cyclic acetals than that of vinyl ethers, it shows at least qualitatively the importance of bases in ionization of covalent species, which may be responsible for reinitiation in the cationic polymerization of vinyl ethers.