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

Abstract

At a temperature Tcc well below its melting point Tm, hydrogenated ring-opened polynorbornene (hPN) is known to exhibit a crystal-crystal transition; above Tcc, 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. Tcc 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 Tcc 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. Tcc approaches Tm with increasing m:r, indicating that r dyads stabilize the rotationally disordered structure. When heated above Tcc, 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 Tcc and Tm yields highly aligned macroscopic specimens. © 2010 Wiley Periodicals, Inc.