Production of polycaproamide fiber from polycaproamide reacted with cyclic tetracarboxylic acid dianhydride

In a process for the formation of polycaproamide fiber from a fiber-forming polycaproamide polymer having excess number of carboxyl end groups over amino end groups, by melt-extruding the polymer through an orifice into a quenching medium and thereafter stretching the resulting filament, the improvement wherein said fiber-forming polycaproamide polymer is prepared by a process comprising:

• a. continuously forming a molten polymerization mixture at 240.degree.-290.degree. C. from e-caprolactam;
• b. continuously smoothly stirring said polymerization mixture while flowing over the surface thereof an inert gas at a flow rate of at least 2 unit volumes of said gas, measured at standard temperature and pressure, per hour per each unit volume of said polymerization mixture, until the total primary amino group plus carboxyl group analysis of the resulting hot water washed and dried polymer is between 105 and 115 milliequivalents per kilogram of polymer;
• and
• c. continuously reacting said polymer at 250.degree.-290.degree. C. with about 0.1-0.7 mol per 100 mols of lactam in the polymer, of a cyclic tetracarboxylic acid dianhydride selected from the group consisting of pyromellitic dianhydride;
• 3,3',4,4'-benzophenone tetracarboxylic dianhydride;
• 1,4,5,8-naphthalenetetracarboxylic dianhydride;
• bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride;
• and tetrahydrofuran-2,3,4,5-tetracarboxylic dianhydride;
• until the primary amino group analysis of the polymer is between 18 and 34 milliequivalents per kilogram of polymer and the carboxyl group analysis of the polymer is between 50 and 90 milliequivalents per kilogram of polymer, thereby reducing the occurrence of nubs in the fiber.