Secondary and tertiary oxonium ions in the polymerization of 1,3-dioxolane initiated with HOSO2 CF3: 1H-NMR studies

In the polymerization of 1,3-dioxolane (DXL) initiated with trifluoromethanesulfonic acid (TFAc) the proton originated from acid [underlined in the scheme (1)] may form either the hydroxy end-group or the secondary oxonium ion: (1) where is a part of linear or cyclic macromolecule (or monomer). In ¹H-NMR spectra, due to the fast exchange, only one averaged signal of proton is observed, its chemical shift reflecting the relative concentration of both species. The chemical shift of proton, measured at different monomer and initiator concentrations changes, indicating that the proportion of macromolecules bearing OH end-groups increases with increasing [monomer]_o/[initiator]_o ratio. This observation is fully consistent with a mechanism, according to which secondary oxonium ions are formed by inter-or intramolecular reaction between growing species and OH end-groups of linear macromolecules.