R1 R2).O.CO.(Zn)-- where Y and Z, which may be the same or different, are --O--, --S-- or --NR3 --; R1 and R2, which may be the same or different, are hydrogen atoms or carbon-attached monovalent organic groups or together represent a carbon-attached divalent organic group, R3 is a hydrogen atom or an organic group, and the symbols n, which may be the same or different, are zero or 1.