Till Gruendling

Mass spectrometry in polymer chemistry: a state-of-the-art up-date

Two decades after the introduction of matrix assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI), soft ionization mass spectrometry represents a powerful toolset for the structural investigation of synthetic polymers. The present review highlights the current state-of-the-art, covering the latest developments of novel techniques, enabling instrumentation as well as the important applications of soft ionization MS from the beginning of 2008. Special attention is paid to the role that soft ionization MS has played in the mechanistic investigation of radical polymerization processes since 2005.

Quantitative LC-MS of polymers: Determining accurate molecular weight distributions by combined size exclusion chromatography and electrospray mass spectrometry with maximum entropy data processing

We report on the successful application of size exclusion chromatography (SEC) combined with electrospray ionization mass spectrometry (ESI-MS) and refractive index (RI) detection for the determination of accurate molecular weight distributions of synthetic polymers, corrected for chromatographic band broadening. The presented method makes use of the ability of ESI-MS to accurately depict the peak profiles and retention volumes of individual oligomers eluting from the SEC column, whereas quantitative information on the absolute concentration of oligomers is obtained from the RI-detector only. A sophisticated computational algorithm based on the maximum entropy principle is used to process the data gained by both detectors, yielding an accurate molecular weight distribution, corrected for chromatographic band broadening. Poly(methyl methacrylate) standards with molecular weights up to 10 kDa serve as model compounds. Molecular weight distributions (MWDs) obtained by the maximum entropy procedure are compared to MWDs, which were calculated by a conventional calibration of the SEC-retention time axis with peak retention data obtained from the mass spectrometer. Comparison showed that for the employed chromatographic system, distributions below 7 kDa were only weakly influenced by chromatographic band broadening. However, the maximum entropy algorithm could successfully correct the MWD of a 10 kDa standard for band broadening effects. Molecular weight averages were between 5 and 14% lower than the manufacturer stated data obtained by classical means of calibration. The presented method demonstrates a consistent approach for analyzing data obtained by coupling mass spectrometric detectors and concentration sensitive detectors to polymer liquid chromatography.

Rapid UV light-triggered macromolecular click conjugations via the use of o-quinodimethanes.

Shining a light on click chemistry: The use of UV-radiation as trigger signal provides a facile means to obtain spatial and temporal control over polymer conjugation reactions in addition to providing a further means of achieving orthogonality in click transformations. In the current contribution, UV-radiation was employed to induce a highly efficient Diels-Alder conjugation of polymeric building blocks via the photo-induced in situ formation of highly reactive cis-dienes from a 2-methylbenzophenone precursor.

Facile conversion of RAFT polymers into hydroxyl functional polymers: a detailed investigation of variable monomer and RAFT agent combinations

We report the systematic investigation of a recently introduced one-pot radical transformation of methacrylate and acrylate-type polymers prepared via reversible addition fragmentation chain transfer (RAFT) polymerization into hydroxyl functional polymers. The simple reaction procedure involves stirring a solution of the RAFT functional polymer and an azo-initiator in tetrahydrofuran at elevated temperatures (T = 60 °C) in the presence of ambient air. Subsequent reduction of the formed hydroperoxide functional polymers to hydroxyl functional polymers is achieved in a one-pot procedure using triphenylphosphine. Polymers investigated in the current study are poly(methyl acrylate) (pMA), poly(butyl acrylate) (pBA), poly(isobornyl acrylate) (piBoA) and poly(tert-butyl acrylate) (ptBA) carrying a dithiobenzoate or phenyldithioacetate end terminius as well as a symmetrical trithiocarbonate mid chain function. Quantitative conversion into the hydroperoxyl and hydroxyl terminated product is observed when trithiocarbonate functional polymers are employed. In the case of dithiobenzoate and phenyldithioacetate functional acrylic polymers, some minor side products due to the oxidation of the RAFT end-group are generated. Size exclusion chromatography (SEC) and size exclusion chromatography–electrospray mass spectrometry (SEC-ESI-MS) were employed to monitor the progress of the reaction and to investigate the proposed reaction mechanism for the model polymers. When trithiocarbonate functional polymers are employed in the transformation reaction, the SEC analysis shows a bisection of the initial Mn. Collision induced dissociation (CID) MS experiments of the intermediate reaction products were conducted to gain in-depth information about the chemical structure. The new backbone linked hydroxyl group provides a versatile anchor for chemical end-group conversions and conjugation reactions with RAFT prepared polymers, alleviating problems with the rather limited ability of the dithioester end-group to undergo non-radical transformations.

Design of Experiment (DoE) as a Tool for the Optimization of Source Conditions in SEC-ESI-MS of Functional Synthetic Polymers Synthesized via ATRP

Design of experiment (DoE) is applied to establish the optimum ionization conditions for analyzing synthetic polymers via coupled size exclusion chromatography electrospray ionization mass spectrometry (SEC-ESI-MS) yielding maximum ionization efficiency. The ion source conditions were optimized with regard to the ionization efficiency, the amount of fragmentation, as well as the formation of salt adducts. A D-optimal experimental design was employed for this purpose and the recorded data were evaluated by a quadratic response surface model, accounting for possible interactions between the individual source settings. It was established that the ionization efficiency can be improved by up to one order of magnitude without compromising the softness of the ionization process and that optimal ionization conditions are found at similar source settings regardless of the charge state. The present optimization exercise therefore provides a hands-on guide for the use of experimental design to determine optimum ionization conditions during the SEC-ESI-MS of functional polymers.

A Novel One-Pot Procedure for the Fast and Efficient Conversion of RAFT Polymers into Hydroxy-Functional Polymers

We report on the successful quantitative transformation of methacrylate and acrylate-type polymers prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization carrying a dithioester-end-group into hydroxy-functional polymers. The simple reaction procedure involves stirring a solution of the dithioester-capped polymer and an azo-initiator in tetrahydrofuran at elevated temperatures (T = 60°C) in the presence of air. This reaction quantitatively yields hydroperoxide functionalities that can be efficiently reduced to hydroxy groups in a one-pot procedure using triphenylphosphine. Size exclusion chromatography–electrospray mass spectrometry was employed to monitor the progress of the reaction. The new backbone-linked hydroxy group provides a versatile anchor for chemical end-group conversions and conjugation reactions with prepared RAFT polymers, which alleviates problems with the rather limited ability of the dithioester-end-group to undergo non-radical transformations.

Modular design of glyco-microspheres via mild pericyclic reactions and their quantitative analysis

The facile and efficient functionalization of porous poly(glycidyl methacrylate) (pGMA) microspheres via hetero Diels–Alder (HDA) chemistry with poly(3-O-acryloyl-1,2:5,6-di-O-isopropylidene-α-D-glucofuranoside) (pAIpGlc) prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization employing electron deficient thiocarbonylthio compounds (benzyl pyridin-2-yldithioformate (BPDF)) is described in detail. The efficiency of the employed ‘grafting to’ approach is qualitatively and quantitatively analyzed. Initially the microspheres are functionalized with a highly reactive diene – cyclopentadiene (Cp) – in one step with sodium cyclopentadienide, and subsequently reacted with a protected glycopolymer (number-average molecular weight, Mn = 4200 g mol−1; polydispersity index, PDI = 1.2) that carries a thiocarbonyl moiety functioning as a dienophile. The functionalization of the microspheres is achieved under mild conditions (T = 50 °C) with trifluoroacetic acid (TFA) as a readily removable catalyst. Deprotection of the grafted pAIpGlc to poly(3-O-acryloyl-α,β-D-glucopyranoside) (pAGlc) can be performed after functionalization in one pot with formic acid at ambient temperature. The obtained loading capacity is 2.63 × 1019 chains per g and the grafting density is close to 0.16 chains per nm2. Quantitative analysis of the grafting densities is achieved via elemental analysis; the pore size distribution before functionalization was analyzed by inverse size exclusion chromatography (iSEC). Further employed characterization techniques include scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and high resolution attenuated total reflectance (ATR) FT-IR microscopy supporting the successful modification of the microspheres.