Ring-opening metathesis polymerization (ROMP) was used to synthesize novel ROMP polyphenethylnorbornene.  Hydrogenation, which we use to impart thermooxidative stability to unsaturated ROMP polymers, can produce two different derivatives of ROMP polyphenethylnorbornene.  Both derivatives have saturated backbones, but, depending on the type of catalyst used for hydrogenation, the phenyl groups can remain intact or be saturated to cyclohexyl groups.  The synthesis of these two different hydrogenated products allowed us to compare the effect on the glass transition temperature (T_g) of replacing the pendant phenyl groups with cyclohexyl groups.  We found that T_g decreased by only 2°C (from 28°C to 26°C) upon replacement (hydrogenation) of the phenyl groups.  This is in contrast to what we observed upon hydrogenation of the phenyl groups in the addition polymer of phenethylnorbornene,  where the T_g increased by 39°C from 353°C to 392°C upon hydrogenation, a remarkable difference in magnitude and direction compared with the ROMP polymers.