The ability of multidimensional mass spectrometry (MS) approaches, interfacing different ionization methods with tandem mass spectrometry (MS2) fragmentation and ion mobility (IM) separation, to characterize synthetic polymers is demonstrated for poly(α-peptoid)s synthesized by N-heterocyclic carbene (NHC)-mediated zwitterionic ring-opening polymerization. Matrix-assisted laser desorption ionization (MALDI) causes elimination of the NHC initiator, if performed in the presence of cationizing salts. Electrospray ionization (ESI) is softer, allowing for the detection of the intact sample. It also shows that in proper solvents self-assembly of the poly(α-peptoid) occurs to form supramacromolecules; since these noncovalent self-assemblies overlap with the main product, separation by IM MS is essential for their conclusive identification. MS2 confirms the connectivity of the poly(α-peptoid)s, whereas MS2 combined with IM separation renders valuable insight into the binding interactions in the supramolecular assemblies and on the structures and conformations of the poly(α-peptoid) resulting after NHC elimination. Performing all analyses inside the mass spectrometer (“top-down”) enables fast, sensitive, and cost-effective analysis of polymer composition, structure, and architecture without prior derivatization, separation, or degradation.