k and t are equal to 0 or 1, the sum k+t being equal to 1; R9 is chosen from a hydrogen atom and a methyl radical; R7 and R8, independently of each other, are chosen from alkyl groups having from 1 to 6 carbon atoms, such as from 1 to 4 carbon atoms, hydroxyalkyl groups in which the alkyl group has 1 to 5 carbon atoms, for instance, a lower (C1-C4) amidoalkyl group; or alternatively, R7 and R8 are