Sequential Epitaxial Organization of Poly(9,9-di‑<i>n</i>‑octyl-2,7-fluorene) in an Eutectic System

The phase diagram of a binary eutectic system of poly­(9,9-di-<i>n</i>-octyl-2,7-fluorene) (PFO) and hexamethylbenzene (HMB) has been established via onset melting temperatures. Upon homogeneous mixing, the growth of PFO nematic phase in binary mixtures exhibits both lyotropic and thermotropic features. During cooling, a smectic phase of PFO developed epitaxially on the surface of prior-developed HMB crystals, establishing layer stacking via the packing of side chains. Upon the annealing at 120 °C, further epitaxial organization of PFO smectic packing on HMB crystalline substrate results in the growth of β crystalline. Both smectic and β phases of PFO yield characteristic ultraviolet (UV) absorption at 434 nm wavelength and photoluminescence at the wavelength of 441 nm. While HMB crystals were sublimated prior to the annealing treatment, α crystalline form developed instead through the transformation of smectic packing. The UV absorption and fluorescence of α form are relatively shorter wavelengths. As attributing the rise of energy bandgaps to the effect of backbone conformation, organization of less coplanar backbones within α crystalline form is proposed as a result of the lack of epitaxial effect. After the growth of smectic phase via the monotropic behavior of PFO in eutectic mixture, the following routes of phase transformation upon annealing hence unveiled the growth mechanisms of both α and β crystalline forms of PFO

On the Stereoselectivity of Ring-Opening Metathesis Polymerization (ROMP) of <i>N</i>‑Arylpyrrolidine-Fused Cyclobutenes with Molybdenum– and Ruthenium–Alkylidene Catalyst

Ring-opening metathesis polymerization (ROMP) of cyclobutenes fused with <i>N</i>-arylpyrrolidene with Schrock–Hoveyda catalyst containing a racemic biphenolate ligand [Mo­(N-2,6-<i>i</i>-Pr₂C₆H₃)­(CHCMe₂Ph)­(biphenolate)] gives polycyclobutenes with homogeneous tacticity and predominantly double bonds in <i>Z</i>-configuration. Reactions of the same substrates with the first-generation Grubbs catalyst [(Cy₃P)₂Cl₂RuCHPh] or Schrock molybdenum carbene with monodendate alkoxy ligands [Mo­(N-2,6-<i>i</i>-Pr₂C₆H₃)­(CHCMe₂Ph)­(OCMe­(CF₃)₂] yield the corresponding polycyclobutene containing a mixture of <i>Z</i>- and <i>E</i>-double bonds. Upon diimide reduction, all these polycyclobutenes give the same tactic hydrogenated polymers, indicating that the stereochemistry at the asymmetric carbons remains the same in all these reactions. The stereoselectivities of ROMP with cyclobutenes and with norbornenes are compared, and the plausible mechanisms are proposed