It has been discovered that N-alkoxymethyl polypyrrolidone and N-alkthiomethyl polypyrrolidone in which from about 14% to about 22% of the amide nitrogen 'atoms are substituted by N-alkoxymethyl or N-alkthiomethyl groups, respectively, are admirably suited for the purposes and ends above described, and that such polymers are far superior to all others for such purposes as packaging and the like ha