. . . . . . "7] (7) polymers corresponding to the general formula (XX) as described, for example, in French patent 1 400 366 (the disclosure of which is incorporated herein by reference): [0188] in which R29 represents a hydrogen atom, a CH3O, CH3CH2O or phenyl radical, R30 denotes hydrogen or a lower alkyl radical such as methyl or ethyl, R31 denotes hydrogen or a lower alkyl radical such as methyl or ethyl," .