A procedure for the synthesis of well-defined random copolymers bearing multiple thiolate functionalities is presented. Reactive copolymers of vinylbenzyl chloride (VBC, 15 mol%) with methyl methacrylate (MMA, 85 mol%) were synthesized via free radical and RAFT polymerizations. The microstructure of these copolymers was characterized by monitoring the polymerization reaction. Post-polymerization modification of this reactive precursor was conducted by an initial substitution of VBC chloride with thiouronium (TU) followed by conversion of TU into thiolate via reaction with either KOH or NaBH₄. The reaction with NaBH₄, which was found to yield better result, was further investigated by monitoring the changes of the copolymer functional groups throughout the entire process using NMR. Further mechanistic insight was obtained by studying a model reaction between benzyl thiouronium, which is the small-molecule counterpart of TU-bearing vinylbenzyl repeating units, and NaBH₄. Analysis of this model reaction confirmed the formation of thiolate groups and their subsequent transformation into inter or intra-molecular disulfide bonds.