A study of gas transport properties of novel polynorbornenes with increasing length of an aliphatic pendant group R (CH3-, CH3(CH2)(3), CH3(CH2)(5)-, CH3(CH2)(9)-) has been performed. These polymers were synthesized using novel organometallic complex catalysts via an addition polymerization route. This reaction route maintained the bridged norbornene ring structure in the final polymer backbone. Gas permeability and glass transition temperature were found to be higher than those for polynorbornenes prepared by ring-opening metathesis and reported in the literature. It was shown that for noncondensable gases such as Hz and He the selectivity over N-2 decreased when the length of the pendant group increased, but remained relatively stable for the more condensable gases (O-2 and CO2). The permeability coefficient is correlated well to the inverse of the fractional free volume of the polymers. The more condensable gases showed a deviation from this correlation for the longest pendant group, probably due to an increase of the solubility effect. This polymer series demonstrated a simultaneous increase in permeability and selectivity, uncommon for polymers.