The chain-end structure of the photopolymerized acrylate using benzophenone as an initiator was investigated as well as polymerization behavior. Dodecyl acrylate was used as a monomer in this study. Gelation occurred during ultraviolet (UV) irradiation, whereas a cross-linker was not employed. Conversion-time profile below gel point gave a linear first-order plot suggesting that the steady-state was held throughout polymerization. Matrix-assisted laser desorption/ionization time-of-flight mass spectra of the resultant polymer indicated that most polymers had an acryloyl group at one of the chain-ends, while some polymers had an acryloyl group at each chain-end. The cross-linking reaction leading to gelation would have been caused by the subsequent copolymerization of the residual monomer with the latter polymer having two acryloyl groups. Dissolved oxygen in the monomer solution influenced the polymer structure giving hydroxyl group at chain-end. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1545-1553