Abstract—: Using the commercial derivative of norbornene—5-ethylidene-2-norbornene—a series of three isomeric homopolymers with different structure of the backbone is synthesized, and the effect of the chain structure on the gas-transport behavior of polynorbornenes is studied. The gas-transport characteristics of the polymers are presented together with data on the conformation properties of molecules of addition and metathesis polynorbornenes. The effect of the structure of side substituents on the equilibrium rigidity of addition polynorbornenes is confirmed by the example of poly(5-ethylidene-2-norbornene) and poly(5-vinyl-2-norbornene).