Tuning the phase behavior of semicrystalline hydrogenated polynorbornene via epimerization

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 Pd⁰ 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.