Complete end-capping during the copolymerization of trioxane and ethylene oxide in the bulk polymerization state using boron trifluoride dibutyl ether as the initiator and methylal as the chain transfer agent was accomplished. By using highly purified trioxane as the starting material and by completely deactivating the catalyst residue, an almost completely end-capped polymer (degree of stability was over 99.5%) was obtained.