A comprehensive mechanistic scheme that accounts for the unique experimental features of the copolymerization of bulk trioxane (TOX) with 2% (wt/wt) ethylene oxide (EO) was developed. The formation of the primary initiating species is shown as the diffusion‐limited reaction of trace water with boron triflouride dibutyletherate [BF3O(Bu)2] to form a Bronsted acid. This acid complexes principally with the more basic EO and partly with the less basic TOX. The acid‐complexed TOX depolymerizes to formaldehyde which can react with acid‐complexed EO in an insertion reaction to form an acid‐complexed dioxolane. Further insertion of formaldehyde yields an acid‐complexed trioxepane. This sequence is generalized into a propagation scheme that involves propagation by expansion and ring opening. Displacement of complexed dioxolane and trioxepane can occur in the event that the more basic EO attacks the oxonium‐active site at the reactive position outside the oxonium ring. These displacement reactions account for the observation of formation of dioxolane and trioxepane. The polymerization of formaldehyde is not considered significant until all EO has been consumed. During the latter stages of polymerization, cyclic oxonium‐active sites are transformed into oxocarbenium sites that are stabilized by complexation with the polymer chain. This complexation is the origin of the phenomena of transacetalization and hydride transfer.

synopsisAcetal copolymers prepared from trioxane and ethylene oxide consist of polyoxymethylene segments with oxyethylene units distributed in the chain. These oxyethylene units profoundly affect the thermal and chemical behavior of the polymer. The presence of the oxyethylene units renders the copolymer amenable to thermal or base hydrolytic treatment which results in a final product with stable endgroups. The copolymers so obtained are significantly more stable, thermally and chemically, than acetylated polyoxymethylene homopolymer.

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