AbstractThe polymerization of 4‐methyl‐, 4,4‐dimethyl‐,cis‐4,5‐dimethyl‐, andtrans‐4,5‐dimethyl‐1,3‐dioxolanes was investigated with boron trifluoride etherate and the perchloric acid‐acetic anhydride binary system as initiators. The polymerization of 4‐methyl‐1,3‐dioxolane (1) involves an equilibrium between monomer and polymer, and the polymer consists of alternating oxypropylene and oxymethylene units. The thermodynamic parameters for the polymerization were determined from the temperature dependence of the equilibrium monomer concentrations in the temperature range from −20 to 5°C: ΔH1c= −3,2±0,2kcal/mol and ΔS 01c= −12,7±0,8 cal mol−1K−1. Dimethyl substituted 1,3‐dioxolanes are very reluctant to polymerize: only a liquid oligomer was obtained in poor yield fromcis‐4,5‐dimethyl‐1,3‐dioxolane, and neither polymer nor oligomer was produced from 4,4‐dimethyl‐ andtrans‐4,5‐dimethyl‐1,3‐dioxolanes at temperatures above −;78°C. The effect of methyl substituents on the cationic polymerizabil