A process is provided for preparing an imidized polymer in a mixing or kneading device, by a reaction in the presence of an acid of a mixture containing a polymer having anhydride groups or derivatives thereof and an aromatic polyamine compound. A solid multifunctional viscosity modifier is also provided which includes a polymer having between 0.1 and 10 functional groups per 1000 C-atoms. The functional groups include aromatic amine based imide groups with a ratio of imide groups to the functional groups being more than 0.85.