A polymacromonomer consisting of polybutadiene backbone and polystyrene side chains (BS-PM) with the degree of polymerization of side chain n = 20 was synthesized by anionic polymerization. Five samples with different molecular weight or main-chain length were prepared. Light scattering and viscosity measurements were taken on dilute solutions of these BS-PM samples in toluene at 15.0 degrees C and in cyclohexane at 34.5 degrees C and 29.0 degrees C. The second virial coefficients of BS-PM in cyclohexane vanished at 29.0 degrees C, which is lower than the theta temperature, 34.5 degrees C, of linear polystyrene and polymacromonomer consisting of polystyrene (PS-PM) in the same solvent. This shows that the interaction between side and main chains is appreciable in cyclohexane at 34.5 degrees C. The mean-square radius of gyration and the intrinsic viscosity in toluene and cyclohexane determined as functions of molecular weight were analyzed with the aid of the theories for the wormlike chain model to determine the stiffness parameter lambda(-1) of the main chain. The values of lambda(-1) for BS-PM in both solvents were smaller than those for PS-PM with the same n, reflecting the wider side-chain spacing. These results are consistent with the prediction of the first-order perturbation theory of Nakamura and Norisuye [Polym J 2002;33:874], which gives the contribution of the segmental interactions among side chains lambda(-1)(b) to lambda(-1). However, the difference of lambda(-1) between BS-PM and PS-PM in each solvent is appreciably larger than the difference of the calculated lambda(-1)(b) between BS-PM and PS-PM, suggesting that the difference of the contribution of steric hindrance among side chains to lambda(-1) between these two polymers is appreciable. (C) 2013 Elsevier Ltd. All rights reserved.