selected from the following divalent groups: --COO-- or --CO--NH--, R11 and R12 are selected from the following divalent groups: --(CH2)n --, where 1???n???3, --C(CH3)2 -- or --C(CH3)2 --(CH2)2 -- and G- is a negatively charged carboxylate or sulphonate group; monomers J, having general formula 10: ##STR16## where R13 is a hydrogen atom or a methyl group, R14 is selected from the divalent groups -