When characterized with DMA it was found that Phenylnorbornene-ethylene copolymers with equivalent comonomer concentrations had very different storage moduli in the glassy state as well as glass transition temperatures. NMR analysis of the copolymers revealed that they had different ratios of Exo- and Endo-diastereomers even if the same comonomer composition had been used. The Exo/Endo-ratio in the polymer was surprisingly higher than in the monomer and increased with higher incorporations of the comonomer. Copolymers with higher Exo/Endo-ratios also had a tendency for lower storage moduli in the glassy state and higher tan δ peaks. These properties are valuable because more flexible and impact-resistant copolymers are obtained.