Collision-induced dissociation of synthetic polymers containing hydride groups: the case of poly(methylhydrosiloxane) homopolymers and poly(methylhydrosiloxane)-co-(dimethylsiloxane) copolymers Thierry Fouquet 1,3 , Christophe Chendo 2 , Valérie Toniazzo 1 , David Ruch 1 and Laurence Charles 3 * 1 Department of Advanced Materials and Structures, Public Research Centre Henri Tudor, 5 rue Bommel/ZAE Robert Steichen, L-4940 Hautcharage, Luxembourg 2 Aix-Marseille Université – CNRS, Fédération des Sciences Chimiques, FR 1739, F-13397 Marseille, France 3 Aix-Marseille Université – CNRS, Institut de Chimie Radicalaire ICR, UMR 7273, F-13397 Marseille, France RATIONALE: When substituting one methyl moiety by a hydrogen atom in each end-group of a trimethylsilyl-terminated poly(dimethylsiloxane) (PDMS), dissociation reactions of oligomers adducted with ammonium were observed to proceed at a much higher rate, evidencing the high reactivity of hydride groups. Polymeric molecules containing methylhydrosiloxane (MHS) units could thus be expected to exhibit a different tandem mass spectrometric (MS/MS) behavior from PDMS. METHODS: Trimethylsilyl-terminated PMHS and trimethylsilyl-terminated poly(MHS)-co-(DMS) were electrosprayed in the gas phase either as ammonium adducts or lithium adducts. Product ions generated upon collision-induced dissociation (CID) were accurately mass measured in an orthogonal acceleration time-of-flight mass analyzer. RESULTS: In contrast to PDMS adducted with lithium, useful structural features could be obtained from product ions generated upon CID of lithium adducts of PMHS. The presence of multiple hydride groups in PMHS induced numerous rearrangements when activating ammonium adducts of these oligomers. MS/MS reactions observed for cationic adducts of MHS-DMS co-oligomers were clearly a combination of major dissociation routes established for the corresponding homopolymers. However, the concerted loss of H 2 and ammonia typically observed from ammonium adducts of PMHS was always shown to generate a quite abundant product ion even from co-oligomers enriched with DMS units. CONCLUSIONS: The high reactivity of hydride moieties, previously evidenced when these groups were at the end of PDMS chains, is also at work in PMHS, where each monomer contains a Si-H function. The presence of these hydride groups would increase the nucleophilic character of the oxygen atoms, favoring a tight bonding of lithium, and hence allowing in-chain cleavages to occur. In PMHS ammonium adducts, the particular reactivity of hydride moieties was illustrated by multiple hydride transfers but also by a dehydrogenation reaction systematically observed to proceed, together with the loss of ammonia, from all precursor ions. This latter reaction remained a very competitive process even from MHS/DMS co-oligomers with a low relative number of MHS units. Copyright © 2012 John Wiley & Sons, Ltd. The fragmentation chemistry of poly(dimethylsiloxane) (PDMS) ions had been reported to strongly depend on the nature of the end-groups, [1] but also on the cationizing agent. [2–4] When adducted with lithium, the unique dissociation reaction experi- enced by trimethylsilyl-terminated PDMS (CH 3 -PDMS) oligo- mers was reported by Mayer and co-workers to be the expulsion of one monomer, either as a neutral or as a lithiated molecule. [2] We observed the same reaction to occur from a,o-dihydride PDMS (H-PDMS) (as shown in Supplementary Fig. S1, Support- ing Information) (unpublished data). In contrast, collision- induced dissociation (CID) of a,o-dihydroxy-PDMS adducted with lithium was shown to give rise to two major product ion distributions after truncation of the chain, [1] while a,o-bis(3- carboxypropyl)- or a ,o-bis(3-aminopropyl)-PDMS dissociated primarily via intramolecular nucleophilic substitutions at the end-groups. [5] When ammonium was used to cationize PDMS oligomers, very different dissociation behavior was observed. As found by our group in the cases of large CH 3 -PDMS and H-PDMS, [3,4] the same tandem mass (MS/MS) spectra were produced regardless of the number of monomers in the precur- sor ion (Supplementary Fig. S2, Supporting Information). In both cases, the major product ion series could be explained by two main mechanisms. The first pathway consisted of the elimination of silanol species containing the o-termination once the precur- sor ion had lost ammonia. The second process was the release of a cyclic siloxane-containing species once the precursor ion had eliminated the neutral pair NH 3 /HX (X = CH 3 or H in the case of CH 3 -PDMS or H-PDMS, respectively). The latter process * Correspondence to: L. Charles, Aix-Marseille Université – CNRS, Institut de Chimie Radicalaire ICR, UMR 7273, F-13397 Marseille, France. E-mail: laurence.charles@univ-amu.fr Copyright © 2012 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2013, 27, 88–96 Research Article Received: 12 August 2012 Revised: 3 October 2012 Accepted: 3 October 2012 Published online in Wiley Online Library Rapid Commun. Mass Spectrom. 2013, 27, 88–96 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6432 88