Exchange reactions between covalent and carbocationic species in polymerization of vinyl ethers in the presence of lewis acids: dynamic NMR studies

The ionization of 1-chloro-1-isobutoxyethane (IBVCI) in the presence of BCI₃ in CD₂Cl₂ and CD₂Cl₂/CDCl₃ mixtures at variable temperatures was studied by ¹H NMR spectroscopy. The presence of only one NMR signal for the methine proton in the intermediate stage of ionization indicated a fast exchange between covalent and cationic species. The dependence of its chemical shift on the [BCI₃]₀/[IBVCI]₀ ratio was used to calculate the equilibrium constant of ionization at various temperatures, assuming only the ionization-recombination process. The dependence of the calculate the equilibrium constant (K^app) on the [BCI₃]₀/[IBVCI]₀ ratio indicated the involvement of additional side equilibria, including the formation of oxonium ions by the reaction between the carbocation and the ether functionality, and the complexation of Lewis acid by the ether group. The width at half-height of the methine proton signal was used to calculate the apparent rate constant of the recombination of the counterions in the ion pairs (K). It was found that K is larger in CDCI₃/CD₂CI₂ mixtures than in CD₂CI_2. When BCI₃ is present in excess, the calculated rate constants for ionization and recombination are higher than the reported rate constant of propagation for the cationic polymerization of isobutyl vinyl ether (IBVE). Attempts to use this finding in order to prepare well-defined polymers were unsuccessful because of the consumption of BCI₃ through the addition reaction to the double bond of the monomer.