Increasing the length of an alkyl side chain on a rigid polynorbornene (PNB) backbone is shown to decrease the glass-transition temperature of the resultant polymer, decrease the density of the bulk polymer, decrease the number and average free-volume element size present, and decrease the permeability of gases through the polymer. Methyl-, butyl-, and hexyl-substituted PNBs were investigated. Experimental results were compared with predictions based on molecular modeling. By using models that provided good agreement between the experimental and simulated wide-angle diffraction patterns, the distributions of free-volume elements were predicted. These predictions clearly indicate that the number of large free-volume elements decreases as the length of the side chain increases, suggesting that the flexible aliphatic side chains can be largely accommodated within the free volume between the rigid PNB backbones. (c) 2005 Wiley Periodicals, Inc.