n atom or a group represented by the formula: RCO-, wherein R is as defined above; one of X4, X5 and X6 is a hydrogen atom, and two of X4, X5 and X6 are each a group represented by the formula: RCO-, wherein R is as defined above; 0 to 15% by weight of ##STR17## wherein R1, R2 and R3 each represents a linear or branched alkyl group having 9 to 22 carbon atoms, an alkyl group having a cycloalkyl mo