In this study, a mechanistic transformation was achieved by taking advantage of coupling reaction of reversible addition-fragmentation chain transfer (RAFT) agents. Polymers prepared by different polymerization modes were coupled to form polystyrene-b-poly(ethylene glycol) (PS-b-PEG) block copolymer. For this purpose, two prepolymers, namely azo and RAFT functional polymers were prepared independently. In the first step, a new macroazoinitiator (MI)-containing PEG units was synthesized by direct esterification of 4,4-azobis(4-cyanopentanoic acid) (ACPA) with poly(ethylene glycol) (PEG) at ambient conditions. The other component, polystyrene (PS) was synthesized by atom transfer radical polymerization and modified with a reversible addition-fragmentation chain transfer (RAFT) agent (xanthate moiety) and used as a polymeric RAFT agent for the block copolymer formation. In the block copolymerization process, polymeric radicals stemming from the thermolysis of PEG-MI were coupled with the RAFT moiety of PS. The PEG-MI, PS, PS-XANTH, and PS-PEG block copolymers were characterized by spectral analysis using FT-IR, H-1 NMR, GPC, and DSC.