Abstract: Film-forming polymers with side substituents involving carbon–carbon double bonds are of great interest as compounds of promise for further modification. In this work, the possibility of synthesizing one of the polymers of this type—additive poly(5-ethylidene-2-norbornene) from a commercially available monomer—has been shown and the molecular properties of a series of the polymer samples in the range of molecular masses of (1–16) × 10⁵ have been studied by methods of hydrodynamics and light scattering in chloroform. The molecular mass dependences of the hydrodynamic characteristics of the polymer have been determined. It has been established that the presence of ethylidene side substituents is responsible for an increase in the equilibrium rigidity of this carbon-chain polymer to 6.2 nm, which is much higher as compared with the typical flexible-chain vinyl polymers and is higher than the value for the known poly(5-trimethylsilylnorbornene-2), the significant value of the Kuhn segment of which approximating 5 nm is due to the presence of bulk silicon-containing side groups.