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 degrees 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. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 68-79, 2011