The copolymerization of trioxane and ethylene oxide in the bulk polymerization state using highly purified trioxane as the starting material, boron trifluoride dibutyl ether as the initiator, and methylal as the chain-transfer agent was investigated for the industrialization of the acetal copolymer, and the various important polymerization conditions were checked. The acetal exchange reaction during the copolymerization gave a uniform acetal copolymer with randomized comonomer distribution. To obtain the polymer with a high degree of stability, several checks were made. The important points were the high purification of the monomer and complete deactivation of the catalyst residue. Thus, the polymer as polymerized showed a degree of stability over 99.5%.