Poly(2-oxazoline) (POx) based materials have experienced renewed interest, due to their biocompatibility and broad functional diversity. Although research pertaining to cationic ring opening polymerization (CROP) of 2-oxazoline derivatives spans upwards of 5 decades, modern characterization techniques, specifically Matrix Assisted Laser-Desorption Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS), were not available during early studies to aid in identifying polymer end-groups. Through careful synthesis and analytical characterization, evidence supports an alternative mode of nucleophilic attack during ammonia termination, with nucleophilic attack occurring at a different site on the terminal oxazolinium than where polymer propagation occurs. Furthermore, investigations employing targeted end-group modification, in conjunction with MALDI and NMR analysis, determined the structure of the resultant terminal group was a hydroxyethylamino end group. Based on the observed data, a mechanistic explanation for the observed 2-oxazoline ring-opening termination is proposed. (C) 2017 Wiley Periodicals, Inc.