Extra pure grade polyethylene glycol (Mn = 6000, PEG6000), was dried before usage at 80 °C under 0.1–0.2 MPa for 6 h. 4,4-Diphenylmethane diisocyanate (MDI, analytical grade), 1,4-butanediol (BDO, analytical grade), 4-octocyloxy benzoic acid (OOBA, analytical grade) and dimethylformamide (DMF, high performance liquid chromatography grade, solvent) were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China) and were used without further purification.