a two - stage evaporative method for recovering polymerizable 2 - pyrrolidone from alkaline aqueous solutions such as those obtained by washing the catalyst from an incompletely reacted polypyrrolidone polymerizate , includes the steps of neutralizing the base with mineral acid , removing most of the water in an initial reduced pressure evaporation and recovering the pyrrolidone from a salt / pyrrolidone slurry by vaporization under reduced pressure .