Process for preparing oxymethylene polymers

In a process for preparing oxymethylene polymers which comprises polymerizing trioxane or both trioxane and a cyclic ether of the following formula ##STR20## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4, identical or different, represent a hydrogen atom, an alkyl group containing 1 to 5 carbon atoms, or an alkyl group containing 1 to 5 carbon atoms and substituted by 1 to 3 halogen atoms;

• and R.sub.5 represents a methylene group, a methylene group substituted by an alkyl group, a methylene group substituted by a haloalkyl group, an oxymethylene group, an oxymethylene group substituted by an alkyl group or an oxymethylene group substituted by a haloalkyl group, in which case n is 0 or an integer of 1 to 3, the substituent alkyl group is an alkyl group containing 1 to 5 carbon atoms and the substituent haloalkyl group is an alkyl group containing 1 to 5 carbon atoms and substituted by 1 to 3 halogen atoms, or R.sub.5 alternatively represents --(CH.sub.2).sub.m -- OCH.sub.2 --, or --(OCH.sub.2 --CH.sub.2 ).sub.m --OCH.sub.2 -- in which m is an integer of 1 to 3, in which case n is 1,
• in the presence of a catalytic amount of a boron fluoride-type catalyst, stopping the polymerization, and then subjecting the resulting polymer to a terminal stabilizing treatment, the improvement which comprises stopping the polymerization by adding a trivalent organic phosphorous compound of the following formula ##STR21## wherein each of R.sub.1, R.sub.2 and R.sub.3 represents an alkyl, haloalkyl, aryl, substituted aryl, alkoxy, substituted alkoxy, aryloxy, substituted aryloxy, alkylthio, substituted alkylthio, arylthio or substituted arylthio group,
• and then performing the terminal stabilizing treatment without removing the catalyst from the polymer by washing.