A complete library of poly(2-oxazoline) block copolymers was synthesized via cationic ring opening polymerization for the characterization by two different soft ionization techniques, namely matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-Q-TOF MS). In addition, a detailed characterization was performed by tandem MS to gain more structural information about the block copolymer composition and its fragmentation behavior. The fragmentation of the poly(2-oxazoline) block copolymers revealed the desired polymer structure and possible side reactions, which could be explained by different mechanisms, like 1,4-ethylene or hydrogen elimination and the McLafferty +1 rearrangement. Polymers with aryl side groups showed less fragmentation due to their higher stability compared to polymers with alkyl side groups. These insights represent a further step toward the construction of a library with fragments and their fragmentation pathways for synthetic polymers, following the successful examples in proteomics. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5533-5540, 2010