AbstractThe polymerization of 4‐methyl‐1,3‐dioxepane (1a), 2,4‐dimethyl‐1,3‐dioxepane (1b), and 4,4‐dimethyl‐1,3‐dioxepane (1c) was investigated in 1,2‐dichloroethane at different temperatures ranging from −60 to 0°C by using boron trifluoride etherate as initiator. The polymerization of1ainvolves an equilibrium between monomer and polymer, and gives a viscous polymer along with some cyclic oligomers of different sizes.1H NMR and13C NMR analysis of the polymer revealed that the bond cleavage of the unsymmetrical 1,3‐dioxepane ring in the polymerization occurred nearly randomly at the two acetal CO bonds. The thermodynamic parameters for the polymerization were determined from the temperature dependence of the equilibrium monomer concentrations: ΔH ss0=−9,3±1,3 kJ · mol−1and ΔS ss0=−38,8±7,8J · mol−1· K−1. Dimethyl substituted 1,3‐dioxepanes were found to be very reluctant to polymerize: Only a liquid oligomer was formed in very low yield from1b, and even oligomer was not obtained from1c. The effect of methyl substituents on the cationic polymerizability of 1,3‐dioxepane