Anionic ring-opening polymerization of glycidyl phthalimide, initiated with alcohol-phosphazene base systems and based on monomer activation with a Lewis acid (iBu(3)Al), has been studied. No propagation occurred for initiator: iBu(3)Al ratios less or equal to 1:3. For larger Lewis acid amounts, the first anionic ring-opening polymerizations of glycidyl phthalimide were observed. Polymers were carefully characterized by NMR, MALDI-TOF mass spectrometry, and size exclusion chromatography and particular attention was given to the detection of eventual transfer or side-reactions. However, polymer precipitation and transfer reaction to aluminum derivative were detrimental to monomer conversion, polymerization control, and limited polymer chain molar masses. The influence of reaction temperature and solvent on polymer precipitation and transfer reactions was studied and reaction conditions have been optimized leading to afford end-capped poly(glycidyl phthalimide) with narrow molar mass distributions. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1091-1099