cyclic polyoxymethylene having a number - average molecular weight of 500 to 5 , 000 is obtained by solid state cyclization reaction of an alkalidegradative linear oxymethylene homopolymer in the presence of a medium containing a cationic catalyst . the cyclization is effected at 10 ° to 150 ° c . for 1 to 500 minutes in an organic liquid medium . the reaction mixture obtained by the cyclization is treated with an alkaline medium to hydrolyze and remove away unreacted linear polyoxymethylene homopolymer , and isolate the cyclic polyoxymethylene . the product has a sharp molecular weight distribution and is useful as a resin improver .