when R2 stands for a hydrogen atom, and that R3 is a methyl or ethyl group when R2 stands for a methyl group, or wherein R2 and R3 form --CH2)4 or --CH2)2 O--CH2)2 in combination; or a copolymer of at least one monomer selected from the acrylamides or methacrylamides and another copolymerizable monomer.