Hans Joachim Räder

Two-dimensional graphene nanoribbons

A new synthetic strategy toward novel linear two-dimensional graphene nanoribbons up to 12 nm has been established. The nanoribbons are characterized by MS, UV/vis, and scanning tunneling microscopy (STM). Various microscopic studies of these novel structures showed a high tendency to self-assemble.

Synthesis of a Giant 222 Carbon Graphite Sheet

In this paper we present the synthesis and characterization of the so far largest polycyclic aromatic hydrocarbon (PAH), containing 222 carbon atoms or 37 separate benzene units. First a suitable three-dimensional oligophenylene precursor molecule is built up by a sequence of Diels-Alder and cyclotrimerization reactions and then planarized in the final step by oxidative cyclodehydrogenation to the corresponding hexagonal PAH. Structural proof is based on isotopically resolved MALDI-TOF mass spectra and electronic characteristics are studied by UV/Vis spectroscopy.

MALDI-TOF Mass Spectrometry in the Analysis of Synthetic Polymers

Matrix-assisted laser desorption/ionization mass spectrometry has been applied over the past 5 years with increasing success to the analysis of synthetic polymers. Basic principles and instrumental parameters influencing the spectra are described. Examples from different areas of polymer research with emphasis on technically relevant systems elucidate the highly useful features of this kind of mass spectrometry, i. e. determination of absolute molar masses, high mass range, reasonable mass resolution, high sensitivity, relative ease of operation even for polar molecules. Molar mass distributions can be obtained in a mass range that is problematic for other mass determining techniques. A detailed characterization of the underlying chemistry can be obtained in many cases by an endgroup analysis. Especially in combination with other separation techniques, a comprehensive characterization of chemical heterogeneity can be achieved. Recent developments, e. g. delayed extraction to enhance the mass resolution obtainable, will further increase the favorable prospects of this technique.

Synthesis and noncovalent protein conjugation of linear-hyperbranched PEG-poly(glycerol) alpha,omega(n)-telechelics.

Linear-hyperbranched, heterobifunctional alpha,omega(n) telechelic block copolymers consisting of a linear poly(ethylene glycol) (PEG) chain and a hyperbranched polyglycerol (PG) block have been prepared in five steps, using a protected amino-functional initiator. The polyfunctionality omega(n) (OH groups) can be adjusted by the degree of polymerization (DP(n)) of the polyglycerol block. Subsequent introduction of a single biotin unit by amidation in alpha-position permitted noncovalent bioconjugation with avidin.

Why does matrix-assisted laser desorption/ionization time-of-flight mass spectrometry give incorrect results for broad polymer distributions?

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is known to give reliable results for narrow molecular weight distributions, though in the case of polydisperse polymers the values obtained are not in agreement with those from conventional methods. In this paper we simulate broad polymer distributions with increasing polydispersity by mixing equimolar amounts of either poly(styrene) (PS) or poly(methylmethacrylate) (PMMA) standards in a mass range between 5000 and 100 000 Da with increasing difference in molecular weight. This allows us to investigate the problems of MALDI-TOF measurements of polydisperse polymers. We found that several reasons are responsible for the false results. High molecular weight polymers require higher laser power for the desorption/ionization process than do low molecular weight polymers. This causes smaller peak areas for the high mass component and can also be responsible for fragmentation of the low molecular weight part of a mixture, if the molecular weights of the polymers are widely distributed. The observed dependence upon laser power is different for PS and PMMA polymers and changes with the matrix used. Furthermore, doubly charged molecular peaks and molecular cluster peaks appear in the spectra and influence the experimentally measured distribution.

Poly[2]catenanes and Cyclic Oligo[2]catenanes Containing Alternating Topological and Covalent Bonds: Synthesis and Characterization

Large bifunctionalized catenate and catenand, composed of macrocycles containing 45 atoms, have been synthesized and copolymerized with a terephthalic acid derivative to afford a cyclic oligo[2]catenand and a linear poly[2]catenate. Poly[2]catenand was obtained by demetalation of poly[2]-catenate. The molecular weight distribution of these polymers, which are composed of alternating topological and covalent bonds, has been characterized by gel permeation chromatography (GPC) with universal calibration, viscometry, and MALDI-TOF mass spectrometry. We were able to synthesize a poly[2]catenate and a poly[2]catenand with a degree of polymerization (DP) equal to 8-9. In addition, an upper value of the Kuhn segment length of poly[2]catenand has been determined and is discussed in the light of its structure.

Synthesis and characterization of a poly(para-phenyleneethynylene)-block-poly(ethylene oxide) rod-coil block copolymer

The synthesis of the poly(para-phenyleneethynylene)-block-poly(ethylene oxide) block copolymer (PPE-b-PEO) (1) via condensation of endfunctionalized poly(para-phenyleneethynylene) (PPE) (5) and poly(ethylene oxide) monomethyl ether (PEO) is reported. This is achieved by the initial synthesis of a PPE homopolymer with quantitative terminal functionalization, as proven by 1H NMR and field desorption mass spectrometry (FD-MS). Reaction of the latter with PEO affords the block copolymer 1, which was characterized by 1H NMR spectroscopy, FD-MS and gel permeation chromatography (GPC). Furthermore it is shown that matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF-MS) is a suitable method to investigate PPE-b-PEO with respect to molecular weights and copolymer composition.

Subliming the Unsublimable: How to Deposit Nanographenes

Quite sublime: Thin-layer fabrication of unsublimable large polycyclic aromatic hydrocarbons (PAHs) by pulsed laser deposition was used to prepare samples for scanning tunneling microscopy. Giant PAHs with up to 222 carbon atoms can be visualized-a size that was previously not possible because of the lack of suitable deposition methods.

Characterization of Large Synthetic Polycyclic Aromatic Hydrocarbons by MALDI- and LD-TOF Mass Spectrometry

Polycyclic aromatic hydrocarbons (PAHs) with high molecular weights represent a class of compounds with are difficult to analyze with conventional analytical methods because of their insolubility. In this paper, we describe their characterization by laser desorption mass spectrometry (LDMD). The huge PAHs show a fragmentation pathway which is unknown so far. It can be explained by the loss of cyclic C 6 -units and a subsequent rehydrogenation of the resulting radical sites of the measured fragments. With these findings we are able to detect structure defects in a PAH induced by rearrangement reactions during synthesis. Since the analysis of large PAHs by LDMS is generally hindred by the formation of fragments we also developed an improved method of sample preparation analogous to matrix assisted laser desorption/ionization (MALDI) mass spectrometry. By this method, the isotopic distributions of the PAH molecular ions can now be measured without interfering signals of fragments to enable a reliable identification. Fragmentation mechanisms proposed for the loss of a cyclic C 6 unit. The mass differences after fragmentation depend on the peripheral structure of the PAHs. The overall mass difference between the molecular ion and the fragment ion is assumed to result from the cleavage of one or more C-C bonds and the subsequent attachment of hydrogen atoms to the reactive radical sites left behind on the bigger fragment. A mass difference of Δm = 72 results from cleavage of three C-C bonds.

Recalcitrance of poly(vinylpyrrolidone): evidence through matrix-assisted laser desorption-ionization time-of-flight mass spectrometry

The aerobic biodegradability of an extensively used synthetic polymer was monitored the first time on a laboratory-scale fixed-bed bioreactor (FBBR) applying matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS). Polymeric poly(vinylpyrrolidone) (PVP) was spiked at concentrations of 10 mg l(-1) onto the FBBR run with river water and the biodegradation monitored after lyophilization of aliquots of the test liquor applying MALDI-TOF-MS. The latter proved to be a powerful tool for qualitative screening purposes of PVP in a molecular mass range <20 kDa in particularly yielding a high sensitivity and shot-to-shot reproducibility. The sample-to-sample reproducibility was enhanced applying the anchor target device. Post-source decay-MALDI-TOF-MS fragmentation investigations determined the unknown end groups of PVP unambiguously. Poor biodegradability of PVP can be assumed, since even after 30 days, no oxidation of the terminal groups and no difference in the repeating units was observed. A decrease in the molecular mass distribution can be drawn back rather to adsorption of PVP in the FBBR other than to biodegradation. This was further investigated performing an adsorption experiment with sewage sludge as solid matrix and analyses of the aqueous phase and sludge samples. Extrapolating these results to the situation in wastewater treatment plants, it is highly likely that PVP is eliminated from the dissolved phase by adsorption onto sludge particles.