This study describes the synthesis of functionalized RAFT-derived poly(n-butyl acrylate) polymers via the use of a continuous-flow microreactor, in which aminolysis as well as thiol–ene reactions are executed in reaction times of just 20 minutes. Poly(n-butyl acrylate) (Mn = 3800 g mol−1, PDI = 1.10) with a trithiocarbonate end group was prepared via a conventional RAFT process. The polymer was then functionalized via aminolysis/thiol–ene reactions in the micro-flow reactor with isobornyl acrylate, propargyl acrylate, poly(ethylene glycol) methyl ether acrylate and pentaerythritol tetraacrylate. To optimize the reaction time and reaction temperature of the micro-flow reactor, freshly collected samples were studied with soft ionization mass spectrometry. With this technique, efficient and very fast aminolysis and subsequent thiol–ene reactions take place on the RAFT-precursor polymer, yielding quantitative end group conversion within 20 min and functionalized polymers of 3700–4000 g mol−1, depending on the type of acrylate coupled. The use of a continuous-flow microreactor opens the pathway towards upsizing lab scale methods into larger processes without suffering from problems associated with reproducibility and tedious optimization issues.