**2.2 Measurements**

The Fourier-transform infrared (FTIR) spectra of the compounds were obtained with a Bruker EQUINOX-55 spectrophotometer using a KBr pellet or film. The nuclear magnetic resonance (NMR) spectra of the synthesized compounds were recorded on a Bruker Fourier Transform Avance 400 spectrometer. The chemical shift of the NMR was reported in parts per million (ppm) using tetramethylsilane as an internal reference. Splitting patterns were designated as s (singlet), d (doublet), dd (doublet of doublets), dt (doublet of triplets), t (triplet), q (quartet), or m (multiplet). Elemental analyses (EA) of the synthesized compounds were carried out with a FLASH 2000 series device. The single-crystal diffraction data of the diimide model compound were collected on a Bruker SMART 1000 with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) at 120 K. The inherent viscosities of the polymers were measured using an Ubbelohde viscometer. Gel permeation chromatography (GPC) diagrams were obtained with a Viscotek TDA302 instrument equipped with a packing column (PLgel 10 μm MIXED-B) using tetrahydrofuran (THF) as an eluent at 35°C. The number and weight-average molecular weight of the polymers were calculated relative to linear polystyrene standards. Wide-angle X-ray diffraction (WAXD) measurements were performed at room temperature (ca. 25°C) on a Rigaku D/MAX-2500 X-ray diffractometer with a Cu Kα radiation under graphite monochromatic operation at 40 kV and 300 mA. The scanning rate was 1°/min over a range of 2*θ* = 2–45°. The mechanical properties of the films were measured with an Instron 5567 at a crosshead speed of 2 mm/min on strips approximately 40–50 μm thick and 11 mm wide with a 15 mm gauge length. The average of two individual determinations was used. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were conducted on a TA Instruments TGA Q500 and a DSC Q100 instrument, respectively. The TGA measurements were conducted at a heating rate of 10°C/min in N2 and air. The melting points (m.p.) of the synthesized compounds and the *T*<sup>g</sup> values of the polymers were obtained with DSC instrument at a heating rate of 10°C/min in N2. *T*<sup>g</sup> values were taken from the second heating scan after cooling to 0°C from 400°C. The in-plane linear coefficients of thermal expansion (CTEs) of polymer films were measured by thermomechanical analysis (TMA) using a TA TMA-2940 thermomechanical analyzer. Specimens were 5 mm in width, 10 mm in length, and typically 70 μm thick. The measurements were carried out three times in a heating range up to 300°C at a heating rate of 5°C/min. After the first measurement (first run), the sample was cooled gradually to room temperature in a nitrogen atmosphere, after which the second measurement (second run) was taken. The same operation was carried out

between the second run and the third run. The CTE values were determined as the mean at 50–250°C in the second and third heating runs. UV-visible spectra of the polymer films were recorded on an Optizen POP spectrophotometer in the transmittance mode. The refractive indices *n*TE and *n*TM for the transverse electric (TE) and transverse magnetic (TM) modes of the polymer films were measured with a Sairon SPA-4000 prism coupler equipped with a gadolinium gallium garnet (GGG) prism at a wavelength of 633 nm at room temperature. The birefringence values (Δ*n*) were calculated as the difference between *n*TE and *n*TM. In order to confirm the thermal response behavior of the polymers in organic solvents, another UV-vis spectrophotometer (Shimadzu UV-3600) was used. The transmittance change was measured at a wavelength of 600 nm, and the heating and cooling rates were 0.5°C/min. The clouding point (*T*cp) was determined as the temperature at which 90% transmittance was observed during the heating process. When *T*cp was observed at a temperature above the boiling temperature of the solvent used,*T*cp was measured by the naked eye, while the solutions were placed in screw-cap vials and gradually heated at 5°C intervals in a heating bath equipped with a mercury thermometer.
