o-Phenylenediamine Electropolymerization by Cyclic Voltammetry Combined with Electrospray Ionization-Ion Trap Mass Spectrometry

The anodic oxidation of o-phenylenediamine at a platinum electrode in aqueous buffers at different pH (1−7), where growth of very thin (few nanometers thickness) and self-sealing polymeric films occurs, was investigated by cyclic voltammetry (CV) combined with off-line electrospray ionization-ion trap mass spectrometry (ESI-ITMS). Soluble oligomers (dimers, trimers, and tetramers in different oxidation states) formed in the course of the anodic oxidation were detected/identified by electrospray ionization-ion trap sequential mass spectrometry (ESI-ITMS n , n = 1−5) while their relative abundance could be estimated from full-scan ESI-MS spectra of the quasi-molecular ions. In particular, oligomer abundances and the abundance ratios between different n-mers of the same class (e.g., structures corresponding to the same value of n but having different m/z ratios) were considered, and their variation with pH and potential range adopted for polymerization was studied. A mechanistic pathway for the growth of poly(o-phenylenediamine) film (PPD), relying on the integration of CV and ESI-ITMS, could be formulated for the first time. Chain propagation seems to compete with two different intramolecular oxidation processes, leading to the formation of phenazine or 1,4-benzoquinonediimine units, whose relative importance is, in turn, dictated by polymerization conditions (pH and potential). The proposed mechanism can account for the different properties of PPD films prepared under different experimental conditions, as well as for the structural data obtained on the polymeric film itself by X-ray photoelectron spectroscopy.