Polyphosphazenes are a class of inorganic polymers characterized by the pho
sphorus nitrogen repeating unit that forms the backbone. The phosphorus is
pentavalent and the backbone has alternating single and double bonds. This
leaves two coordination sites on the phosphorus free for substituted with a
variety of nucleophilic groups. Several hundred polyphosphazene formulatio
ns have been reported in the literature. The ease of controlling the type a
nd number of substituents provides a unique opportunity to develop special
series of polymers to investigate structure property relationships. In this
paper the pure gas permeabilities of a series of substituted bisphenoxypho
sphazene polymers is reported. The polymers were exposed to ten different g
ases and the resulting permeabilities were analyzed using the time lag meth
od. The time lag method enables the permeability to be broken down into its
solubility and diffusivity components. Careful examination of the results
makes it possible to determine what types of substituent-gas interactions a
re responsible for the overall permeabilities in the polymers. Some of the
polymers showed low permeabilities for all of the test gases, while others
showed very large differences among the different gases tested. Especially
interesting differences were observed for the series of alkanes tested. The
results permit the prediction of what types of mixed gas separations might
be worth pursuing.