Tandem Mass Spectrometry Characteristics of Silver-Cationized Polystyrenes:  Backbone Degradation via Free Radical Chemistry

Abstract

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 (an•, bn•) or the terminating (yn•, zn•) chain end and primary (an•, yn•) or benzylic (bn•, zn•) 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 β-H• loss. The benzylic radical ions primarily decompose by 1,5-H rearrangement (backbiting) followed by β C−C bond scissions; this pathway leads to either closed-shell fragments with CH2 end groups, internal fragments with 2−3 repeat units, or truncated benzylic bn•/zn• 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 CH2 end groups (an/yn) 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