Hydride ion transfer processes and cationic polymerization of cyclic acetals

Four topics related to the cationic polymerization of cyclic acetals and mainly based on the author's results are reviewed. The kinetics of the H − transfer from linear and cyclic acetals and ethers to the triphenylmethyl cation were examined. A linear correlation between the 1 H‐NMR shift of the H‐atom to be transferred as H − anion and the enthalpy of activation (Δ H ) was observed for H − donors which are linear and cyclic‐planar or deviate slightly from planarity. Puckered rings, like 1,3,5‐trioxane were found to be very poor H − donors. A linear relationship between Δ H and Δ S was observed for all studied compounds. The equilibria carbenium ion ↔ oxonium ion involving the unpaired triphenylmethyl cation and different linear and cyclic acetals and ethers were studied. A linear relationship between the basicity of the involved nucleophile (expressed as p K a or p K b ) and the enthalpy of equilibrium (Δ H ) was found. The chemistry of the elementary reactions leading eventually to the formation of the active species in the polymerization of 1,3‐dioxolane was investigated. The latter was used as a model monomer and initiated with triphenylmethyl salts with various stable anions like AsF and SbF or with the unstable SbCl . The importance of the ambident reactivity of 1,3‐dioxolan‐2‐ylium cations was pointed out and the kinetics of cationation via ring opening of these salts was described. 1,3‐Dioxolan‐2‐ylium salts were shown to be sufficiently stable to be slowly transformed into the linear cations (k̄ cat ≈ 10 −3 to 10 −4 mol −1 dm 3 s −1 at 25°C). This slow reaction was partially responsible for the induction periods observed in the polymerization of 1,3‐dioxolane. The kinetics and mechanism of the chain propagation of 1,3‐dioxolane were described and a structure ( 20 ) was proposed for the growing centres. It was indicated that the growing species, being the tertiary oxonium ions use the polymer segments as ligands and in contrast to the oxycarbenium ions do not abstract hydride anions (H − ).