A polyimide-graft-polystyrene (PI-g-PS) copolymer with a polyimide backbone and polystyrene side chains was synthesized by the grafting from method using styrene polymerization on a polyimide multicenter macroinitiator via ATRP mechanism. The side chain grafting density z=0.86 of PI-g-PS is rather high for graft-copolymers synthesized by the ATRP method. Molecular characteristics and solution behavior of PI-g-PS were studied in selective solvents using light scattering and viscometry methods. In all solvents, the backbone tends to avoid contact with a poor solvent. To describe the conformation and hydrodynamic properties of PI-g-PS macromolecules in thermodynamically good solvents for side chains and PI-g-PS, the wormlike spherocylinder model is used. Macromolecules of the studied graft-copolymer are characterized by high equilibrium rigidities (Kuhn segment length >20 nm). In -conditions, PI-g-PS macromolecules may be modeled by a rigid prolate ellipsoid of revolution with a low asymmetry form and a collapsed backbone as the ellipsoid core. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1539-1546