ula (I): ##STR3## wherein R1 and R2 are individually a hydrogen atom or methyl group and R3 is a methyl, ethyl or propyl group, with the proviso that R3 is an ethyl or propyl group when R2 is a hydrogen atom, and that R3 is a methyl or ethyl group when R2 is a methyl group, or wherein R2 and R3 are --CH2)4 or --CH2)2 O--CH2)2 in combination; or a copolymer of at least one monomer selected from sai