Influence of Tacticity on the Crystal Structures of Hydrogenated Ring-Opened Poly(norbornene)s

Abstract

The crystal structures of highly crystalline isotactic and syndiotactic hydrogenated poly(norbornene)s [H-poly(NB)s], synthesized by the stereospecific ring-opening metathesis polymerizations of norbornene followed by complete hydrogenation, have been determined for the first time on the basis of the X-ray diffraction data analyses. The similarity and difference of the chain conformation and the chain packing mode in the crystal lattice have been clarified among the three kinds of H-poly(NB) samples (syndiotactic, isotactic, and atactic species). The molecular chains were found to take essentially the planar zigzag conformation at room temperature with slight torsional angle fluctuations around the skeletal C-C bonds. The cyclopentylene rings were found to protrude from the plane of the skeletal polymer chain in various different ways depending on the tacticity, resulting in the difference of the whole shape of the chains and then the difference of the chain packing structure. The melting point was found to be different depending on the tacticity: 178, 148, and 134 °C for the isotactic, atactic, and syndiotactic H-poly(NB)s, respectively. The difference in the thermal behavior has been discussed qualitatively on the basis of these structural pieces of information.