Study of the gas-phase decomposition of multiply lithiated polycaprolactone, polytetrahydrofurane and their copolymer by two different activation methods: Collision-induced dissociation and electron transfer dissociation

Abstract

Collision-Induced Dissociation and Electron Transfer Dissociation experiments were carried out from various lithium adducts ([M+2Li]2+, [M+3Li]3+) from polycaprolactone diol (pCL), polytetrahydrofurane (pTHF), and one triblock copolymer (pCL-pTHF-pCL). In both cases (pCL and pTHF), CID of triply lithiated precursors led to complex mass spectra compared to corresponding ETD spectra, which remained relatively simple because CID product ions exhibit multiple charge states whereas ETD mainly led to singly charged fragment ions. CID of pCL involves charge-remote rearrangements over the ester groups and intramolecular transesterification reactions, whereas ETD leads to radical and charge induced cleavages leading globally to structurally different product ions but accounting for the same bond cleavages: (CO)O-C and (CO)-O respectively. Both CID and ETD can produce a low amount of undesirable reactions such as consecutive fragmentations especially from 3+ precursors but these fragmentations are absent in ETD from 2+ species. CID of a triply lithiated pTHF involved charge-induced and charge-remote fragmentations. In contrast, under ETD conditions, in the absence of suitable chemical functionality, pTHF did not undergo any backbone fragmentations at all. Nevertheless, proton abstraction by the fluoranthene reagent anion allowed the formation of species that could further be collisionally activated, leading to a depolymerization process from the ends. This strategy combining sequentially ETD and CID led to dramatically simplified product ion spectra. Concerning the supposed triblock copolymer, which was commercially purchased, both CID and ETD led to the same conclusion that at least a part of the copolymer was a diblock rather a triblock.