Typical examples thereof are those represented by the following formulas,andwherein R.sup.12, R.sup.13, R.sup.14, R.sup.15 and R.sup.16 are independently of one another a hydrocarbon group having 1 to 8 carbon atoms, Y is a halogen atom, a hydrogen atom or an alkoxy group, r is a number satisfying 2.ltoreq.r.ltoreq.3,and d is a number satisfying 1.ltoreq.d.ltoreq.30.Specific examples of said compo