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.
We report the synthesis and characterization of thermoplastic elastomers (TPEs) containing both crystalline and glassy hard segments, with the aim of capturing the mechanical properties of conventional all-amorphous triblock TPEs, while forming the solid-state structure by crystallization from a single-phase melt. To accomplish this, we used living ring-opening metathesis polymerization (ROMP) and subsequent hydrogenation to synthesize symmetric pentablock copolymers with the architecture crystalline-glassy-rubbery-glassy-crystalline. Analogous crystalline-rubbery-crystalline triblocks show a high initial modulus, yielding, and poor recovery, resulting from platelike crystalline hard blocks. By contrast, with the pentablock architecture and appropriate selection of block lengths, crystallization from a singlephase melt causes a layer rich in the glassy block to form around the crystallites, limiting their lateral growth and generating composite hard domains with both crystalline and glassy components. The pentablocks show the low initial modulus, strain-hardening behavior, and small permanent set desired for TPEs, while retaining an easily processed single-phase melt.
Ring‐opening metathesis polymerization (ROMP) was used to synthesize novel ring‐opened poly[5‐(2‐phenylethylnorbornene)]. Hydrogenation can produce two different saturated derivatives, with the phenyl groups either preserved or saturated to cyclohexyl groups. These two different hydrogenated products allowed us to compare the effect of replacing the pendant phenyl groups with cyclohexyl groups on properties such as the glass transition temperature (Tg). We found that Tg decreased by only 2 °C (from 28 to 26 °C) upon replacement of the phenyl groups. This contrasts with the effect of saturating the phenyl groups in the addition polymer of 5‐(2‐phenylethylnorbornene), where the Tg increased by 34 °C, from 215 to 249 °C, a remarkable difference in magnitude and direction compared with the ROMP polymers.
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