In formula (I) ##STR1## Z is hydrogen or alkyl; one of Z1 and Z2 is represented by the group --CH2 --X--X1 --X2 wherein X is phenylene, --C.tbd.C--, cis or trans --CH???CH-- or --CH2 --CQ2 -- in which each Q is independently selected from hydrogen and alkyl such as ethyl or the two Q's together form an alkylene radical having four, five or six carbon atoms; X1 is a covalent bond or a straight or b