End-group Determination Research Articles

Rapid and simple determination of average molecular weight and composition of synthetic polymers via electrospray ionization-mass spectrometry and a Bayesian universal charge deconvolution.

Simple, affordable, and rapid methods for identifying the molecular weight (MW) distribution and macromolecular composition of polymeric materials are limited. Current tools require extensive solvent consumption, linear calibrations, and expensive consumables. A simple method for the determination of average MW (Mn , Mw ) and chain end groups is demonstrated for synthetic homopolymer standards using direct injection electrospray ionization-mass spectrometry (ESI-MS) and an open-sourced charge deconvolution (CDC) algorithm. Five homopolymer standards in the 1-7kDa MW range were analyzed using direct-injection ESI-MS on a quadrupole/time-of-flight mass spectrometer. The samples investigated, viz. two poly(ethylene oxide) (PEO) and two poly(styrene sulfonic acid) (PSS) standards with narrow polydispersity and one poly(d,l-alanine) (pAla) standard with undefined polydispersity, were chosen to illustrate challenges with ESI-MS quantitation. Using the UniDec program, weight average MWs (Mw ) obtained from the charge-deconvoluted spectra were compared to the reported Mw data of the standards from size exclusion chromatography (SEC) measurements. The MW data derived for the PSS, PEO, and pAla standards agreed well with the corresponding reported Mw or MW range values. The method was able to provide MW, degree of polymerization (DP), and polydispersity index (PDI) information for polymers with narrow (PSS, PEO) as well as broader (pAla) molecular weight distribution; this feature provides an advantage over MW analysis via matrix-assisted laser desorption/ionization (MALDI) for ESI-compatible materials. PSS standards differing in average MW by only a few repeat units could be confidently distinguished. Additionally, the oligomeric resolution observed for all samples studied unveiled chain-end information not available through chromatographic analysis. Overall, the free and easy-to-use UniDec CDC algorithm provides a simple, alternative method to measuring MW and DP for polymeric materials without high solvent consumption, expensive ionization sources, or calibration curves. Information about the masses of individual oligomers and the possibility to further characterize these oligomers using tandem mass spectrometry and/or ion mobility techniques constitutes additional benefits of this approach vis-à-vis traditional MW and PDI elucidation through SEC.

Масс-спектрометрия как современный метод анализа полимеров

Работа посвящена изучению места современных масс-спектрометрических методов в области исследования полимеров. Исследованы вопросы развития методов и показано, что практически до конца ХХ века масс-спектрометрия применялась для изучения полимеров крайне ограниченно из-за нелетучести полимеров. Рассмотрен метод пиролитической масс-спектрометрии как наиболее распространённый для определения молекулярно-массового распределения, определения концевых групп и других общих задач. Также описаны более современные методы “мягкой” ионизации, позволяющие ионизировать полимеры из конденсированного состояния с одновременной десорбцией в газовую фазу. Обзор литературы показал, что подобные методы практически полностью вытеснили или значительно дополнили инструментарий исследователя полимерных материалов. В работе со ссылками на литературу описаны матрично-активированная лазерная десорбция/ионизация (MALDI), полевая десорбция (FD), бомбардировка быстрыми атомами (SIMS), ионизация электораспылением (ESI). Также в качестве более классического метода описана пиролитическая масс-спектрометрия с прямым вводом и в сочетании с газохроматографическим разделением. Показано, что масс-спектрометрия наиболее универсальный и чувствительный метод при любых задачах в области анализа и исследования полимерных материалов.

MSPolyCalc: A web-based App for polymer mass spectrometry data interpretation. The case study of a pharmaceutical excipient.

In contrast to biological polymers, synthetic macromolecules consist of distributions of sizes, chemical compositions, functionalities and eventually architectures. The mass spectrum of a synthetic polymer may exhibit a tremendous number of signals. The availability of suitable IT tools to support interpretation is key. A web-based tool is presented: MSPolyCalc. It offers a set of functionalities, including the calculation of polymer distributions, molecular formulae and a match evaluation for peak assignment based on both mass and spectral accuracy (similarity score). The software was successfully tested with mass spectra exhibiting resolutions ranging from 10,000 to 240,000. The molecular characterization of a synthetic poly(ethylene glycol)-based excipient was achieved. MSPolyCalc allowed the discrimination of six polymer compositions of variable relative abundance. Secondary ionization adducts with very low intensity consisting of matrix-analyte clusters were also successfully identified. MSPolyCalc offers assisted data interpretation to target the needs of polymer chemists. It facilitates structure characterization, ionization adduct identification, and end-group determination together with visual result reporting.

Characterization of Synthetic Polymers via Matrix Assisted Laser Desorption Ionization Time of Flight (MALDI-TOF) Mass Spectrometry.

There are many techniques that can be employed in the characterization of synthetic homopolymers, but few provide as useful of information for end group analysis as matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS). This tutorial demonstrates methods for optimization of the sample preparation, spectral acquisition, and data analysis of synthetic polymers using MALDI-TOF MS. Critical parameters during sample preparation include the selection of the matrix, identification of an appropriate cationization salt, and tuning the relative proportions of the matrix, cation, and analyte. The acquisition parameters, such as mode (linear or reflector), polarization (positive or negative), acceleration voltage, and delay time, are also important. Given some knowledge of the chemistry involved to synthesize the polymer and optimizing both the data acquisition parameters and the sample preparation conditions, spectra should be obtained with sufficient resolution and mass accuracy to enable the unambiguous determination of the end groups of most homopolymers (masses below 10,000) in addition to the repeat unit mass and the overall molecular weight distribution. Though demonstrated on a limited set of polymers, these general techniques are applicable to a much wider range of synthetic polymers for determining mass distributions, though end group determination is only possible for homopolymers with narrow dispersity.

Characterization of Synthetic Polymers via Matrix Assisted Laser Desorption Ionization Time of Flight (MALDI-TOF) Mass Spectrometry

There are many techniques that can be employed in the characterization of synthetic homopolymers, but few provide as useful of information for end group analysis as matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS). This tutorial demonstrates methods for optimization of the sample preparation, spectral acquisition, and data analysis of synthetic polymers using MALDI-TOF MS. Critical parameters during sample preparation include the selection of the matrix, identification of an appropriate cationization salt, and tuning the relative proportions of the matrix, cation, and analyte. The acquisition parameters, such as mode (linear or reflector), polarization (positive or negative), acceleration voltage, and delay time, are also important. Given some knowledge of the chemistry involved to synthesize the polymer and optimizing both the data acquisition parameters and the sample preparation conditions, spectra should be obtained with sufficient resolution and mass accuracy to enable the unambiguous determination of the end groups of most homopolymers (masses below 10,000) in addition to the repeat unit mass and the overall molecular weight distribution. Though demonstrated on a limited set of polymers, these general techniques are applicable to a much wider range of synthetic polymers for determining mass distributions, though end group determination is only possible for homopolymers with narrow dispersity.

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

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. 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. 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. 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.

Study of fluoroalkylsilanes, a new class of amphiphilic oligomers: Determination of end groups and chain length

Radiation-initiated polymerization of tetrafluoroethylene (TFE) in the liquid alkylsilanes Si(CH3)4 (I), C2H5Si(CH3)3 (II), and ClSi(CH3)3 (III) produces fluoroalkylsilane oligomers with the general formula R(C2F4) n X, where X = H or Cl in I, II, and III and R is one of the radicals generated by the abstraction of H from the methyl and ethyl groups in I and II or Cl detachment from III. The nature of R has been determined using an NMR technique, and the molecular mass distribution (MMD) has been found from abundances in anion progressions in mass spectra and from DTG and GPC data. At initial TFE concentrations (C 0) changed from 0.4, 1, and 0.13 to 1, 2.7, and 0.7 mol/L in I, II and III, respectively, the average chain length 〈n〉 increases from 4–5 to 10–12 and the MMD changes from the unimodal to the bimodal pattern in which the proportion of oligomers with n < 6 decreases with increasing C 0. As 〈n〉 increases and the MMD changes, the homogeneous solution becomes colloidal and then turns into a gel. The colloidal particles are largely composed of long oligomers, and the number of solvent molecules per C2F4 unit decreases with an increase in C 0 and is 4–7 in the gel.

Atmospheric pressure photo-ionization mass spectrometry for the detection of labile end groups in poly(styrene)

Mass spectrometry is a powerful tool for end group determination. Several ionization techniques are available. Unfortunately for labile-end-group-containing polymers and especially for polystyrene, cleavage of the end group may easily occur upon ionization. An alternative ionization method suitable for non-polar polymers is atmospheric pressure photo-ionization (APPI). High-energy photons can eliminate an electron out of certain compounds forming radical cations. Additionally in the presence of halogenated solvents the released electrons can be captured forming chloride ions. Hence, adducts can be seen in the negative mode as well. In order to analyze the end groups in PS prepared by ATRP APPI was applied to minimize elimination. For all PS-Cl samples the existence of Cl-end groups as sole end group could be shown. For PS-Br partial preservation of the end groups were obtained. Hence, APPI-ToF-MS in negative mode has been proven a suitable tool to analyze polymers with labile end groups.

Poly(N-alkyl-3,6-carbazole)s via Suzuki–Miyaura Polymerization: From Macrocyclization toward End Functionalization

The metal-catalyzed polymerization of 3,6-carbazoles is explored with emphasis of end-group fidelity of the telechelic polymers. The Suzuki–Miyaura coupling polymerization using asymmetric bifunctional (AB-type) monomers was selected to circumvent the inherent polycondensation character of symmetric (AA/BB-type) monomers. Common synthetic protocols were screened to facilitate the chain growth vs step growth character during the polymerization, e.g., several activation reagents for the boronic ester and various catalytic systems and their associated reaction conditions. The reactions were analyzed in detail by size exclusion chromatography (SEC), 1H NMR spectroscopy, and mass spectrometry including isotope analysis for end-group determination. It was found that the intermediate oligomers are susceptible for macrocycle formation, particularly at low concentration, while longer polymer chains continue to polymerize. The synthetic methodology was extended to utilize functionalized precatalysts and functional terminating agents in order to explore the scope of chain growth vs step growth. The concomitant formation of macrocycles was observed, which indicates the catalyst dissociation and, thus, also the polycondensation pathway. Importantly, the applied low concentration leads to trapping of the undesired nonfunctionalized chains as macrocycles, which can be readily removed from the telechelic linear polymer. In summary, the developed protocols demonstrate the versatility to prepare telechelic poly(3,6-carbazole)s via Suzuki–Miyaura coupling. The azide-decorated poly(carbazole) exemplifies the potential to serve as building blocks for further copper-mediated azide–alkyne cycloaddition reactions.

13C NMR spectroscopy for the quantitative determination of compound ratios and polymer end groups.

13C NMR spectroscopic integration employing short relaxation delays was evaluated as a quantitative tool to obtain ratios of diastereomers, regioisomers, constitutional isomers, mixtures of unrelated compounds, peptoids, and sugars. The results were compared to established quantitative methods such as 1H NMR spectroscopic integration, gas chromatography, and high-performance liquid chromatography and were found to be within <3.4% of 1H NMR spectroscopic values (most examples give results within <2%). Acquisition of the spectra took 2–30 min on as little as 10 mg of sample, proving the general utility of the technique. The simple protocol was extended to include end group analysis of low molecular weight polymers, which afforded results in accordance with 1H NMR spectroscopy and matrix-assisted laser desorption-ionization time-of-flight spectrometry.

Matrix effect in matrix‐assisted laser desorption/ionization mass spectra of derivatized oligomeric polyols

Herein we describe a strong matrix effect observed in the matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectra of silylated glycerol alkoxylates and manifested in the loss of the silyl groups in the presence of carboxyl-containing matrices. Commercially available glycerol alkoxylates containing three end OH groups as well as three matrices - 2,5-dihydroxybenzoic acid (DHB), 3-indoleacrylic acid (IAA) and 1,8,9-anthracenetriol (dithranol) - were chosen for the investigation. N,O-Bis(trimethylsilyl)trifluoroacetamide containing 1% trimethylchlorosilane, acetic anhydride and a formylation mixture (formic acid/acetyl chloride) were used for derivatization. Initial oligomers and derivatized products were analyzed by MALDI-ToF-mass spectrometry (MS) on an Autoflex II instrument, equipped with a nitrogen laser (λ 337 nm), in positive ion reflectron mode. Only [M + Na](+) ions were observed for underivatized polymers and for completely derivatized polymers in the presence of DHB and dithranol, respectively. In the case of IAA the mass spectra revealed sets of peaks for underivatized, and for partially and completely derivatized oligomers. No similar 'matrix effect' was observed in the case of acylated glycerol alkoxylates (acyl = formyl, acetyl): only peaks for completely derivatized oligomers were obtained in all matrices: DHB, IAA and dithranol. Using 1,9-nonandiol, we showed that the 'matrix effect' was due to trans-silylation of carboxyl-containing matrices (DHB and IAA) during co-crystallization of silylated oligomers and matrices. The obtained results show that matrix molecules can participate as reactive species in MALDI-ToF-MS experiments. The matrix should be carefully chosen when a derivatization approach is applied because the analysis of spectra of the completely derivatized products is particularly desirable in the quantitative determination of functional end-groups.

LC-Multistage Mass Spectrometry for the Characterization of Poly(Butylene Adipate-co-Butylene Terephthalate) Copolyester

A comprehensive study using liquid chromatography electrospray ionization multistage mass spectrometry (LC-ESI MS(n)) was performed to get detailed structural information on poly(butylene adipate-co-butylene terephthalate) co-polyester and its product of partial degradation. LC-MS and LC-MS(n) identified the existence of cyclic structures in the original samples that disappear completely during the degradation. The occurrence of methanol transesterification in the degradation process was confirmed. MS(2) on the first (13)C isotope peak helped to determine the elemental composition of the fragments and facilitated end group determination. The method can be used to provide an alternative for high mass accuracy MS(2) experiments. Sequence information was also revealed for certain copolymeric structures.

A rapid and reversible colorimetric assay for the characterization of aminated solid surfaces

The covalent immobilization of synthetic or natural macromolecular compounds containing amino groups onto polystyrene (PS) solid surfaces is of great interest in diagnostic applications. A sensitive assay allowing the determination of reactive end groups is therefore a powerful tool for predicting the performance of the active surface. Recently, we reported the use of the Coomassie brilliant blue (CBB) colorimetric reagent to quantify protonated groups (N(+)) in linear and dendritic structures in solution (Coussot et al., Polym Int 58(5):511-518, 2009). In this work, a simple method using CBB dye for the characterization of PS aminated solid surfaces is developed. The proposed amino density estimation by colorimetric assay (ADECA) method is based on the reversible complexation of the dye with the N(+) groups on solid surfaces. The assay measures the released dye thanks to the use of a unique sodium carbonate-methanol buffer. Thereby, for the first time, the same surface can be used for characterization and for further coupling applications. A surface density of four N(+) groups per square nanometer can be measured in PS microwell format, the whole characterization being done within 30 min. Performances of this new colorimetric-based method are detailed. The ADECA method is further demonstrated to be useful for the characterization of aminated polypropylene and glass materials with various sizes and shapes.

Synthesis of PPP-b-PS block copolymers using a combination of Suzuki-polycondensation and nitroxide-mediated radical polymerization

The synthesis of π-conjugated Nitroxide-Mediated Radical Polymerization (NMRP)-macroinitiators by Suzuki-polycondensation in a one-step reaction has been investigated using conventional and microwave-heating in presence of a suitable terminating agent. Alkoxyamine-functionalized poly(para-phenylene)s were initially synthesized by Suzuki-polycondensation and then its block copolymer with styrene by NMRP. Molecular weight and molecular weight distribution of the polymers have been determined in SEC-measurements, while end-group determination was performed with MALDI-ToF-MS. Thin-layer-morphologies of the block copolymers were investigated using tapping-mode AFM.

Systematic MALDI‐TOF CID Investigation on Different Substituted mPEG 2000

AbstractMethoxy poly(ethylene glycol) 2000 (mPEG 2000) samples were substituted via esterification reactions to convert the hydroxyl end group of the mPEG into potential initiators for the cationic ring opening polymerization of 2‐oxazolines. These substitution products were investigated by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS); in addition, detailed MALDI‐TOF/TOF‐collision induced dissociation (CID) MS studies were performed to introduce this method as complementary structural characterization tool allowing the detailed analysis of the prepared macromolecules. The CID of the macroinitiators revealed 1,4‐hydrogen and 1,4‐ethylene eliminations forming very regular fragmentation patterns which showed, depending on the end groups, different fragmentation series. Furthermore, very pronounced McLafferty + 1 rearrangements (1,5 hydrogen‐transfer) of the introduced ester end groups were observed leaving the mPEG molecule as neutral acid. This incisive loss revealed the exact molar mass for each end group and, therefore, represents an important tool for end group determination of functionalized polymers.magnified image

Derivatization aids in the determination of end groups in poly(alkylene glycol)s by matrix‐assisted laser desorption/ionization mass spectrometry

A method for end-group characterization by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry following preliminary derivatization with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) and capryloyl chloride is described and applied to poly(alkylene glycol)s. The MALDI mass spectra of the products revealed peaks of sodiated derivative cations, whose shift by the respective increments allowed the determination of the number of end functional groups with active hydrogens. This approach is particularly efficient for the distinction of cyclic and linear dehydration products among minor components.

Dopant-Assisted Atmospheric Pressure Photoionization Mass Spectrometry of Polyisobutylene Derivatives Initiated by Mono- and Bifunctional Initiators

Nine polyisobutylene (PIB) derivatives with different end groups (chlorine, vinyl, isobutenyl, 2,2-diphenylvinyl, and carboxyl) and molecular weights (1000 to 4500 g/mol), initiated by monofunctional and aromatic bifunctional initiators were studied by atmospheric pressure photoionization mass spectrometry (APPI-MS) in both negative and positive ion modes. Consistent with previous findings, negative ion APPI-MS revealed end-group identities through the formation of PIB adducts with chloride ions formed in situ from a chlorinated solvent (e.g., CCl4) in the presence of a dopant (toluene). In positive ion mode, considerable fragmentation of these PIB derivatives was observed, rendering end-group determinations very difficult. The M(n) values obtained by APPI(-)-MS were considerably lower than those determined by SEC for PIB derivatives with M(n) higher than 2000 g/mol. PIBs containing carboxyl termini can undergo collision-induced dissociation, yielding structurally important product ions. The resulting APPI-MS/MS intensities were found to reflect the "arm-length" distribution for PIBs with bifunctional aromatic moieties. In positive ion mode, [M + COCl]+ adducts were observed for PIBs with an aromatic initiator moiety. The origin of the COCl+ species is also discussed.

Reassessment of Free-Radical Polymerization Mechanism of Allyl Acetate Based on End-Group Determination of Resulting Oligomers by MALDI-TOF-MS Spectrometry

Reassessment of Free-Radical Polymerization Mechanism of Allyl Acetate Based on End-Group Determination of Resulting Oligomers by MALDI-TOF-MS Spectrometry

A new photoiniferter/RAFT agent for ambient temperature rapid and well‐controlled radical polymerization

AbstractPhenacyl morpholine‐4‐dithiocarbamate is synthesized and characterized. Its capability to act as both a photoiniferter and reversible addition fragmentation chain transfer agent for the polymerization of styrene is examined. Polymerization carried out in bulk under ultra violet irradiation at above 300 nm at room temperature shows controlled free radical polymerization characteristics up to 50% conversions and produces well‐defined polymers with molecular weights close to those predicted from theory and relatively narrow poyldispersities (Mw/Mn ∼ 1.30). End group determination and block copolymerization with methyl acrylate suggest that morpholino dithiocarbamate groups were attained at the end of the polymer. Photolysis and polymerization studies revealed that polymerization proceeds via both reversible termination and RAFT mechanisms. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3387–3395, 2008

Novel Software for the Assignment of Peaks from Tandem Mass Spectrometry Spectra of Synthetic Polymers

Novel software has been developed to aid the interpretation of tandem mass spectrometry (MS/MS) data from synthetic polymers. The software is particularly focused toward aiding the end-group determination of these materials by significantly speeding up the interpretation process. This allows information on the initiator and/or chain transfer agents, used to generate the polymer, and the mechanism of termination to be inferred from the data much more rapidly. The software allows the validity of hypothesized structures to be rapidly tested by automatically annotating the data file using previously proposed fragmentation rules for synthetic polymers. Low-energy collision-induced dissociation (CID) data from methacrylate, styrene, and polyether oligomers are used as example data for the software. Exact-mass CID information was used to aid the understanding of the dissociation mechanism of the polymers. The software can use exact-mass data to provide more confidence in the results. The MS/MS results indicate that the fragmentation pathways are those previously proposed for these polymers.