p and q have the value 1, Z2 and Z3 represent oxygen, Z1 represents oxygen or the group in which A is an unsubstituted or substituted divalent group from an aliphatic or cycloaliphatic C2-C18 hydrocarbon, Z4 represents oxygen or the group in which A3 is an unsubstituted or substituted C2-C18 group from an aliphatic or cycloaliphatic hydrocarbon, A1 is an ethylene or propylene-1,2 group and A2, A3