Novel reactions between trioxane and ethylene oxide were discovered, and three novel cyclic formals were isolated and identified. These novel cyclic compounds clarified the initiation mechanism of the copolymerization of trioxane and ethylene oxide. This type of reaction was not limited to the reaction between trioxane and ethylene oxide but was also generalized to the reaction between the cyclic formal and ethylene oxide. Although an NMR method for analyzing the ethylene oxide sequences of the acetal copolymer from trioxane and ethylene oxide has not yet been established, the three newly found novel cyclic compounds, composed of 1 mol of ethylene oxide and 1 mol of trioxane, 2 mol of ethylene oxide and 1 mol of trioxane, and 3 mol of ethylene oxide and 1 mol of trioxane, were useful for analyzing the ethylene oxide sequences. These compounds gave only one consecutive oxyethylene unit, two consecutive oxyethylene units, and three consecutive oxyethylene units in three consecutive oxymethylene units, respectively, and gave different H-1 NMR spectra for each oxyethylene unit. Considering these data, we synthesized three polymeric model compounds that had one consecutive oxyethylene sequence, two consecutive oxyethylene sequences, and three consecutive oxyethylene sequences in an oxymethylene main chain. By a linear combination of the H-1 NMR spectrum of each oxyethylene unit of the three polymeric model compounds, we succeeded in determining the ethylene oxide sequences by the H-1 NMR method for the copolymer from trioxane and ethylene oxide. (C) 2003 Wiley Periodicals, Inc.