Aminolysis of polystyrene and poly(methyl methacrylate) (PMMA) prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization was investigated. The product of the former contains predominantly double molecular weight species by the formation of disulfide bond, whereas the latter formed coupled species which consequently cleaved to unimolecular weight species. MALDI-TOF MS (matrix-assisted laser desorption ionization time-of-flight mass spectrometry), elemental analysis and H-1 NMR indicated that thiolactone terminus was formed after aminolysis of PMMA. We propose that the thiol end groups generated during the aminolysis of PMMA tend to cyclize through "backbiting" to form thiolactone structure. A similar reaction was observed in the case of poly(N,N-dimethylaminoethyl methacrylate) and poly(laury methacrylate). Despite this, the preparation of thiol-end functionalized PMMA was achieved by introducing a short block of polystyrene after the RAFT polymerization of MMA.