Fluorene-Containing Hydrogenated Polypentenamers

Cyclopentenes spiro-fused to fluorene, namely spiro[cyclopentane-1,9′-fluoren]-3-ene (1) and its substituted analogues (2–4), are introduced as a new class of monomers for ring-opening metathesis polymerization (ROMP), and the physical properties of the resulting hydrogenated polypentenamers, including H-poly1, are systematically investigated. Equilibrium ROMP of 1 using Grubbs catalysts, conducted at low temperatures and high monomer concentrations, afforded a high-molecular-weight polypentenamer, poly1. Precise molecular-weight control and successful chain extension were demonstrated. Furthermore, the polymerization enthalpy of 1 (−21.6 kJ/mol) indicates slightly lower ROMP reactivity than cyclopentene (Cp), presumably due to the gem-disubstituent effect. As the fluorene content increased in H-poly(1-co-Cp)s, the glass transition temperature (Tg), Young’s modulus, and refractive index increased, while the orientation birefringence became more negative owing to preferential alignment of the fluorene units perpendicular to the drawing direction. H-poly1 exhibited a semicrystalline nature, together with a Tg of 144 °C, a melting point of 268 °C, a Young’s modulus of 2020 ± 83 MPa, and a refractive index of 1.644. In contrast, H-poly(1-co-Cp) containing 4 mol % Cp was amorphous and retained comparable Tg, Young’s modulus, and refractive index values (137 °C, 1856 ± 4 MPa, and 1.637), while exhibiting a toughness approximately six times greater. This work establishes spiro-fused cyclopentenes as unprecedented ROMP monomers and provides fundamental insights into the relationships between polymer structure and the thermal, mechanical, and optical properties of fluorene-containing polyolefin analogues.