Role of the Adducted Cation in the Release of Nitroxide End Group of Controlled Polymer in Mass Spectrometry

Automatic peak assignment and visualisation of copolymer mass spectrometry data using the 'genetic algorithm'

Copolymer analysis is vitally important as the materials have a wide variety of applications due to their tunable properties. Processing mass spectrometry data for copolymer samples can be very complex due to the increase in the number of species when the polymer chains are formed by two or more monomeric units. In this paper, we describe the use of the genetic algorithm for automated peak assignment of copolymers synthesised by a variety of polymerisation methods. We find that in using this method we are able to easily assign copolymer spectra in a few minutes and visualise them into heat maps. These heat maps allow us to look qualitatively at the distribution of the chains, by showing how they alter with different polymerisation techniques, and by changing the initial copolymer composition. This methodology is simple to use and requires little user input, which makes it well suited for use by less expert users. The data outputted by the automatic assignment may also allow for more complex data processing in the future.

New mass spectrometry concepts for characterization of synthetic polymers and additives

RATIONALE

New ionization processes have been developed for biological mass spectrometry (MS) in which the matrix lifts the nonvolatile analyte into the gas phase as ions without any additional energy input. We rationalized that additional fundamental knowledge is needed to assess analytical utility for the field of synthetic polymers and additives.

METHODS

Different mass spectrometers (Thermo Orbitrap (Q-)Exactive (Focus); Waters SYNAPT G2(S)) were employed. The formation of multiply charged polymer ions upon exposure of the matrix/analyte(/salt) sample to sub-atmospheric pressure directly from the solid state and surfaces facilitates the use of advanced mass spectrometers for detection of polymeric materials including consumer products (e.g., gum).

RESULTS

Astonishingly, using nothing more than a small molecule matrix compound (e.g., 2-methyl-2-nitropropane-1,3-diol or 3-nitrobenzonitrile) and a salt (e.g., mono- or divalent cation(s)), such samples upon exposure to sub-atmospheric pressure transfer nonvolatile polymers and nonvolatile salts into the gas phase as multiply charged ions. These successes contradict the conventional understanding of ionization in MS, because can nonvolatile polymers be lifted in the gas phase as ions not only by as little as a volatile matrix but also by the salt required for ionizing the analyte through noncovalent metal cation adduction(s). Prototype vacuum matrix-assisted ionization (vMAI) and automated sources using a contactless approach are demonstrated for direct analyses of synthetic polymers and plasticizers, minimizing the risk of contamination using direct sample introduction into the mass spectrometer vacuum.

CONCLUSIONS

Direct ionization methods from surfaces without the need of high voltage, a laser, or even applied heat are demonstrated for characterization of detailed materials using (ultra)high-resolution and accurate mass measurements enabled by the multiply charged ions extending the mass range of high-performance mass spectrometers and use of a split probe sample introduction device. Our vision is that, with further development of fundamentals and dedicated sources, both spatial- and temporal-resolution measurements are within reach if sensitivity is addressed for decreasing sample-size measurements.

Smart Control of Nitroxide-Mediated Polymerization Initiators' Reactivity by pH, Complexation with Metals, and Chemical Transformations

Because alkoxyamines are employed in a number of important applications, such as nitroxide-mediated polymerization, radical chemistry, redox chemistry, and catalysis, research into their reactivity is especially important. Typically, the rate of alkoxyamine homolysis is strongly dependent on temperature. Nonetheless, thermal regulation of such reactions is not always optimal. This review describes various ways to reversibly change the rate of C–ON bond homolysis of alkoxyamines at constant temperature. The major methods influencing C–ON bond homolysis without alteration of temperature are protonation of functional groups in an alkoxyamine, formation of metal–alkoxyamine complexes, and chemical transformation of alkoxyamines. Depending on the structure of an alkoxyamine, these approaches can have a significant effect on the homolysis rate constant, by a factor of up to 30, and can shorten the half-lifetime from days to seconds. These methods open new prospects for the application of alkoxyamines in biology and increase the safety of (and control over) the nitroxide-mediated polymerization method.

Adduction of ammonium to polylactides to modify their dissociation behavior in collision-induced dissociation

RATIONALE

The goal of this work was to modify the dissociation pathways of polylactide (PLA) holding benzyl and hydroxyl terminations, in order to circumvent coincidence of product ions generated during collisional activation of sodiated chains, which prevented their reliable characterization.

METHODS

Benzyl-, hydroxyl-terminated PLAs were ionized as ammonium adducts in positive ion mode electrospray and subjected to collision-induced dissociation (CID). Tandem mass spectrometry (MS/MS) experiments were conducted in a quadrupole time-of-flight (QTOF) instrument for safe assignment of product ions based on their elemental composition derived from accurate mass measurements.

RESULTS

Adduction of ammonium to PLAs was found to induce chain fragmentation via charge-assisted processes, in great contrast to the charge-remote mechanisms experienced by sodiated molecules. The main reaction produced ions containing the ω termination only, hence allowing straightforward end-group determination. Other minor pathways were studied in detail to establish dissociation rules for ammoniated PLAs. Some reactions were found to be end-group specific, highlighting the higher reactivity of ammonium than alkali ion adducts.

CONCLUSIONS

Changing the usually employed sodium-cationizing agent to ammonium was shown to induce dramatic changes in the CID behavior of PLAs. This was a simple and efficient approach to address issues encountered for end-group analysis of the particular PLA studied here.

C-ON Bond Homolysis of Alkoxyamines, Part 11: Activation of the Nitroxyl Fragment

A few years ago, Bagryanskaya and colleagues (J. Org. Chem. 2011) showed that protonation of the nitroxyl fragment deactivated the alkoxyamine C-ON bond. Conversely, our group showed that protonation (Chem. Commun. 2011), as well as other chemical reactions such as oxidation or amine quaternization (Org. Lett. 2012), of the pyridyl moiety carried by the alkyl fragment was suitable to activate the homolysis of the C-ON bond. To pursue our goal of applying alkoxyamines as theranostic agents (Org. Biomol. Chem. 2014 and Mol. Pharmaceutics 2014) by activation of the C-ON bond homolysis, we turned our interest to the chemical activation of the nitroxyl fragment by oxidation/reduction of selected functions. Conversion of a hydroxyl group located close to the nitroxyl moiety successively into aldehyde, then acid, and eventually into ester, led to a successive decrease in kd.

Tandem mass spectrometry sequencing in the negative ion mode to read binary information encoded in sequence-defined poly(alkoxyamine amide)s

RATIONALE

Digitally encoded oligomers composed of two distinct amide coding units spaced by a nitroxide moiety were recently decrypted using a tandem mass spectrometry (MS/MS) sequencing approach developed for protonated oligomers. Here, the MS/MS behavior of deprotonated oligomers was explored in the negative ion mode to provide both structural and mechanistic complementary information.

METHODS

Binary-encoded oligo(alkoxyamine)amides, containing coding 0/1 amide units spaced by a TEMPO nitroxide moiety, were ionized in negative ion mode electrospray thanks to their α end-group containing a carboxylic acid function. Deprotonated molecules were subjected to collision-induced dissociation in MS/MS and MS(3) experiments, combined with accurate mass measurements, for a thorough investigation of their dissociation behavior.

RESULTS

Deprotonated oligomers readily dissociated upon collisional activation via competitive homolytic cleavages of all fragile alkoxyamine linkages between any coding 0 or 1 monomers and a nitroxide moiety. As expected, only product ions holding the deprotonated α end-group were detected while complementary moieties containing the ω termination were released as radicals. The so-formed distonic radical anions were observed to further depolymerize according to a radical-induced process, as evidenced by MS(3) experiments.

CONCLUSIONS

Messages encoded in oligo(alkoxyamine)amides were readily decrypted by MS/MS sequencing performed in the negative ion mode. When compared with results obtained in positive ion mode ESI-MS/MS, these data provided further evidence regarding the influence of adducted proton on the charge-remote homolytic cleavage of alkoxyamine linkages.

Elucidation of a side reaction occurring during nitroxide-mediated polymerization of cyclic ketene acetals by tandem mass spectrometric end-group analysis of aliphatic polyesters

RATIONALE

In order to prevent side reactions while developing new polymerization processes, their mechanism has to be understood and one first key insight is the structure of the end-groups in polymeric by-products. The synthetic method scrutinized here is the nitroxide-mediated polymerization (NMP) of a cyclic ketene acetal, a promising alternative process to the production of polyesters.

METHODS

Polymer end-group characterization was performed by mass spectrometry (MS), combining elemental composition information derived from accurate mass data in the MS mode with fragmentation features recorded in the MS/MS mode. Electrospray was used as the ionization method to ensure the integrity of original chain terminations and a quadrupole time-of-flight (QTOF) instrument was employed for high-resolution mass measurements in both MS and tandem mass spectrometry (MS/MS) modes.

RESULTS

Occurrence of side reactions in the studied polymerization method, first evidenced by an unusual increase in dispersity with conversion, was confirmed in MS with the detection of two polymeric impurities in addition to the expected species. Fragmentation rules were first established for this new polyester family in order to derive useful structural information from MS/MS data. In addition to a usual NMP by-product, the initiating group of the second polymeric impurities revealed the degradation of the nitroxide moiety.

CONCLUSIONS

Unambiguous MS/MS identification of end-groups in by-products sampled from the polymerization medium allowed an unusual side reaction to be identified during the NMP preparation of polyesters. On-going optimization of the polymerization method aims at preventing this undesired process.

Labile alkoxyamines: past, present, and future

Alkoxyamines--per-alkylated derivatives of hydroxylamine R(1)R(2)NO-R(3)--can undergo C-ON bond homolysis to release a persistent nitroxyl radical R(1)R(2)NO˙ and a transient alkyl radical R(3)˙. Although they were considered as an oddity when discovered in 1974, their properties have been extensively studied since the seminal work of Solomon, Rizzardo and Cacioli (Chem. Abstr., 102, 221335q), who patented the key role of alkoxyamines in nitroxide-mediated polymerization (NMP) in 1985. This feature article surveys and assesses the various applications of alkoxyamines: in tin-free radical chemistry, e.g., for the elaboration of carbo- or hetero-cycles, for the development of new reactions, for total synthesis of natural products; in polymerization under thermal conditions (NMP) or photochemical conditions (nitroxide-mediated photo-polymerization, NMP2); and in the design of smart materials. In this feature article, we also describe our recent findings concerning the chemical triggering of the C-ON bond homolysis in alkoxyamines, affording the controlled generation of alkyl radicals at room temperature. Based on these results, we describe herein some new opportunities for applications in the field of smart materials, and of course, some possible developments as new initiators for NMP as well as an entirely new field of application: the use of alkoxyamines as theranostic agents. Indeed, each of the radicals released after homolysis can play an appealing role: the nitroxide, through dynamic nuclear polarization (DNP), can be used for imagery purposes (diagnostic properties), while the alkyl radical can be used to induce cellular disorders in abnormal cells (therapeutic activity).

MALDI of synthetic polymers with labile end-groups

Mass spectrometry is increasingly used in the field of synthetic polymers as a fast and accurate technique for end-group analysis. More particularly, matrix-assisted laser desorption/ionization (MALDI) has gained much popularity because it allows quite simple mass spectra to be obtained, displaying a single distribution for each polymeric species present in the sample, in contrast to electrospray ionization (ESI) which readily promotes multiple charging for most polymers. A soft ionization process, ensuring the integrity of the species upon transfer into gas phase ions, is however mandatory for polymer end-group analysis since information about the chain terminations mainly rely on the m/z values measured for polymer adducts. As compared to ESI, MALDI is sometimes suspected to be a quite "hard" ionization technique, leading to spontaneous dissociation of ionized species either in the source or during their flight time. This issue is of particular concern for polymers carrying so-called fragile end-groups arising from their mode of synthesis. In particular, controlled radical polymerization (CRP) processes, one of the most important advances in the field of polymer science during the last 20 years, allow the production of polymers with well-defined molecular distribution and low polydispersities, but they are all based on the low dissociation energy of the chemical bond between the last monomer and the terminating group. As a result, if macromolecules are activated while being ionized, this end-group is prone to fragmentation and ions measured in the mass spectra do no longer reflect the original chain composition. However, different results are reported in the literature about the ability of MALDI to generate intact ions from CRP synthetic polymers. This article reviews MALDI MS data reported for synthetic polymers produced by atom transfer radical polymerization (ATRP), reversible addition-fragmentation transfer polymerization (RAFT), and nitroxide-mediated polymerization (NMP), the three most studied CRP techniques. The general principle of each polymerization process, which defines the structure of the end-groups in both targeted macromolecules and species arising from eventual side-reactions, is first briefly presented. An overview of MALDI data reported for samples obtained upon polymerization of different monomers are then commented for each polymerization techniques with regards to the success of the ionization method to generate intact cationic adducts and its propensity to distinguish in-source fragments from polymerization side-products.

End-group cleavage in MALDI of ATRP-made polystyrene: a silver-catalyzed reaction during sample preparation

Cleavage of the labile halide termination upon matrix-assisted laser desorption/ionization (MALDI) has always been reported as a major concern in mass analysis of polystyrene prepared by atom transfer radical polymerization (ATRP). By studying this issue using nuclear magnetic resonance (NMR) and electrospray ionization-mass spectrometry, we evidence here that the ionization step is not involved in this deleterious process. Instead, removal of the halogen was shown to readily occur upon interaction of the silver salt (AgTFA) used as the cationizing agent in mass spectrometry, either in solution or in the solid-state when performing solvent-free sample preparation. In solution, this silver-induced reaction mostly consists of a nucleophilic substitution, leading to polystyrene molecules holding different terminations, depending on relative nucleophilicity of species present in the liquid-phase solution composition. In chloroform supplemented with AgTFA, trifluoroacetate-terminated PS were evidenced in ESI-MS spectra but experienced end-group cleavage in MALDI. In contrast, the major methoxy-terminated PS macromolecules formed when the silver-catalyzed nucleophilic substitution was performed in methanol were generated as intact gas-phase ions using both ionization techniques. This controlled and fast modification could hence be advantageously used as a rapid sample pretreatment for safe MALDI mass analysis of ATRP-made polystyrene.

Structural identification of hindered amine light stabilisers in coil coatings using electrospray ionisation tandem mass spectrometry

Hindered amine light stabilisers (HALS) are the most effective antioxidants currently available for polymer systems in post-production, in-service applications, yet the mechanism of their action is still not fully understood. Structural characterisation of HALS in polymer matrices, particularly the identification of structural modifications brought about by oxidative conditions, is critical to aid mechanistic understanding of the prophylactic effects of these molecules. In this work, electrospray ionisation tandem mass spectrometry (ESI-MS/MS) was applied to the analysis of a suite of commercially available 2,2,6,6-tetramethylpiperidine-based HALS. Fragmentation mechanisms for the [M + H](+) ions are proposed, which provide a rationale for the product ions observed in the MS/MS and MS(3) mass spectra of N-H, N-CH(3), N-C(O)CH(3) and N-OR containing HALS (where R is an alkyl substituent). A common product ion at m/z 123 was identified for the group of antioxidants containing N-H, N-CH(3) or N-C(O)CH(3) functionality, and this product ion was employed in precursor ion scans on a triple quadrupole mass spectrometer to identify the HALS species present in a crude extract from of a polyester-based coil coating. Using MS/MS, two degradation products were unambiguously identified. This technique provides a simple and selective approach to monitoring HALS structures within complex matrices.

Tuning block copolymer structural information by adjusting salt concentration in liquid chromatography at critical conditions coupled with electrospray tandem mass spectrometry

Different cationic adducts of poly(ethylene oxide)/polystyrene block co-oligomers could be produced by adjusting the salt concentration in the mobile phase using a coupling between liquid chromatography at critical conditions and electrospray ionization mass spectrometry. Formation of doubly lithiated adducts was observed at high LiCl concentration (1 mM) while lowering the salt concentration down to 0.1 mM allowed co-oligomers to be ionized with both a proton and a lithium. The fragmentation pathways observed to occur upon collision-induced dissociation of ionized copolymers were shown to be highly dependent on the nature of the cationic adducts. As a result, complementary structural information could be reached by performing MS/MS experiments on different ionic forms of the same co-oligomer molecule. On one hand, release of the nitroxide end-group as a radical from [M+2Li](2+) was followed by a complete depolymerization of the polystyrene block, allowing both this end-group and the polystyrene segment size to be determined. On the other hand, [M+H+Li](2+) precursor ions mainly dissociated via reactions involving bond cleavages within the nitroxide moiety, yielding useful structural information on this end-group.