Stabilization of oxymethylene copolymers

A solid polymer containing oxymethylene groups and groups of greater stability to alkaline hydrolysis is subjected in solid particulate form to treatment with water and/or an organic hydroxy compound at a temperature high enough to cause degradation of terminal oxymethylene groups but too low to cause the polymer particles to become tacky enough for them to agglomerate, the treatment being effected while a mixture of the polymer particles and liquid reagent is being forwarded without back-mixing. The polymer may contain 85-99.9 mole per cent oxymethylene and 0.1 to 15 mole per cent oxyethylene groups e.g. those derived from cyclic ethers-a third monomer may also be present and the polymer may be end-capped. The liquid reagent is preferably aqueous. The water may be replaced wholly or in part by a liquid organic water-soluble compound having an oxygen atom directly bonded to a carbon atom e.g. ketones, esters and ethers, or such a compound may be used together with the organic hydroxy compound. The polymer is preferably treated under alkaline conditions i.e. in the presence of a strongly basic hydroxide, a salt of a strong base and weak acid or ammonia or an organic base. The polymer may be ground with reagent liquid to form a slurry and the slurry diluted and heated to the desired stabilization temperature after which it may be concentrated and then forwarded through a zone in which at least the major part of the stabilization takes place.