Copyright © 2018 American Chemical Society. Some N-phenyl monosubstituted mono-1,3-benzoxazines can be polymerized under appropriate UV irradiation in aqueous solutions at room temperature. Photoinduced intramolecular electron transfer from the amine to the phenyl ring in aqueous media produces the mesolitic O-alkyl bond cleavage governed by "topologically controlled Coulombic interactions". Irradiation of EWG-substituted benzoxazines produces opening of the oxazine ring as expected, but no propagation of the polymerization is observed probably due to the low nucleophilicity of the intermediate phenolic species. However, with more nucleophilic electron-donor substituted benzoxazines the photopolymerization proceeds smoothly, with excellent conversions and modest degrees of polymerization. The produced materials show a similar degree of polymerization to the observed in the non-cross-linked fraction when monobenzoxazines are polymerized under thermal conditions (>150 °C) but having a lower dispersity.