The crystal-crystal transition in hydrogenated ring-opened polynorbornenes: Tacticity, crystal thickening, and alignment

At a temperature T_cc well below its melting point T_m, hydrogenated ring-opened polynorbornene (hPN) is known to exhibit a crystal-crystal transition; above T_cc, the hPN chains are rotationally disordered. This transition is examined in a series of hPNs polymerized with different Mo- and Ru-based catalysts, each of which imparts a slightly different tacticity to the polymer. T_cc is found to correlate well with the ratio of meso to racemo dyads (m:r); small changes in m:r (from 0.8 to 1.1) are sufficient to raise T_cc by nearly 20 °C. For the homogeneous Mo-based "Schrock-type" catalyst examined, such a change in m:r is easily achieved by simply adding the reversibly binding ligand trimethylphosphine during polymerization. T_cc approaches T_m with increasing m:r, indicating that r dyads stabilize the rotationally disordered structure. When heated above T_cc, hPN crystals thicken at a rate much greater than conventional three-dimensionally ordered crystals, but below the rates shown by the two-dimensional hexagonal (columnar) phase formed by some polymers, reflecting the intermediate level of order and chain mobility present in the high-temperature hPN crystal phase. Solid-state processing of hPN between T_cc and T_m yields highly aligned macroscopic specimens.