wherein w is an integer of at least 1 and y, z, n, R and R' are the same as defined in the above formula (I), characterized by containing 5 to 95 percent by weight, preferably 5 to 50 percent by weight of polysiloxane units and 95 to 5 percent by weight, preferably 95 to 50 percent by weight of polyoxyalkylene units, having a polysiloxane chain and a polyoxyalkylene chain bonded with a C--Si bond