be the same as or different. ##STR7## wherein R3 is a cyclic saturated hydrocarbon residue; and R4 and R5, which may be the same or different, each is a cyclic saturated hydrocarbon residue, an alkyl group or an aryl group. ##STR8## wherein R6 is an alkyl group containing 1 to 10 carbon atoms, which may be substituted with a phenyl group; R7 is an alkyl group containing 1 to 12 carbon atoms, whic