Role of the Adducted Cation in the Release of Nitroxide End Group of Controlled Polymer in Mass Spectrometry

Abstract

The role of cations in the fragmentation of polymer nitroxide labile end groups, as often deplored in matrix-assisted laser desorption/ionization, was investigated. A combination of different spectroscopic techniques and theoretical calculations was implemented to address the mechanistic aspect of the end-group cleavage in a nitroxide-terminated poly(ethylene oxide) macroinitiator agent upon activation in mass spectrometry. When occurring during the ionization process, the homolytic cleavage of a C-ON bond in the end group results in the release of a nitroxide moiety and prevents the intact oligomer from being observed. In contrast, electrospray allowed oligomer ionization in a more gentle process and was used here in conjunction with tandem mass spectrometry to identify factors influencing the release of the end group. Collision-induced dissociation of intact electrosprayed oligomers showed that the elimination of the nitroxide radical depends on the cationizing agent. This fragmentation readily occurs from alkali adducts, whereas it is hardly observed from oligomer adducted with divalent cations such as Cu 2+ , Zn 2+ , or Ca 2+ . Nuclear magnetic resonance experiments actually revealed that the presence of divalent cations in solution would allow the formation of carboxylic salts, inducing an internal proton transfer in the end group. Upon protonation, the C-ON bond dissociation energy increases by about 25 kcal/mol, as indicated by theoretical calculations. This suggests that the release of the nitroxide radical would no longer efficiently compete with alternative dissociation pathways, as observed in MS/MS experiments. On the basis of the structural details provided by NMR and calculations, dissociation mechanisms supported by accurate mass data were proposed to account for all detected fragment ions.

Structural identification of hindered amine light stabilisers in coil coatings using electrospray ionisation tandem mass spectrometry

“…This increase in NO-C BDE strengthens the argument for Scheme 4 which favours the heterolytic NO-C and homolytic N-OC bond cleavage pathways in the event of nitroxide protonation. [31] Several prominent product ions in Fig. 5, including m/z 496 from HALS (5) and m/z 576 from HALS (6) are consistent in mass with the loss of both ether moieties from these bifunctional species.…”

Mass spectrometry in polymer chemistry: a state-of-the-art up-date

“…73 Mazarin et al later used quantum chemical calculations in addition to NMR-measurements to rationalize the observed fragmentation reactions of nitroxide functional PEOs with different adducted cations. 74 This interesting study showed that an advanced theoretical understanding of the complex ionization and dissociation processes of synthetic polymers can help in tuning and interpreting the structural information obtained in (co)polymer analysis by MS/MS. The prediction of polymer fragmentation patterns based (partially) upon quantum chemical calculations and computational modelling studies may be of increasing value in the future.…”

MS/MS Sequencing of Digitally Encoded Poly(alkoxyamine amide)s

“…In the case of synthetic polymers prepared by nitroxide-mediated polymerization, protonation of the nitrogen atom of the nitroxide termination was demonstrated to induce an increase of the dissociation energy of the C−ON bond between the nitroxide end-group and the last monomer, hence preventing its spontaneous cleavage. 24 Accordingly, the y 2 •+ product ion would be generated only from precursor ions holding the proton on the nitroxide nitrogen in the second T moiety, whereas formation of the c 2…”