Process for preparing molding materials MM with improved optical and color properties, consisting of a polymer PM which contains units of the formula I in which R1 and R2 are each hydrogen and methyl and R3 is hydrogen, C1-C18-alkyl, C5-C8-cycloalkyl, C6-C10-aryl, C6-C10-aryl-C1-C4-alkyl, where these radicals may be up to trisubstituted by radicals selected from the group consisting of C1-C4-alkoxy and halogen, prepared in an imidation reaction known per se in a reactive extruder, in which, on completion of the imidation reaction, an amount of one or more reducing phosphorus compounds P which is effective for color stabilization is added to the polymer PM, characterized in that the phosphorus compounds are organic phosphorus compounds OP selected from the group consisting of compounds of the formula II in which R4 and R5 are each independently hydrogen or an aromatic 6-membered ring which may be up to pentasubstituted by a C1-C4-alkyl and/or C5-C8-cycloalkyl groups. Shaped bodies obtainable by the process, especially light waveguides, have a low tendency to yellowing under thermal stress, coupled with a low haze, and also a stable and high transparency.