Linear and star-shaped polyisoprene were synthesized and hydrogenated to form the targeted poly(ethylene-co-propylene) copolymers, (EP)(lin) and (EP)(star). Hydrodynamic properties of the polymers in THF at 40degreesC have been studied by gel permeation chromatography, using multiple-angle laser light scattering and differential refractive index detectors. The radius of gyration R-g of (EP)(star) was found proportional to the molecular weight to a power of 0.34 corroborating a globular architecture and the R-g of (EP)(lin) to a power of 0.60 indicating a random coil conformation in a good solvent. The intrinsic viscosity [eta] of (EP)(lin) was much higher than that of (EP)(star) due to the compact globular structure of star molecules. While the [eta] of (EP)(lin) increased with an increase in the molecular weight, the molecular weight had little effect on the [eta] of (EP)(star) as long as the ann length was fixed. Based on the measured R-g and [eta], the coefficient q for the scaling law, the Mark-Houwink constant a, and the hydrodynamic radius R-e have been calculated. The hydrodynamic radius R-e was approximately 0.78 times of R-g for (EP)(lin) and was nearly identical to R-g for (EP)(star). The value of R-e/R-g appeared to be independent of the molecular weight for both linear and star polymers and was almost equal to those of polystyrene and polyisoprene in good solvents. (C) 2004 Elsevier Ltd. All rights reserved.