In a poly(ethylene glycol) derivative of the formula Y-Poly-X, the first functional group X and the second functional group Y are preferably different from each other, thus ensuring the polymer to be heterobifunctional. [0051] Preferably, when X is Ar???CH???CH???CH???CH???CO2??? where Ar is as defined above, Y is ???O???(CH2)n???CHO or ???O???(CH2)n???CH(ZR5)2 where n is a number of 1-6, Z is O o