. "The group R may be for example --(CH2)3 --, --(CH2)4 --, --CH2 CH2 CHCH3 CH2 -- or --(CH2)6 -- but is preferably --(CH2)3 -- or --CH2 CH.CH3 CH2 --; and each R' may be for example methyl, ethyl, propyl, decyl, tetradecyl or octadecyl." . . .