Hydrogenated polynorbornene (hPN) synthesized via ring-opening metathesis polymerization exhibits a thermotropic polymorphic transition to a rotationally-disordered state at a temperature Tcc below the melting point Tm. hPN is able to crystallize even while atactic, and the ability of hPN to accommodate stereodefects within the crystal allows for the investigation of the effect of chain defects on Tcc and Tm. The method of hydrogenation from polynorbornene to hPN can introduce additional chain defects along the polymer backbone via epimerization of the cyclopentylene ring, from the all-cis structure inherent from the polymerization to a mixture of cis and trans rings. Hydrogenation via diimide induces no epimerization along the chain, while catalytic hydrogenation over noble metals (e.g. Pd or Pt/Re) induces significant (8 – 36%) epimerization. The rate of epimerization initially increases roughly logarithmically as a function of extent of saturation, but then increases much more strongly as the last few units of the polymer backbone are hydrogenated. The epimerization level has little effect on Tm but a profound effect on Tcc, which ranges from 136 °C (no epimerization) to 71 °C (27% epimerization) to no observable Tcc (36% epimerization). The epimerization level of hPN also influences the room-temperature mechanical behavior of hPN, increasing elongation at break and reducing Young’s modulus.

Control of phase behavior and mechanical properties of hydrogenated polynorbornene via epimerization

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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