The alkene enone [2 + 2] photocycloaddition reaction between polymer-bound maleimide and functional alkenes has been tested in a UV-flow reactor, demonstrating a very high efficiency of the reaction. As a test reaction, polymer end group modifications were carried out on maleimide-functional poly(butyl acrylate). The polymer was prepared by atom transfer radical polymerization (ATRP) using a N-hydroxysuccinimide-functionalized initiator, followed by exchange reaction of the activated ester with a maleimide amine linker. Good control over the polymerization and successful exchange of the end group under mild reaction conditions was confirmed by electrospray ionization mass spectrometry (ESI-MS). The terminal maleimide group was then reacted quantitatively with alkenes in a custom-made tubular UV-flow reactor within a minute reaction time. Compared to reaction times in batch of several hours to days, the reaction proceeds to full conversion within 1 min under flow conditions. This significant acceleration of the reaction was achieved by employing a photosensitizer in combination with the flow reactor technique. Via the cycloaddition a variety of functional groups can be introduced to the polymer chain ranging from multifunctional allyl compounds to hydroxyl-functional alkenes, demonstrating the general versatility and high potential of the approach for polymer modification reactions.