The elastomeric properties of networks of stereoregular polypropylenes (PP) filled with spherical nanoparticles have been modeled in an attempt to obtain better insights into elastomer reinforcement. The polymers were either isotactic or syndiotactic PP in the amorphous state, and the simulations were based on rotational isomeric state (RIS) theory combined with the largest eigenvalue method for deriving conditional bond probabilities. Monte Carlo simulations gave distributions of the end-to-end distance of these chains in the presence of the particles, and these were used in the Mark-Curro theoretical approach to calculate values of the normalized stress, and the reduced stress (shear modulus) under uniaxial stretching. The simulations were calculated for PP chains having 100-200 skeletal bonds, for several temperatures from 481 to 650 K, and for varying filler particle sizes (up to 100 A). The presence of the filler nanoparticles was found to influence chain conformations, frequently leading to significant chain extensions, which significantly affect the elastomeric properties of the nanocomposites. (c) 2005 Elsevier Ltd. All rights reserved.