Radical-Induced Single-Chain Collapse of Passerini Sequence-Regulated Polymers Assessed by High-Resolution Mass Spectrometry

We assess the versatility of high-resolution Orbitrap electrospray mass spectrometry (HR ESI MS) coupled to size exclusion chromatography (SEC) by challenging it with the to date most complexfor mass spectrometric analysissingle chain nanoparticle (SCNP) collapse system in terms of both precursor complexity and collapse mechanism. Here, sequence-regulated ter- and quaterpolymers prepared by the Passerini multicomponent step-growth polymerization serve as ideal mass spectrometric complex precursor structures to identify critical parameters for successful MS mapping and thus understanding of the radical intrachain compaction process. As Passerini monomers for the terpolymerization, 1,6-diisocyanohexane (1 equiv), HO2C-PEG10–11-CO2H (1 equiv), and 4-formylcoumarin equipped with an acrylate (2 equiv) were utilized for subsequent radical-induced single-chain collapse. We evidence the characteristic mass species indicating successful collapse, confirming the collapse mechanism via bimolecular coupling or propagation. A quaterpolymer containing 1,6-diisocyanohexane (1 equiv), HO2C-PEG10–11-CO2H (1 equiv), benzaldehyde (1.7 equiv), and 4-formylcoumarin acrylate (0.3 equiv) is also collapsed, yet cannot be mapped via SEC-ESI MS, as the number of chains entailing a reactive unit is too small, while diffusion ordered NMR spectroscopy (DOSY) can readily witness the compaction. On the basis of these two precursor systems, we are able to identify criteria that the precursors and the collapse system have to fulfill for their successful SEC-ESI MS characterization. The collapse mechanism can only be elucidated via mass analysis if (i) a minimum of four to five intra SCNP linkages are present and, thus, (ii) the precursor formation needs to lead to a sufficient number of chains that feature reactive coumarin acrylate units. Finally, (iii) a chemically too diverse system such as the quarterpolymer leads to a high repeat unit diversity that challenges the MS analysis given the contemporary resolution limits.