This mixed ester is preferably of the of the formula: ##STR3## wherein X is a sulfur atom or a carbonyl group; each Y independently is a hydrogen atom, a halogen atom, a --CO--OR4 or a --CO--NHR4 group, where R4 is an alkyl group containing not more than about 10 carbon atoms; R is a hydrogen atom or an --O--C(???O)--O--CR5 R6 R7 group; and R1, R2, R3, R5, R6 and R7 are each an alkyl group, subjec