where w is an integer of at least 1, y, z, n, R and R' are the same as defined in the above formula (III), characterized by containing more than 95 percent by weight of polysiloxane units and less than 5 percent by weight of polyoxyalkylene units, having a polysiloxane chain and a polyoxyalkylene chain bonded with a C--Si bond (instead of a C--O--Si bond) and having a molecular weight of 500 to 1