Tuning the Phase Behavior of Hydrogenated Polynorbornene via Epimerization

Hydrogenated polynorbornene (hPN) synthesized by ring-opening metathesis polymerization exhibits a thermotropic polymorphic transition at a temperature T_cc below the melting point T_m. Though atactic, hPN is able to crystallize, and the ability of hPN to accommodate defects within the crystal allows the influence of a variety of defects on T_cc to be studied. Polynorbornene (PN) can be saturated via multiple pathways, and the choice of hydrogenation route influences both the crystal structure and T_cc. Three hydrogenation routes for complete PN saturation are compared: a homogeneous route that generates a diimide molecule in situ, a Ni/Al catalyst complex, and a supported Pd catalyst. The resulting polymer chains show zero, minor (< 4 %), and major (> 8 %) amounts of epimerization of the cyclopentylene ring (from cis to trans) for the diimide, Ni/Al, and Pd-hydrogenated hPNs respectively, but all show similar degrees of crystallinity. The window of stability of the rotationally-disordered polymorph (T_m - T_cc) increases with increasing epimerization. The crystal structure of the rotationally-ordered polymorph – observed below T_cc – changes sharply with low levels of epimerized units along the chain, but is weakly influenced by further epimerization.