bon atoms in one or more rings, e.g. a phenyl group or a naphthyl group.Where R1 or R2 is an aralkyl group, it is an alkyl group as defined above that is substituted with one or more aromatic groups having from 6 to 14 carbon atoms in one or more rings, e.g. a benzyl group or a triphenylmethyl group.Each group R1 and R2 may be selected from alkyl groups having from 1 to 25 carbon atoms, and alkeny