To investigate the structures of the prepolymers, a Bruker (Zurich, Switzerland) AV600 NMR spectrometer was employed to determine the 1H-NMR spectra; it worked at 600.13 MHz for protons in chloroform-d6 with tetramethylsilane as the standard. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and polymer distribution index (PDI) of the prepolymers were reported by gel permeation chromatography (GPC) measurements on a Waters instrument equipped with three columns (Styragel HT3, HT5, and HT6E), with tetrahydrofuran as the eluent at a flow rate of 1 mL/min, and a Waters 2410 refractive-index detector, and a polystyrene standard was used for calibration.