Structure and Properties of Tactic Hydrogenated Polynorbornenes

Tacticity is one of the most important structural parameters for determining the physical properties of a polymer. A high degree of steroregularity typically promotes crystallization, with different tacticities giving rise to differences in crystal structure, melting point, and degree of crystallinity. In polynorbornene (PN) made by ring-opening metathesis polymerization (ROMP), tacticity is determined by the relative configuration of the nonplanar cyclopentylene rings enchained in the backbone. Traditional ROMP initiators yield \textit{atactic} polymers ($a$PN); however, recent advances in catalyst design have produced both \textit{isotactic} and \textit{syndiotactic} PN. Newly reported \textit{cis,isotactic}- and \textit{cis,syndiotactic}-PNs were catalytically hydrogenated (abbreviated $i$hPN and $s$hPN, respectively) without altering the tacticity. The thermal and structural characteristics of $i$hPN and $s$hPN were studied by differential scanning calorimetry (DSC) and wide-angle x-ray scattering (WAXS) and compared to that of $a$hPN. Remarkably, all three polymers are semicrystalline, each with a distinct crystal structure. $i$hPN has a nominal melting point of 165 C, more than 20 C above that of $a$hPN. WAXS patterns of melt-drawn fibers of $i$hPN show few strong reflections indicative of either a highly symmetric unit cell or poor long-range order. $i$hPN fibers also exhibit a crystal-crystal transition near 130 C, which is not fully reversible on subsequent cooling. On the other hand, $s$hPN has a nominal melting point some 15 C below that of $a$hPN, and $s$hPN fibers show no evidence of polymorphism.