Preparation of PEB: The PEB was synthesized through a condensation polymerization reaction ( Fig. 1). First, the predetermined amounts of PP, BDA and DMSO were added into a 50 ml round-bottomed flask with a magnetic stirrer. This mixture was stirred at 30 °C for 1 h. Then, EDTAD, the molar number of which was equal to the total molar value of PP and BDA was added into the above-mentioned mixed solution. The reaction temperature was raised to 40 °C and kept fixed for 24 h under nitrogen environment. After that, the mixture was added into excessive methanol to produce precipitates. The filtered precipitates were washed with methanol for three times, and subsequently vacuum dried to constant weight at room temperature.