Synthesis and property of 9,9′-spirobifluorene-containing aromatic polyesters as optical polymers with high refractive index and low birefringence

C2-Symmetric 9,9′-spirobifluorene-containing polyesters (PEs) were synthesized by polycondensation of 2,2′-dihydroxy-9,9′-spirobifluorene (1) with bis(acyl chloride)s (2) at 230 °C in diphenylether. The molecular weights of PEs 3a-3f were sufficiently high (Mw 13,400-41,600). PEs displayed high thermal stability. The glass transition temperatures (Tg) estimated by differential scanning calorimetry analysis appeared in a range 177-352 °C depending on the spacer structure, while the 5% decomposition temperatures (Td5) measured by thermogravimetric analysis were over 416 °C both under nitrogen atmosphere and in air. PEs showed good solubility in typical organic solvents such as CHCl3 and THF easily to afford the tough, transparent, and flexible cast films. The transmittance of the polymer films reached over 90% in the wavelength range from ca. 410-900 nm. In addition, PEs exhibited higher refractive index rather than that of commercially available 9,9-diarylfluorene-containing PE, in addition to very low degree of birefringence presumably due to the C2-symmetric structure.