All-atom molecular dynamic simulations were performed on isolated polybutadiene chains to study the retraction velocity of a free end of an idealized network chain from extended conformations due to thermal collisions. We compare the snap-back velocity calculated from these simulations with experimental measurements on bulk rubber samples. Over a range of chain lengths, extension ratios and temperatures, we find that the average retraction velocity of a free end seen in the simulations is about two orders of magnitude less than the experimental value. (c) 2008 Elsevier Ltd. All rights reserved.