wherein R3 is a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, R4 is an alkylene group having 1 to 10 carbon atoms, R5 is a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, n is an integer of 1 to 6, preferably 1, and m is such a number that causes the average of m???n to be 6 to 80. (Refer to Japanese Patent Application Laid-Open No. 305893/1990.)