in a process for producing a prepolymer epoxy resin mixture with oxazolidinone structures , an insoluble reaction resin powder which has no isocyanate groups and consists of an epoxy resin mixture containing a reaction accelerator and filler and having isocyanurate structures , is fed to a continuously working reactor and reacted at temperatures up to 200 ° c ., with reactor temperature at 140 °- 190 ° c ., and then the extruded material is cooled down to a temperature of & lt ; 50 ° c . with the aid of a cooling device mounted at the outlet die of the reactor .