A process for preparing a polymer, which comprises a) reacting a mixture (I) containing 6-aminocapronitrile and water in the presence of a catalyst to obtain a mixture (II) containing caprolactam, ammonia, water, high boilers and low boilers, then b) removing ammonia from mixture (II) to obtain a mixture (III) containing caprolactam, water, high boilers and low boilers, then c) removing all or some of the water from mixture (III) to obtain a mixture (IV) containing caprolactam, high boilers and low boilers and then d) feeding mixture (IV) to a polymerization reaction, and also the polymer obtainable by this process, the use of the polymer for producing fibers, sheetlike structures and moldings, and also fibers, sheetlike structures and moldings obtainable using such a polymer.