Polymer end group modifications via [2 + 2] Paterno-Büchi reactions were carried out. Polyisobornyl acrylate (PiBoA, 3100 g•mol-1) and polystyrene (PS, 2800 g•mol-1) was synthesized by atom transfer radical polymerization (ATRP) employing an aldehyde functional initiator. Good control over the polymerizations was achieved, and material with high aldehyde functionality was obtained. The terminal aldehyde was then reacted with alkenes under UV irradiation in [2 + 2] cycloaddition reactions. Good yields of the modification reaction was confirmed via NMR and by electrospray ionization mass spectrometry (ESI-MS) analysis and generally conversion of the aldehyde function into the respective oxetanes of 90% or higher was observed. With the oxetane, a variety of functional groups were introduced to the polymer chain ranging from multifunctional allyl-compounds to disubstituted alkenes and amino- or hydroxyl-functional alkenes. In the photoreaction, the integrity of the bromine end group of the ATRP polymers is retained. ATRP chain extensions can be performed after the cycloaddition, demonstrating the versatility of this newly introduced polymer modification reaction.