(6) The polymers corresponding to the general formula (VI) are described for example in French Patent 1 400 366 and comprising the repeating unit below: in which R20 represents a hydrogen atom, a CH3O, CH3CH2O or phenyl group, R21 denotes a hydrogen atom or a lower alkyl group such as methyl or ethyl, R22 denotes a hydrogen atom or a C1-C6 lower alkyl group such as methyl or ethyl, R23 denotes a C