A Cu-catalyzed multi-component reaction (CuMCR) of terminal alkynes, sulfonyl azides, and amines was employed in a post-polymerization modification process. The reaction was conducted on polystyrene bearing 20 mol % alkyne groups with 4-toluenesulfonyl azide and dihexylamine in the presence of a Cu(PPh3)3Br/diisopropylethylamine catalyst. The CuMCR on polymer proceeded efficiently, as confirmed by the obvious gas evolution during the reaction, and the conversion of alkyne moiety reached almost 100%, yielding polymeric N-sulfonylamidine derivatives, as proven by 1H NMR measurements. In addition, the precise structural assignment was proven by synthesis and analysis of the model monomeric compound (styrene N-sulfonylamidine) and the respective model polymer. We have successfully demonstrated the utiliztation of this new post-modification reaction by employing various amines including dipeptide and 1-aza-15-crown-5-ether.