On-line monitoring of transient radicals and oligomers: o-Phenylenediamine electrooxidation mechanism study by mass spectrometry

Mechanistic insight into the electrooxidation of o-phenylenediamine (oPD) has been achieved by on-line electrochemistry-mass spectrometry (EC-MS). A serious of oxidation products include radical cation as well as soluble oligomers (i.e. dimers, trimers, tetramers, pentamers and hexamers in different oxidation states) were successfully detected and further identified with high-resolution and tandem MS methods. The oligomer itself can undergo dehydrogenation reaction and the products would not participate in the oPD chain propagation. Based on the solid evidences, a new mechanistic pathway for poly(o-phenylenediamine) was formulated. Moreover, the electrooxidation was dependence on the solution pH as well as oxidation voltages. The favored pH environment for oPD polymerization was at around 7.0, whereas only the dimer could be formed in strong acidic (pH ≤ 2.6) and alkaline (pH ≥ 10.6) solution. A high oxidation voltage can accelerate the chain propagation speed, but by-products has generated when the voltage increased to 8.0 V.