Bis(halophthalimides) are prepared in mixture in an organic liquid such as ortho-dichlorobenzene or anisole, by a reaction at a temperatire of at least 150° C. between at least one diamine compound and at least one halophthalic anhydride in the presence of imidization catalyst. The reaction mixture is maintained at about 15% by weight solids content and rich in the halophthalic anhydride by constantly monitoring the reaction mixture using analytical methods such as high performance liquid chromatography. The product mixture may be directly employed in the direct preparation of polyetherimides, and similar slurries may be employed to prepare other polyether polymers.