Process for the polymerization of formaldehyde

A method for preparing a polyoxymethylene, comprising the step of supplying anhydrous monomeric formaldehyde to a reaction medium comprising an organic diluent which is a liquid and non-solvent for the polyoxymethylene under the reaction conditions and inert towards the other constituents of the reaction medium, and comprising from 0.001 to 0.5 percent by weight of a catalyst based upon the polyoxymethylene, dispersed in said medium, said catalyst having a molecular weight of from 1,000 to 50,000, containing block A in a proportion of from 1 to 89 weight percent, and consisting of a block copolymer carrying ionic couples on its macromolecular chain, said block copolymer being defined by the general structure A-B in which:

• A is a polylactonic block consisting of recurring units:
• ##EQU5## obtainable from at least one monomeric lactone of the general formula:
• ##EQU6## B is a polylactam block consisting of recurring units:
• ##EQU7## obtainable from at least one monomeric lactam of the general formula:
• ##EQU8## wherein PM.sub.1 and PM.sub.2 are polymethylene chains having from 2 to 13 and 3 to 13 carbon atoms, respctively, non-substituted or having at least one hydrogen atom replaced by a radical selected from the group consisting of alkyl, aryl, alkylaryl and cycloalkyl radicals.