ABSTRACT:2-Oligodimethylsiloxanyl substituted 5-norbornene-2-carbonitriles and carboxylates were synthesized from 5-norbornene-2-carbonitrile and methyl 5-norbornene-2-carboxylate by metalation at the 2-position followed by reaction with monochloro functionalized oligodimethylsiloxane. Polymers were obtained by the ring-opening polymerization of the monomers by a metathesis catalyst. Permeation of oxygen through the polymer films was highly selective over nitrogen. The diffusion coefficient was shown to play the principal role in the permeation.KEY WORDS Norbornene / Oligosiloxane / Oxygen Permeation / Permselectivity / Membrane / Molecular Design / Diffusion Coefficient / One basic idea for obtaining a highly gas permeable membrane material is to use semirigid backbone polymers with bulky substituents which cannot pack densely. An example of such a polymer is poly(l-trimethylsilylpropyne), reported by Higashimura, which has conjugated double bonds along with the polymer backbone and bulky trimethylsilyl groups as substituents. This polymer was reported to show the highest P 02 among the organic polymers, although relaxation was seen by heat treatment. 1 • 2 We have proposed three basic principles for the molecular design of selectively oxygen permeable membrane materials.