wherein R3 is hydrogen or linear or branched C1-C20 alkyl or alkenyl group, preferably methyl, ethyl, propyl, butyl, pentyl or hexyl, in particular methyl or ethyl, examples of suitable compounds according to formula (V) are formaldehyde (HCHO), asetaldehyde (CH3CHO) and butyraldehyde (CH3CH2CH2HO),