n atom, or an alkyl group containing up to 20 carbon atoms, or a phenyl group; Y2 is a hydrogen atom or an alkyl group containing up to 20 carbon atoms, or Y1 and Y2, together with the group ##STR11## form a saturated or unsaturated ring system; and R1 to R14, Y1 to Y2 are the same or different and together contain in total from 10 to 60 saturated carbon atoms. 3.