Free radical initiated polymerization of methyl methacrylate containing 1-15 wt% of dissolved polyimide is studied by a calorimetric method. It is shown that polyimide (up to 10 wt%) addition leads to a shifting of the initial autoacceleration and initial autoretardation conversions to lower conversion values than those for polymerization of plain methyl methacrylate. It is stated that polyimide macromolecules do not create additional initiation reaction centres, but may take part in chain transfer and/or termination reactions during methyl methacrylate polymerization. Besides polyimides some other aromatic polymers, namely polyesters, polyamides, poly(ether ether ketone)s and poly(phthalide)s, have been used in similar processes. The new copolymers obtained differ in properties (solubility, thermal resistance, etc) compared to corresponding polymer components.