Hydrogenated polynorbornene (hPN) synthesized by ring-opening metathesis polymerization (ROMP) exhibits a thermoreversible change in crystal polymorph at a temperature T cc below its melting point, T m . The polymorphic transition corresponds to a sharp increase in rotational disorder around the chain axis as the temperature is increased above T cc . Saturation of ROMP polynorbornene (PN) to hPN can be achieved through both catalytic and noncatalytic approaches. Here, three different hydrogenation routes were employed on the same precursor polymer: catalytic routes over either supported Pd 0 or a Ni/Al complex, and noncatalytic saturation with diimide. The different hydrogenation routes result in hPNs with varying degrees of epimerization of the cyclopentylene ring (from cis to trans); these epimerized units are included in the hPN crystals. The crystal structure of the rotationally ordered hPN polymorph, observed below T cc , changes sharply at low levels of epimerization and then is weakly influenced by further increases in trans content. The stability of the rotationally ordered hPN polymorph decreases with increasing epimerization, as reflected in a reduction of T cc from 134 C to 92 C at 22% epimerization. T cc is less affected by epimerization than by the inclusion of a similar content of 5-methylnorbornene units, reflecting the smaller size of the trans defect.

Though polynorbornene synthesized by ring‐opening metathesis polymerization has an intrinsically all‐cis configuration of the cyclopentylene backbone rings, a fraction of these rings can be epimerized to the trans configuration during hydrogenation over suitable catalysts. By varying the method of hydrogenation, semicrystalline hydrogenated polynorbornenes (hPNs) with trans levels between 0 and 36% were obtained. With increasing trans content, the glass transition temperature, melting point, and enthalpy of melting decrease modestly. By contrast, the temperature at which the hPN crystal transitions into a rotationally disordered polymorph varies strongly with trans content, ranging from 126 °C (all‐cis) to 71 °C at 27% trans; at trans contents of 34% and above, no rotationally‐ordered phase is observed at any temperature. The room‐temperature Young's modulus shows no dependence on trans content, while the yield stress drops by 20% at 1% trans content and slowly decreases further with additional epimerization. The temperature dependence of the Young's modulus differs for trans‐containing versus all‐cis polymers, while the temperature dependence of the yield stress is set by the polymorph type (rotationally ordered vs. disordered).