Structural characterization of polymethoxymethylsiloxanes by electrospray ionization tandem mass spectrometry

Accepted Manuscript Title: Structural characterization of polymethoxymethylsiloxanes by electrospray ionization tandem mass spectrometry Author: Thierry Fouquet Fabio Ziarelli Hiroaki Sato Laurence Charles PII: DOI: Reference:

S1387-3806(16)30002-1 http://dx.doi.org/doi:10.1016/j.ijms.2016.03.006 MASPEC 15586

To appear in:

International Journal of Mass Spectrometry

Received date: Revised date: Accepted date:

14-12-2015 4-3-2016 10-3-2016

Please cite this article as: T. Fouquet, F. Ziarelli, H. Sato, L. Charles, Structural characterization of polymethoxymethylsiloxanes by electrospray ionization tandem mass spectrometry, International Journal of Mass Spectrometry (2016), http://dx.doi.org/10.1016/j.ijms.2016.03.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Structural characterization of polymethoxymethylsiloxanes by electrospray ionization tandem mass spectrometry

1

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Thierry Fouquet,1* Fabio Ziarelli2, Hiroaki Sato1 and Laurence Charles3 National Institute of Advanced Industrial Science and Technology (AIST)

Environmental Measurement Technology Group, Environmental Management Research Institute

2

Aix-Marseille Université – CNRS

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Tsukuba, Japan.

Marseille, France.

Aix-Marseille Université – CNRS

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3

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Fédération des Sciences Chimiques de Marseille FR 1739

Institut de Chimie Radicalaire UMR 7273 Marseille, France.

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*Correspondence to: [email protected] Running title

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Abstract

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CID of PMMS

Poly(methoxymethylsiloxane)s (PMMS) are silicon-based polymers usually prepared

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according to a poorly controlled synthesis route, yielding samples of high structural dispersity. Collision induced dissociation of PMMS was hence studied in details in order to propose MS/MS rules to be usefully employed for end-group characterization. Lithium adduction did not provide meaningful insights through charge remote dissociation while ammonium adduction allowed PMMS to readily dissociate according to two main information-rich charge driven routes. On one hand, after precursor ions have eliminated methanol together with ammonia, a series of 46 Da (CH2O+CH4) losses was observed, the number of which could be used to decipher the polymerization degree of the dissociating precursor. On the other hand, cleavage of Si–O bonds in the polymer backbone generated two series of product ions of low m/z values, each containing one or the other end-group found to be methoxy and tetramethyltetramethoxycyclotetrasiloxy terminations. Proposed mechanisms were all supported by accurate mass measurements and MS3 experiments. Based on these two highly complementary routes, the three main species of a PMMS sample were thoroughly characterized, allowing unique insights on the synthesis mechanism. Page 1 of 1 Page 1 of 23

1. Introduction Polymethoxymethylsiloxane (PMMS), also known as polymethylmethoxysiloxane (PMOS), designates a family of polysiloxane derivatives used as reactive backbones to produce, after further

functionalization

via

the

methoxy

group

[1],

cross-linkable

agents

for

polydimethylsiloxanes (PDMS) [2,3] or porous ceramic nano/microspheres [4]. Prior to

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operate any chemical modification of a polymeric substrate, a detailed description of its backbone and end-groups is mandatory. PMMS are usually characterized by Fourier transform infrared (FTIR) or nuclear magnetic resonance (NMR), both providing useful

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information but eventually failing at discriminating species in complex mixtures such as those

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expected during the preparation of PMMS (Scheme 1).

Scheme 1. One-step synthesis route of PMMS from tetramethylcyclotetrasiloxane and

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methanol in the presence of diethylamine.

Indeed, PMMS could be synthesized in a one-pot reaction by the dehydrocoupling of

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tetramethylcyclotetrasiloxane and methanol, followed by the ring-opening polymerization of the so-formed tetramethyltetramethoxycyclotetrasiloxane [5]. Because this synthesis route is

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not controlled, the degree of polymerization and the nature of the α and ω end-groups are not predictible. In addition, presence of methylhydrosiloxane units (MHS) due to incomplete dehydrocoupling or of residual cycles arising from incomplete ring-opening would increase the complexity of the PMMS sample. Although FTIR spectroscopy and NMR spectroscopy ensure a PMMS chain has been synthesized based on wavenumbers and chemical shifts [1-4], mass spectrometry (MS) allows individual chains to be mass measured within distinct distributions corresponding to each polymer present in a sample, hence providing an insight into the polymerization degree and the nature of the end-groups. Characterization of individual end-groups can then be performed by tandem mass spectrometry (MS/MS) to generate, from intact precursor ions, fragments containing either one or the other original chain termination [6]. Although increasingly used in the field of synthetic polymers [7] including silicon-based species [8-16], such MS-based approaches have never been reported for PMMS. Using electrospray ionization (ESI) to ensure molecule integrity upon their transfer as ions in the gas phase, the present study explores the dissociation mechanisms of Page 2 of 2 Page 2 of 23

small PMMS chains submitted to collision induced dissociation (CID), in order to establish fragmentation rules to be usefully applied to PMMS prior to their chemical modification.

2. Experimental 2.1 Chemicals

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Methanol was purchased from SDS (Peypin, France) while tetrahydrofuran (THF) was from Riëdel-de Haen (Seelze, Germany). Ammonium acetate, lithium fluoride and diethylamine were from Sigma Aldrich (St. Louis, MO). Tetramethylcyclotetrasiloxane was purchased from

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ABCR (Karlsruhe, Germany). All chemicals were used as received without further purification. The synthesis of PMMS was carried out based on a procedure described

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elsewhere [5]. In a one pot reaction, tetramethylcyclotetrasiloxane (2 g, ~ 8 mmol) was mixed with methanol (1.1 g, ~ 33 mmol) and diethylamine (2.4 g, ~ 33 mmol) in hexane (30 mL) at

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room temperature and let to react for 24h (Scheme 1). The solution has been washed with water and a colorless liquid of low viscosity was recovered after the evaporation of hexane.

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2.2 NMR spectroscopy

About 15 µL of the synthesized PMMS were placed in a 4 mm HRMAS zirconium dioxide

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rotor (equipped with Teflon insert to seal the samples in the active volume of the probe) for NMR analysis (Bruker Avance III 400 MHz WB Solid State spectrometer). Quantitative

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Single Pulse Magic Angle Spinning (SPMAS) NMR spectra were obtained at the

29

Si

resonance frequency of 79.5 MHz. Typical acquisition parameters included 4.5 µs 90° pulse,

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225 s recycle delays (equal to five times the longer relaxation time), 512 scans and spin rate of 7 kHz. Experiments were performed at ambient temperature. Chemical shifts are referenced to tetramethylsilane whose resonance was set to 0 ppm. Line fitting of the NMR spectra was performed using Gaussian peaks with DMFIT software package [17].

2.3 Mass Spectrometry

High resolution MS and MS/MS experiments were performed using a QStar Elite mass spectrometer (Applied Biosystems SCIEX, Concord, ON, Canada), equipped with a quadrupole and an orthogonal acceleration time-of-flight (oa-TOF) mass analyzers and an electrospray ionization source operated in positive ion mode (further noted ESI(+)). The capillary voltage was set at +5500 V and the cone voltage at +50 V. Ions were measured using the oa-TOF mass analyzer in both MS and MS/MS modes. A quadrupole was used for selection of precursor ions to be further submitted to collision-induced dissociation (CID) in MS/MS experiments. In MS, accurate mass measurements were performed using two Page 3 of 3 Page 3 of 23

reference ions from a poly(ethylene glycol) internal standard, according to a procedure described elsewhere [18]. The precursor ion was used as the reference for accurate measurements of product ions in MS/MS spectra. MS3 experiments were performed using a 3200 QTrap mass spectrometer (Applied Biosystems SCIEX), equipped with a quadrupole and a linear ion trap mass analyzers, and an electrospray ionization source operated in the

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positive ion mode (capillary voltage: +5500 V, cone voltage: +50 V). Primary precursor ions generated in the ion source were selected in the quadrupole analyzer and submitted to CID in a collision cell. Secondary precursor ions produced during collisions were selected and then

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fragmented in a linear ion trap. In both instruments, air was used as the nebulizing gas (10 psi) while nitrogen was used as the curtain gas (20 psi) as well as the collision gas. Collision

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energy was set according to the experiments and are reported below in the center of mass frame. Instrument control, data acquisition and data processing of all experiments were

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achieved using Analyst software (QS 2.0 and 1.4.1 for the QTOF and the QTrap instruments, respectively) provided by Applied Biosystems. The PMMS polymer was first dissolved in THF and further diluted using a methanolic solution of either lithium fluoride (1 mM) or

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ammonium acetate (3 mM) to a final 10 µg.mL-1 concentration. Sample solutions were

3.1 ESI-MS of PMMS

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3. Results and Discussion

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introduced in the ionization source at a 5 µL.min-1 flow rate using a syringe pump.

While PMMS was readily ionized using different ion sources (either ESI or matrix assisted

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laser desorption ionization MALDI) and cationizing agents (NH4+, Li+ and Na+), useful mass data to inventory polymeric species were obtained by ESI-MS using lithium salt [9]. As shown in Fig. 1A, two main distributions of singly charged molecules spaced by ∆m/z = 90.0 (the mass of the C2H6O2Si monomer unit) were detected. This first result indicated that this PMMS sample did not contain any segment of residual MHS (CH4OSi) units or cyclic moieties (C8H24O8Si4) expected to induce ∆m/z = 60.0 or 360.1 peak spacing, respectively.

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Fig. 1. Positive ion mode ESI mass spectra of PMMS in (A) a methanolic solution of lithium fluoride and (B) a methanolic solution of ammonium acetate. (C) Structure proposed for detected PMMS species based on their mass. Adducts of Ian (linear) or Ibn (branched, isomer of Ian) are designated with red squares, IIn are annotated with black circles and IIIn with blue stars (striped: singly charged lithium adducts; filled: singly charged ammonium adducts; empty: doubly charged ammonium adducts).

Based on calculation and accurate mass measurements (Table S1), peaks annotated with striped red squares in Fig. 1A were assigned to linear PMMS with methyl and methoxy endgroups (further designated as Ian, Fig. 1C), with a polymerization degree ranging from n=2 Page 5 of 5 Page 5 of 23

(m/z 233.1) up to n=14 (m/z 1313.2). Alternatively, these ions could be lithium adducts of branched PMMS chains (named Ibn), that is, isomers of linear PMMS chains which hence cannot be differentiated based on their mass. Lithiated oligomers designated by striped black circles in the second PMMS distribution were measured at m/z – 46.0 compared to ions in the I series, and the first member was detected at m/z 367.1: this suggested that they hold a cyclic

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ω end-group as shown by structure II in Fig. 1C. In terms of elemental composition, such a termination (C7H21O8Si4) is indeed equivalent to a four monomer segment (C8H24O8Si4) lacking a methyl group, yielding a C2H6O difference (46.0 Da) between I and II. Accordingly,

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the m/z 367.1 ion would contain no linear skeleton (that is, n=0), and these IIn oligomers were detected up to n=8 (m/z 1087.2). Presence of such species in the PMMS sample could be

the

intermediate

reacting

species

formed

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explained by the condensation of a pristine tetramethyltetramethoxycyclotetrasiloxane (i.e. upon

the

dehydrocoupling

of

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tetramethylcyclotetrasiloxane and methanol) on a ring-opened congener. Keeping in mind such a – 46.0 Da shift, a third distribution was revealed by very low abundance ions (annotated with blue stars in the inset of Fig. 1A) lacking C2H6O compared to species II.

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These IIIn oligomers were observed from n=0 (m/z 681.1) to n=5 (m/z 1131.2). Although the same species could also be observed as ammonium adducts (noted with filled symbols in Fig.

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1B), more complex mass spectra were obtained from PMMS solution supplemented with ammonium salt due to extensive and unavoidable in-source dissociation regardless of the

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ionization parameters (charge directed dissociation readily activated upon the release of ammonia and methanol and further triggering the depolymerization of the oligomer, Table S2

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for the accurate mass measurements). Such features were also previously observed for PDMS carrying basic groups such as hydride, hydroxyl, methoxy and ethoxy [15]. The use of ammonium salt also promotes the formation of multiply charged ions as shown in inset of Fig. 1B with the detection of doubly charged ammonium adducts of Ian/Ibn (empty red squares), IIn (emptry black circles) and IIIn (empty blue stars). It is worth mentioning the 1 Da shifted m/z values between [III + NH4]+, [II + 2NH4]2+ and [I + NH4]+, as well as the same m/z values measured for [IIn + NH4]+ and [II2n+4 + 2NH4]2+ (inset in Fig. 1B). This will lead to interferences in the MS/MS spectra of [I + NH4]+ species with the (limited) detection of product ions from the doubly charged adducts (noted with “x” in Fig. 2B) and the impossibility to record satisfactory MS/MS spectra for [II + 2NH4]2+. The structural assumptions proposed from the ESI-MS spectra were consistent with 29Si SPMAS NMR data obtained for this PMMS sample (Fig. S1), showing the presence of linear and/or branched oligomers (In), cyclolinear molecules (IIn) and dicyclolinear species (IIIn), as named according to the nomenclature used for PDMS [12,19]. Page 6 of 6 Page 6 of 23

3.2 CID of linear PMMS While the higher stability of lithiated PMMS compared to ammonium adducts was a clear advantage in the MS mode, it became an issue in the MS/MS mode where high collision energies were required to induce the formation of product ions with low signal to noise ratio,

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as previously reported for the case of polymethylhydrosiloxanes (PMHS) [16]. A 30 minute acquisition was needed to record the ESI-MS/MS of [I12 + Li]+ depicted in Fig. 2A (recorded at a 2.17 eV collision energy using the high resolution QTOF instrument) which displays a

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main product ion series covering the whole m/z range. According to the accurate mass measurements (Table S3) it corresponds to Li+-cationized cyclic oligomers with no end-

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groups, noted cn according to the nomenclature proposed by Wesdemiotis et al [6]. The complementary linear product ions (i.e. Li+-cationized truncated chains carrying the two end-

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groups) are hardly detected (noted Ln with capital letter to avoid any confusion with the In distribution of PMMS) and echoe the fragmentation chemistry of a dihydroxy-terminated PDMS adducted with lithium [9]. A third product ion series is detected at m/z – 46.0 Da

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compared to the cn congeners (noted with asterisks), suggesting that these fragments carry an additional cyclic moiety (see previous section). The structures of the product ion series are

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depicted in the inset of Fig. 2A. Such a CID behavior – namely charge remote intramolecular Si-O bond translocations yielding linear and cyclic product ions – brings no information

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regarding the end-groups of the oligomers since the fragment ions possess both or none of the pristine end-groups, requiring another cationizing agent to be used to gain insight into the

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microstructure of the PMMS chains.

The low energy activation of PMMS ammonium (0.75 eV in the center of mass frame) adducts gave rise to series of abundant product ions as shown in Fig. 2B for [I12 + NH4]+ (although oligomers I can be either linear or branched PMMS, they will first be considered as linear species for the sake of simplicity). Detection of product ions in the upper m/z range of the MS/MS spectrum was in great contrast with CID data reported for PDMS where only low m/z fragments were observed due to primary product ions experiencing extensive consecutive dissociation [10-15]. This first result confirmed that the nature of the polysiloxane repeating unit strongly influenced CID routes, as previously shown when comparing dissociation of PDMS with PMHS [16].

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Fig. 2. a) [I12+Li]+ at m/z 1133.2 recorded at a 2.17 collision energy (QTOF device), b1) [I12+NH4]+ at m/z

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1144.2 (QTOF device) and b2) [I3+NH4]+ at m/z 334.2 (QTrap device) recorded at a 0.75 collision energy. Product ions generated after one to four 46.0 Da losses from any a) cn and Ln congeners or b1) bn+

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congeners are designated with the corresponding number of asterisks. In b1) and b2), the major ion series generated by the combined ammonia / methanol loss from the precursor ion (17.0 + 32.0 = 49.1 Da) and the iterative methane / formaldehyde expulsions (30.0 + 16.0 = 46.0 Da) are indicated by a filled red triangle (▲). Product ions annotated with x arise from the

13

C2 isotopes of [III5+NH4]+ at m/z 1144.17 and

[II21+2NH4]2+ at m/z 1144.21 interfering with the main [I12+NH4]+ at m/z 1144.24 (See Supplementary Data). Accurate mass measurements and associated assignments are listed in Tables S3 and S4.

In Fig. 2B, the major ion series (designated by red triangles) comprised seven members spaced by 46.0 m/z units, with relative abundances strongly suggesting consecutive dissociations. The largest member of this series, detected at m/z 1095.1, was formed upon a 49.1 Da loss from the m/z 1144.2 precursor ion. This first reaction would consist of a combined loss of ammonia and methanol, as supported by both accurate mass measurements (Table S4) and MS/MS data recorded for very small ammoniated congeners (using the low resolution QTrap device) for which sequential elimination of NH3 and CH3OH could be Page 8 of 8 Page 8 of 23

evidenced (Fig. 2C). The m/z 1095.1 product ion would then eliminate methane together with formaldehyde, a combined loss again clearly shown to occur sequentially in the case of a small precursor ion (Fig. 2C). The mechanism proposed for this reaction is illustrated with the example of I6 in Scheme 2. The primary product ion formed after the precursor ion has released NH3+CH3OH has a positively charged silicon atom, which would experience a

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nucleophilic attack from a methoxy oxygen atom to lead to a six-member ring with a positively charged oxygen atom holding a methyl group. A hydride transfer from a neighboring methoxy moiety to this methyl group in a 6+2 mechanism would lead to the

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release of methane and formaldehyde. This 46.0 Da loss would further repeat until conducting to a highly cross-linked structure containing only one methoxy group. A similar iterative

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formation of (SiO)x cycles was also observed during CID of polyhedral oligomeric silsesquioxanes (POSS) adducted with ammonium [20]. The proposed iterative process, m/z

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1095.2 → m/z 1049.1 → m/z 1003.1 → m/z 957.1 → m/z 911.0 → m/z 865.0 → m/z 818.9, was supported by MS3 experiments (Fig. S2). Based on a similar mechanism (Scheme S1), some of these ions would alternatively eliminate dimethoxymethylsilane (instead of

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formaldehyde) together with methane, accounting for the 136.1 Da loss observed from m/z

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911.0, m/z 865.0, and m/z 818.9 to yield m/z 775.0, m/z 728.9, and m/z 682.8, respectively.

Scheme 2. Mechanism proposed for the combined loss of methane and formaldehyde occurring after the ammonium adduct of a linear PMMS (here [I6 + NH4]+) has eliminated methanol and ammonia.

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Although the preferential –46.0 Da loss iterative process did not provide relevant information regarding the nature of end-groups in linear PMMS, it could be usefully employed to decipher the polymerization degree of the dissociating precursor from the number of members in this ions series. Indeed, any n-mer of linear PMMS contained a total of n+2 methoxy moieties, one in each monomer and one at each chain end. One of these CH3O groups is eliminated

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during the primary NH3/MeOH loss from the precursor ion, hence leading to a product ion containing n+1 CH3O groups. According to the mechanism shown in Scheme 2, release of one CH4/CH2O pair (46.0 Da) involved two methoxy groups: as a result, the [In + NH4 – NH3 –

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MeOH]+ product ion is expected to experience at most (n+1)/2 CH4/CH2O losses. This approach is illustrated in Fig. 3 with MS/MS data obtained for four successive linear PMMS,

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with n ranging from 9 to 12. The 9-mer, which ammonium adduct is detected at m/z 874.2, contained a total of eleven methoxy moieties: the primary fragment formed after the release of

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NH3/MeOH hence possessed ten CH3O groups, allowing up to five CH4/CH2O losses to proceed as observed in Fig. 3A. The same number of 46.0 Da losses was measured starting from the 10-mer (m/z 964.2), because the eleven CH3O groups in the primary fragment at m/z

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915.1 also allowed up to five CH4/CH2O pairs to be released (Fig. 3B). Similarly, a total of six steps were observed in the 46.0 Da loss process from both [I11 + NH4]+ (Fig. 3C) and [I12 +

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NH4]+ (Fig. 3D). In other words, when considering an odd n value, n-mer and (n+1)-mer exhibit the same number of members in this ion series (Fig. 3E, black line). However, such n-

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and (n+1)-mers can still be distinguished by considering relative abundance of the last and the last but one members in this series: when considering an odd n value, this ratio was

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systematically higher for precursor ions containing n+1 MMS units compared to n. As shown in Fig. 3E (red bars), this ratio was 5% and 40% for the 9-mer and 10-mer, respectively, or 15% and 78% for the 11-mer and 12-mer, respectively. This result can be explained by a better statistics for the last CH4/CH2O pair to be eliminated from a product ion containing three methoxy groups compared to two. This trend was systematically observed for linear PMMS, as illustrated in Fig. 3E for n ranging from 5 to 14. This MS/MS approach to determine the polymerization degree of precursor ions can be employed for any synthetic polymers which main dissociation pathway involved pendant groups, as previously reported for poly(methacrylic acid) [21].

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Fig. 3. Truncated ESI-MS/MS spectra (0.75 eV, QTOF device) of linear PMMS adducted with ammonium,

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[In+NH4]+, with a) n=9, b) n=10, c) n=11, and d) n=12, showing the ion series generated by consecutive losses of methane/formaldehyde (46.0 Da) designated by red triangles (▲). e) Number of 46.0 Da loss steps (black line) and intensity ratio calculated for peaks associated to the last and the last but one 46.0 Da

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loss (red bars) as a function of the polymerization degree of the dissociating precursor.

In the case of PMMS, however, an additional product ion series of weak abundance in the low

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m/z range of the CID spectrum was found to provide information about each end-group as it

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was formed after cleavage of Si-O bonds in the oligomer backbone. These ions were detected at m/z 195.1, 285.1, 375.1, and 465.1 in Fig. 2A, and hence contained only one of the two

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original end-groups. Owing to the symmetry of the dissociating linear PMMS, so-formed bi+ and zi+ fragments were equivalent (and further named bi+ according to the product ion nomenclature proposed by Wesdemiotis et al. [6]). Cleavage of Si-O bonds would lead to product ions containing one end-group and a positive charge located on the terminal silicon atom (Scheme S2). Only small size bi+ ions (with i=2-5 in Fig. 2B) were observed in MS/MS because of an efficient depolymerization of the largest bi+ (Scheme S2), as previously shown for other siloxane-based oligomers [10-15]. These bi+ ions were also observed to further eliminate 46.0 Da neutrals, the number of which is indicated by a corresponding number of asterisks to designate so-formed product ions in Fig. 2B. These secondary reactions were confirmed by MS3 experiments (Fig. S3 in the Supplementary Data). Nevertheless, the study of these few bi+ product ions, expected at m/z = 90 i + mα, is sufficient to characterize the mass of each end-group in any [In + NH4]+. For example, considering the MS/MS spectrum of Fig. 2B, b2+ (m/z 195.1), b3+ (m/z 285.1), b4+ (m/z 375.1), and b5+ (m/z 465.1) conducted to mα = 15.0 Da in the m/z 1144.2 precursor ion. The lack of distinct signals for zi+ suggested a Page 11 of 11 Page 11 of 23

symmetrical structure of the dissociating In: the ω termination should then be OCH3 (m=31.0 Da). Alternatively, as the polymerization degree of the m/z 1144.2 precursor ion is known to be n=12 from the iterative -46.0 Da process, the mass of the oligomer equal to 1126.2 Da and mα = 15.0 Da, a simple calculation led to mω=31.0 Da. Taking advantage of the formation of doubly charged species when using ammonium (Fig. 1B), influence of the precursor ion

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charge state on dissociation routes was studied. As shown in Fig. S4 with the example of [I18 + 2NH4]2+ at m/z 851.2 (Table S5 for the accurate mass measurements), the same

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fragmentation pattern previously established for singly charged species was observed. In particular, nine consecutive losses of methane/formaldehyde were clearly observed to

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proceed, consistent with n=18 in this oligomer and ensuring the evaluation of the DP based on the number of methane/formaldehyde losses to be valid for long PMMS chains adducted with

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two ammonium cations. 3.3 CID of cyclolinear and dicylolinear PMMS

Upon collision activation, ammonium adducts of both cyclolinear (IIn) and dicyclolinear

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(IIIn) PMMS were observed to obey the dissociation rules previously established for CH3(MMS)n-OCH3 (In) species. This result implied that cyclic moieties did not induce new

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dissociation reactions, consistent with their location at chain ends. For example, the MS/MS spectrum in Fig. 4A shows that the m/z 828.2 cyclolinear precursor has released NH3/CH3OH

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to generate the m/z 779.1 product ion, which further experienced four successive CH4/CH2O losses. As any IIn cyclolinear species contains a total of n+4 methoxy groups, amongst which

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n+3 can be eliminated in (n+3)/2 steps of 46.0 Da loss, data from Fig. 4A indicated n=5 or 6 in the dissociating oligomer (Fig. S5). In the low m/z range of this CID spectrum, two ion series with peaks spaced by 90.0 Da were observed and assigned to bi+ (m/z = 90 i + mα) and zi+ (m/z = 90 (i-1) + 74 + mω), respectively. The first member of each series allowed each chain termination to be mass characterized: b3+ at m/z 285.1 indicated mα=15.0 Da (b2+ was of too low abundance for accurate mass measurement) while z1+ at m/z 419.0 indicated mω=345.1 Da.

These results were consistent with a precursor of mass 810.2 Da and

containing n=5 MMS units (Table S6 in the Supplementary Data for accurate mass measurements).

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Fig. 4. ESI-MS/MS spectra of singly charged 5-mers a) cyclolinear PMMS [II5+NH4]+ at m/z 828.2, and b)

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dicyclolinear PMMS [III5+NH4]+ at m/z 1142.2, both recorded at a 0.75 eV collision energy (QTOF

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device). Structures of cyclolinear and dicyclolinear are shown as insets together with the product ion nomenclature. Peaks designated by red triangles correspond to product ions generated during the iterative

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methane/formaldehyde elimination process.

Similarly, the ammoniated dicyclolinear precursor at m/z 1142.2 experienced a first 49.1 Da loss followed by successive elimination of five CH4/CH3OH pairs (Fig. 4B), consistent with its n=5 (Fig. S6). In a competitive pathway, Si-O bond cleavages in the backbone of this symmetric oligomer led to only one short ion series (bi+, m/z = 90 i + mα) in the low m/z range of the CID spectrum, composed of m/z 329.0 (assigned to α+), m/z 419.1 (b1+) and m/z 509.1 (b2+), and indicating mα=329.0 Da. Accordingly, as the mass of the dissociating 5-mer was 1124.1 Da, mω was found to be 345.1 Da (Table S7 in the Supplementary Data for the accurate mass measurements).

4. Conclusion The tandem mass spectrometry behavior of ammoniated polymethoxymethylsiloxanes was found to differ from the routes proposed for polydimethylsiloxanes, with the occurrence of product ions of high m/z values formed upon the iterative releases of methane and Page 13 of 13 Page 13 of 23

formaldehyde. Due to their polycyclic structure, these internal fragments did not fully dissociate, in contrast to b+/z+ ions generated upon cleavage of the polymer backbone, and were detected with high abundance. As a result, they could be confidently employed to count the total number of methane/formaldehyde (46.0 Da) losses in MS/MS spectra, which was found to be related to the number of repeating unit within the oligomer submitted to CID.

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Although of limited number and abundance, b+/z+ ions in the low m/z range of MS/MS spectra provided information about the nature of the end-groups. Using these simple rules, polymerization degree and end-groups hold by species present in the studied PMMS samples

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could be characterized, providing meaningful insights into the synthesis route of PMMS. However, due to the lack of authentic standards, linear structures could not be distinguished

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from branched PMMS.

Acknowledgments

T. Fouquet and L. Charles acknowledge support from Spectropole (the analytical facility of

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Aix-Marseille University) for a privileged access to the instruments purchased with European funding (FEDER OBJ2142-3341) and the Fonds National de la Recherche (FNR, the

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Luxembourg Research Funding Agency) for the former financial support. T. Fouquet and H. Sato gratefully acknowledge the ongoing financial support by the Japan Society for the

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Promotion of Science (JSPS), an independent administrative institution in Japan contributing to the advancement of science in all fields of the natural and social sciences and the

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humanities.

Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version.

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• CID of ammoniated polymethoxymethylsiloxane (PMMS) was studied to propose MS/MS rules;

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• In the high mass range, PMMS eliminated CH2O/CH4 neutrals in an iterative manner; • In the low mass range, cleavage of Si–O bond generated product ions containing one end-group;

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The end-group characterization allowed unique insights into the synthesis route.

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• The number of CH2O/CH4 losses provided the polymerization degree of the precursor;

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