Thus, for example, a unit that derives from an acrylic or methacrylic acid ester does not cover a unit of formula ???CH2???CH(COOH)???, ???CH2???C(CH3)(COOH)???, ???CH2???CH(OH)???, respectively, obtained for example by polymerizing an acrylic or methacrylic acid ester or a vinyl acetate ester, respectively, then by hydrolyzing it.