hyl, ethyl or propyl group, with the proviso that R3 is an ethyl or propyl group 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 of a copolymer of at least one monomer selected from the acrylamides or methacrylamides and another copolymerizable monomer, said agent