A process for preparing a thermoplastic polymer may involve reacting at least components (i) to (ii): a polyisocyanate composition including at least one diisocyanate (i); an epoxide composition comprising at least one diepoxide (ii); in the presence of a catalyst composition (iii); where epoxide composition (ii) and catalyst composition (iii) are initially charged as a mixture at a temperature in a first temperature range (T1); the polyisocyanate composition (i) is at least partially added while maintaining a temperature in the first temperature range T1; the temperature is raised to a temperature in a final temperature range (Tf); and the remaining polyisocyanate composition (i) is added in the final temperature range. A thermoplastic polymer obtainable by such a process may be used, e.g., for producing a fiber or a shaped body by injection molding, calendering, powder sintering, laser sintering, melt pressing, or extrusion, or as modifier for thermoplastic material.