the polymerization of the lactam was conducted at 180 u00b0 c in an oven whereby the lactam solution lost its fluidity after about 10 minutes when the reaction product was heated at 180 u00b0 c in the oven for an additional 15 minutes and then allowed to cool an opaque solid polymer having an extremely slight yellow color was obtained the relative viscosity of the polymer was 2 19 and the content of the hot water soluble components was 2 92 in these examples the copolymerization of u03b5 caprolactam and u03c9 lauryllactam was illustrated that is u03b5 caprolactam was mixed with u03c9 lauryllactam in a nitrogen atmosphere in the mol ratio as shown in table 2 and the mixture was melted by heating while maintaining the molten mixture at the temperature as shown in table 2 0 2 mol of sodium hydride 50 oily was dissolved therein and then the promotor was added to the mixture followed by stirring the vessel containing the reaction mixture was closed and the mixture was heated to 180 u00b0 c in an oven in example 4 in which diphenylmethane 4 4 39 bis ethylene urea was used as the promotor the reaction system lost its fluidity within 60 minutes and when the reaction product was allowed to cool an opaque solid polymer having an extremely slight yellow color was obtained the polymerization conditions the properties of the polymers obtained etc are shown in table 2 table 2__________________________________________________________________________ example of copolymerization of u03b5 caprolactam and u03c9 lauryllactam exampleno a b c d e f g h i j mol mol mol mol u00b0 c u00b0 c min min __________________________________________________________________________4 1 2 83 0 41 0 2 0 1 128 180 60 lt 60 2 68 39 24.