Polymerization of formaldehyde in protic media

Polyoxymethylenes prepared in hydroxylic media are produced by a fundamentally different mechanism from that in anhydrous media. Formaldehyde reacts in water in the form of methylene glycol and with methanol to give the hemiacetal. Even in dilute solutions there is a small amount of free formaldehyde present, but, for the purpose of this discussion, the polymerization of formaldehyde in aqueous medium is discussed in the traditional manner as the polymerization of methylene glycol. This means that this polymerization is mechanistically a chain growth polymerization but the polymer formed is a condensation polymer. 

When considering the polymerization of formaldehyde in aqueous media, it must be realized that methylene glycol is not stable and, even in relatively dilute solution, establishes complicated equilibria between a very small amount of free formaldehyde, and low molecular weight 

When a DP of approximately 10 is reached, the oligomeric polyoxymethylene glycols are no more soluble, precipitation occurs and further polymerization proceeds in the heterogeneous, crystalline phase. The heterogeneous equilibria are 

Once crystallization occurs further polymerization proceeds exclusively in the crystalline phase and the driving force of the polymerization is the crystallization of the polymer (see Table 11). 

Clear solution does not precipitate polymer particles 

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