Tandem mass spectrometry characteristics of silver-cationized polystyrenes: backbone degradation via free radical chemistry

The M + Ag](+) ions of polystyrene (PS) oligomers are formed by matrix-assisted laser desorption/ionization, and their fragmentation characteristics are determined by tandem mass spectrometry experiments in a quadrupole/time-of-flight mass spectrometer. Collisionally activated dissociation (CAD) of M + Ag](+) starts with random homolytic C-C bond cleavages in the PS chain, which generate radical ions carrying either the initiating (a(n*), b(n*)) or the terminating (y(n*), z(n*)) chain end and primary (a(n*), y(n*)) or benzylic (b(n*), z(n*)) radical centers. The fragments ultimately observed arise by consecutive, radical-induced dissociations. The primary radical ions mainly decompose by monomer evaporation and, to a lesser extent, by beta-H(*) loss. The benzylic radical ions primarily decompose by 1,5-H rearrangement (backbiting) followed by beta C-C bond scissions; this pathway leads to either closed-shell fragments with CH(2) end groups, internal fragments with 2-3 repeat units, or truncated benzylic b(n*)/z(n*) radical ions that can undergo anew backbiting. The same internal fragments are produced in all backbiting steps; hence, these fragments and small benzylic radical ions (which cannot undergo backbiting) dominate the low-mass region of the CAD spectra, while the less abundant closed-shell fragments with CH(2) end groups (a(n)/y(n)) dominate the medium- and high-mass regions. The latter fragments are suitable for determining the individual initiating and terminating end groups, whereas the internal ions could be valuable in sequence analyses of styrene copolymers.