curable polyurethane uv pu is further applied as a water resisting material in this preferred embodiment the polyurethane prepolymer is bis cyclic carbonate which is a ppg type cyclic carbonate the epoxy resin is polypropylene glycol diglycidyl ether ppg dge and the catalyst is lithium bromide libr and the method of preparing a pu prepolymer comprises the following steps s 11 material mixing ppg dge 5 moles and lithium bromide 5 mole percents are mixed uniformly until the ppg dge is dissolved completely to form a mixed raw material and s 12 thermal reflux carbon dioxide gas is introduced into the mixed raw material and a thermal reflux is performed at the pressure of one atmosphere and a temperature of 100 u00b0 c for 24 hours to form a bis cyclic carbonate product in this preferred embodiment a large quantity of deionized water and ethyl acetate are used for rinsing the bis cyclic carbonates product to remove remained catalysts and achieve the purification effect so as to obtain a highly pure transparent colorless bis cyclic carbonate liquid with reference to fig1 a and 1b a fourier transformed infrared spectroscopy is used for detecting and tracing the elimination state of the epoxy functional group 910 cm u2212 1 and the formation state of the cyclic carbonate functional group 1800 cm u2212 1 the fourier infrared spectra show that the epoxy functional group is fully converted into the cyclic carbonate functional group 2 method of preparing pu prepolymer containing an amino group at an end the bis cyclic carbonates product produced by the aforementioned method can be used for manufacturing a pu prepolymer containing an amino group.