Mass spectrometry analysis of poly(styrene sulfonate sodium salt), a polyanionic electrolyte

Electrospray ionization (ESI) of poly(styrene sulfonate sodium salt) (PSS) with a molar mass of 1100 Da (weight average) efficiently produces distributions of [M + Na]+ cations in positive mode and [M–Na]−as well as [M–2Na]2−anions in negative mode. From these distributions, repeat unit, end groups, and defects in the poly(electrolyte) can readily be determined by quadrupole ion trap mass spectrometry (QiT MS). During MS analysis, sodium/proton exchanges in the SO3Na substituents of the [M–Na]−ions take place, with the number of exchanges observed being equal to the number of sulfonate groups present in the analyzed oligomer. Such exchanges are less pronounced for the oligomers with 2− charges and absent in the sodium-cationized oligomers (1+ charge). Collisionally activated dissociation (CAD) of the sodiated cations proceeds via charge-remote homolytic C–C bond cleavages in the polymer backbone, ultimately leading to fragments that contain both or neither of the end groups. The same chemistry is observed for the anions if they have not exchanged sodium ions for protons. In sharp contrast, precursor ions that contain SO3H (sulfonic acid) groups due to Na/H exchange dissociate mainly by charge-catalyzed SO3 losses. The number of SO3 units lost is identical to the number of sulfonic acid pendants in the oligomer. The CAD results suggest that PSS may degrade to polystyrene under acidic conditions.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (125 K)Download as PowerPoint slideResearch highlights▶ ESI ionizes efficiently sulfonated styrene poly(electrolytes). ▶ Such compounds readily undergo sodium/proton exchanges in anionic charge states. ▶ Polymers with sulfonate salt pendants decompose by charge-remote homolytic chain cleavages via ion/radical complexes. ▶Polymers with sulfonic acid pendants degrade by proton-catalyzed sulfur trioxide losses.