The invention relates to a process for preparing a thermoplastic polyisocyanate polyaddition product based on polyisocyanate (i) and compounds which are reactive toward polyisocyanate, with or without the use of chain extenders, a first catalyst (iv) and/or auxiliaries and/or additives (v), wherein a second catalyst is initially vaporized and then applied to the polyisocyanate polyaddition product by condensation, and to the products prepared therefrom.