The present invention relates to a method for preparing macromolecular species with a modified surface, comprising a step (e) in which macromolecular species (M), initially carrying —OH and/or —SH functions, are brought into contact with: a catalyst (C) carrying at least one conjugated guanidine function; and reactive species (E), comprising reactive groups including: (i) at least one group including an a,b-unsaturated carbonyl group C═C—C═O and/or an a,b-unsaturated thiocarbonyl group C═C—C═S; and/or (ii) at least one heterocyclic group comprising from 3 to 5 ring members, said group being selected from cyclic ethers, cyclic thioethers and aziridine rings; and/or (iii) at least one group selected from isocyanate —N═C═O or thioisocyanate —N═C═S groups, and trivalent groups of formula >C═CZ—, where Z is an electron-withdrawing group. The invention also relates to the macromolecular species with a modified surface that are obtained in this context.