in which: n is an integer greater than or equal to 2, limits included, R1 and R2 may be identical or different and represent a covalent bond or an aliphatic, arylaliphatic, aromatic or alkylaromatic hydrocarbon radical, R is a linear or branched aliphatic radical, a substituted or unsubstituted cycloaliphatic radical, a substituted or unsubstituted aromatic radical optionally comprising several ar