Tandem Mass Spectrometry Characteristics of Silver-Cationized Polystyrenes:  Internal Energy, Size, and Chain End versus Backbone Substituent Effects

Abstract

The Ag+ adducts of polystyrene (PS) oligomers with different sizes (6−19 repeat units) and initiating (α) or terminating (ω) end groups mainly decompose via free radical chemistry pathways upon collisionally activated dissociation. This reactivity is observed for ions formed by matrix-assisted laser desorption/ionization as well as electrospray ionization. With end groups lacking weak bonds (robust end groups), dissociation starts with random homolytic C−C bond cleavages along the PS chain, which lead to primary and benzylic radical ions containing either of the chain ends. The primary radical ions mainly depolymerize by successive β C−C bond scissions. For the benzylic radical ions, two major pathways are in competition, namely, depolymerization by successive β C−C bond scissions and backbiting via 1,5-H rearrangement followed by β C−C bond scissions. The extent of backbiting decreases with internal energy. With short PS chains, the primary radical ions also undergo backbiting involving 1,4- and 1,6-H rearrangements; however, this process becomes negligible with longer chains. If the polystyrene contains a labile substituent at a chain end, this substituent is eliminated easily and, thus, not contained in the majority of observed fragments. Changes in the PS backbone structure can have a dramatic effect on the resulting dissociation chemistry. This is demonstrated for poly(α-methylstyrene), in which backbiting is obstructed due to the lack of benzylic H atoms; instead, this backbone connectivity promotes 1,2-phenyl shifts in the primary radical ions formed after initial C−C bond homolyses as well as H atom transfers between the incipient primary and benzylic radicals emerging from these homolyses