Dokument-ID Dokumenttyp Autoren/innen Persönliche Herausgeber/innen Haupttitel Abstract Auflage Verlagsort Verlag Herausgeber (Institution) Erscheinungsjahr Titel des übergeordneten Werkes Jahrgang/Band ISBN Veranstaltung Veranstaltungsort Beginndatum der Veranstaltung Enddatum der Veranstaltung Ausgabe/Heft Erste Seite Letzte Seite URN DOI Lizenz Datum der Freischaltung OPUS4-39753 Zeitschriftenartikel Weidner, Steffen; Kricheldorf, H. R. The Role of Transesterification in SnOct 2 - Catalyzed Polymerizations of Lactides l-lactide or meso-lactide are polymerized either at 120 °C where the polymerization process of l-lactide is accompanied by crystallization, or at 180 °C where poly(l-lactide) remains in the molten state. Polymerizations at 120 °C initially yield even-numbered chains (with respect to lactic acid units) having relatively low dispersity, but the fraction of odd-numbered chains increases with time and the entire molecular weight distribution changes. Traces of cyclics are only formed after 7 d. Polymerizations at 180 °C yield equilibrium of even and odd-numbered chains from the beginning, but at low monomer/initiator ratios and short reaction times (<4 h) cyclics are again not formed. They appear at longer reaction times and entail higher dispersities. The results are discussed in terms of five different transesterification mechanisms. Wiley 2017 Macromolecular Chemistry and Physics 218 3 1600331 10.1002/macp.201600331 2017-04-10 OPUS4-43583 Zeitschriftenartikel Kricheldorf, Hans R.; Weidner, Steffen; Scheliga, Felix Cyclic poly(l-lactide) via ring-expansion polymerization by means of dibutyltin 4-tert-butylcatecholate Five new catalysts are prepared from dibutyltin oxide and catechol (HCa), 2,3-dihydroxynaphthalene (NaCa), 4-tert-butyl catechol (BuCa), 4-cyano catechol (CyCa), and 4-benzoyl catechol (BzCa), but only BuCa gives useful results. When benzyl alcohol is used as an initiator, linear chains having benzyl ester end groups are formed in a slow polymerization process. In contrast to cyclic or noncyclic dibutyltin bisalkoxides, neat BuCa yields cyclic poly(l-lactide)s via a fast ring-expansion polymerization. Under certain conditions, a high-melting crystalline phase (Tm = 191 °C) is obtained. At 160 °C and short reaction times even-numbered cycles are slightly prevailing, but, surprisingly, at 120 °C, odd-numbered cycles are predominantly formed. These results definitely prove that a ring-expansion mechanism is operating. Weinheim Wiley VCH 2017 Macromolecular Chemistry and Physics 218 22 1700274, 1 1700274, 10 10.1002/macp.201700274 2018-01-03 OPUS4-39339 Zeitschriftenartikel Billing, Mark; Gräfe, Christine; Saal, Adrian; Biehl, Philip; Clement, Joachim; Dutz, silvio; Weidner, Steffen; Schacher, Felix Zwitterionic Iron Oxide (γ-Fe2O3) Nanoparticles Based on P(2VP-grad-AA) Copolymers This study presents the synthesis and characterization of zwitterionic core-shell hybrid nanoparticles consisting of a core of iron oxide multicore nanoparticles (MCNPs, γ-Fe2O3) and a shell of sultonated poly(2-vinylpyridine-grad-acrylic acid) copolymers. The gradient copolymers are prepared by reversible addition fragmentation chain transfer polymerization of 2-vinylpyridine (2VP), followed by the addition of tert-butyl acrylate and subsequent hydrolysis. Grafting of P(2VP-grad-AA) onto MCNP results in P(2VP-grad-AA)@MCNP, followed by quaternization using 1,3-propanesultone-leading to P(2VPS-grad-AA)@MCNP with a zwitterionic shell. The resulting particles are characterized by transmission electron microscopy, dynamic light scattering, and thermogravimetric analysis measurements, showing particle diameters of ≈70-90 nm and an overall content of the copolymer shell of ≈10%. Turbidity measurements indicate increased stability toward secondary aggregation after coating if compared to the pristine MCNP and additional cytotoxicity tests do not reveal any significant influence on cell viability. Wiley-VCH 2017 Macromolecular Rapid Communications 38 4 1600637-1 1600637-8 10.1002/marc.201600637 2017-03-14 OPUS4-39748 Zeitschriftenartikel Kricheldorf, H. R.; Weidner, Steffen; Scheliga, F. Cyclic poly(L-lactide)s via ring-expansion polymerizations catalysed by 2,2-dibutyl-2- stanna-1,3-dithiolane L-Lactides were polymerized in bulk at 120 or 160 °C with cyclic dibutyltin catalysts derived from 1,2-dimercaptoethane or 2-mercaptoethanol. Only linear chains having one benzyl ester and one OH-end group were obtained when benzyl alcohol was added. When L-lactides were polymerized with neat dibutyl-2-stanna-1,3-dithiolane, exclusively cyclic polylactides were formed even at 120 °C. The temperature, time and monomer/catalyst ratio (M/C) were varied. These results are best explained by a combination of ring-expansion polymerization and ring-extrusion of cyclic oligo- or polylactides with Elimination of the cyclic catalyst. Neither syntheses of linear polylactides nor of cyclic lactides involved racemization up to 20 h at 160 °C. Royal Society of Chemistry 2017 Polymer Chemistry 8 9 1589 1596 10.1039/C6PY02166B 2017-04-10 OPUS4-39288 Zeitschriftenartikel Lauer, Franziska; Seifert, Stephan; Kneipp, Janina; Weidner, Steffen Simplifying the Preparation of Pollen Grains for MALDI-TOF MS Classification Matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS) is a well-implemented analytical technique for the investigation of complex biological samples. In MS, the sample preparation strategy is decisive for the success of the measurements. Here, sample preparation processes and target materials for the investigation of different pollen grains are compared. A reduced and optimized sample preparation process prior to MALDI-TOF measurement is presented using conductive carbon tape as target. The application of conductive tape yields in enhanced absolute signal intensities and mass spectral pattern information, which leads to a clear separation in subsequent pattern analysis. The results will be used to improve the taxonomic differentiation and identification, and might be useful for the development of a simple routine method to identify pollen based on mass spectrometry. Basel, Switzerland MDPI AG MDPI AG 2017 International Journal of Molecular Sciences 18 3 543 554 urn:nbn:de:kobv:b43-392885 10.3390/ijms18030543 http://creativecommons.org/licenses/by/3.0/de/deed.de 2017-03-07 OPUS4-47196 Zeitschriftenartikel You, Zengchao; Meier, Florian; Weidner, Steffen Weidner, Steffen Comparison of miniaturized and conventional asymmetrical flow field flow fractionation (AF4) channels for nanoparticle separation The performance of a miniaturized channel for the separation of polymer and metal nanoparticles (NP) using Asymmetrical Flow Field-Flow Fractionation (AF4) was investigated and compared with a conventional AF4 system. To develop standard separation methods, experimental parameters like cross flow, gradient profile and injection time were varied and optimized. Corresponding chromatographic parameters were calculated and compared. Our results indicate that the chromatographic resolution in the miniaturized channel is lower, whereas significantly shorter analyses time and less solvent consumption were obtained. Moreover, the limit of detection (LOD) and limit of quantification (LOQ) obtained from hyphenation with a UV-detector are obviously lower than in a conventional channel, which makes the miniaturized channel interesting for trace analysis. 2017 Separations 4 1 8, 1 11 10.3390/separations4010008 2019-01-15 OPUS4-42600 Zeitschriftenartikel Kricheldorf, H. R.; Weidner, Steffen; Scheliga, F. Cyclic Polylactides via Simultaneous Ring-Opening Polymerization and Polycondensation Catalyzed by Dibutyltin Mercaptides L-Lactide is polymerized in bulk at 160 8C either with dibutyltin bis(benzylmercaptide) (SnSBzl), dibutyltin bis(benzothiazole 2-mercaptide) (SnMBT), or with dibutyltin bis(pentafluorothiophenolate) (SnSPF) as catalysts. SnSBzl yields linear polylactides having benzylthio-ester end groups in addition to cyclic polylactides, whereas SnMBT and SnSPF mainly or exclusively yield cyclic polylactides. This finding, together with model reactions, indicates that the SnS catalysts promote a combined ring-opening polymerization and polycondensation process including end-to-end cyclization. SnMBT caused slight racemization (3%-5%), when used at 160 8C. With SnSPF optically pure cyclic poly(L-lactide)s with high-molecular weights can be prepared at 160 8C. Wiley Periodicals 2017 Journal of Polymer Science A - Polymer Chemistry 55 22 3767 3775 10.1002/pola.28762 2017-10-23 OPUS4-42407 Zeitschriftenartikel Kricheldorf, H.; Weidner, Steffen; Scheliga, F. Shaplov, A. Cyclization and dispersity of polyesters Two classes of polyesters were prepared by irreversible polycondensations. The dependence of the dispersities on the experimental parameters such as synthetic method, chemical structure, molecular weight and initial monomer concentration was determined. At first it was demonstrated that it is essential for a correct calculation of Mn and Mw to include all oligomers down to the dimers in the evaluation of SEC curves. Furthermore, it was demonstrated for poly(e-caprolactone)s and polylactides that reversible polycondensations and ring-opening polymerizations with equilibration yield identical products. Finally, the dependence of the dispersity on various experimental Parameters was determined for equilibrated poly(e-caprolactone)s and polylactides. Wiley-VCH 2017 Macomolecular Symposia 375 1 Article 1600169, 1 6 10.1002/masy.201600169 2017-10-09 OPUS4-41732 Zeitschriftenartikel Weidner, Steffen; Kricheldorf, Hans R. Transesterification in the solid state of cyclic and linear poly(l-lactide)s Poly(l-lactide)s are synthesized and annealed at 120 °C and changes of the molecular weight distribution (MWD) are monitored by matrix-assited laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. For example, benzyl alcohol+SnOct2 causes equilibration of odd- and even-numbered chains and the final goal of the transesterification is the most probable distribution. The underlying intermolecular transesterification is even observed at 100 and 80 °C in the solid state. However, cyclic tin mercaptide catalysts transform the initial most probable distribution into a MWD with maxima, which display a conspicuous fine structure due to a preferential crystallization of certain ring sizes. The optimum ring sizes for the crystallization are provided by ring-ring equilibration. The gradual formation of a special morphology shifts the melting temperature to values up to 187 °C. Annealing of commercial poly(l-lactide) with a cyclic tin catalyst also yields a distribution of mass peaks with a maximum showing the characteristic fine structure. Wiley-VCH 2017 Macromolecular Chemistry and Physics 218 14 1700114-1 1700114-10 10.1002/macp.201700114 2017-08-29 OPUS4-41733 Zeitschriftenartikel Lauer, Franziska; Seifert, Stephan; Kneipp, Janina; Weidner, Steffen Simplifying the preparation of pollen grains for MALDI-TOF MS classification Matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS) is a well-implemented analytical technique for the investigation of complex biological samples. In MS, the sample preparation strategy is decisive for the success of the measurements. Here, sample preparation processes and target materials for the investigation of different pollen grains are compared. A reduced and optimized sample preparation process prior to MALDI-TOF measurement is presented using conductive carbon tape as target. The application of conductive tape yields in enhanced absolute signal intensities and mass spectral pattern information, which leads to a clear separation in subsequent pattern analysis. The results will be used to improve the taxonomic differentiation and identification, and might be useful for the development of a simple routine method to identify pollen based on mass spectrometry. MDPI 2017 International Journal of Molecular Sciences 18 3 543-1 543-11 10.3390/ijms18030543 2017-08-29 OPUS4-42685 Zeitschriftenartikel Wessig, P.; Schulze, T.; Pfennig, A.; Weidner, Steffen; Prentzel, S.; Schlaad, H. Thiol-ene polymerization of oligospiroketal rods The nucleophilic thiol-ene (thia-Michael) reaction between molecular rods bearing terminal thiols and bis-maleimides was investigated. The molecular rods have oligospiroketal (OSK) and oligospirothioketal (OSTK) backbones. Contrary to the expectations, cyclic oligomers were always obtained instead of linear rigid-rod polymers. Replacing the OS(T)K rods with a flexible chain yielded polymeric products, suggesting that the OS(T)K structure is responsible for the formation of cyclic products. The reason for the preferred formation of cyclic products is due to the presence of folded conformations, which have already been described for articulated rods. Royal Society of Chemistry 2017 Polymer Chemistry 8 44 6879 6885 10.1039/C7PY01569K 2017-11-06 OPUS4-41146 Zeitschriftenartikel Zeegers, G. P.; Steinhoff, R. F.; Weidner, Steffen; Zenobi, R. Evidence for laser-induced redox reactions in matrix-assisted laserdesorption/ionization between cationizing agents and target plate material: a study with polystyrene and trifluoroacetate salts Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is often applied to assess the dispersity and the end groups of synthetic polymers through the addition of cationizing agents. Here weaddress how these cation adducts are formed using polystyrene (PS) as a model polymer. We analyzed PSby MALDI-MS with a 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) as the matrix and a range of trifluoroacetate (TFA) salts as cationizing agents on a range of different targetplate materials (copper, 1.4301 stainless steel, aluminum, Inconel 625, Ti90/Al6/V4 and chromium-, gold-and silver-plated stainless steel). It was found that on a stainless steel substrate the metal cations Al+,Li+, Na+, Cu+and Ag+formed polystyrene adducts, whereas K+, Cs+, Ba2+, Cr3+, Pd2+, In3+, or their lower oxidation states, did not. For the copper and silver substrates, PS and DCTB adduct formation with cations liberated from these target plate materials was observed upon addition of a cationizing agent, which indicates the occurrence of redox reactions between the added TFA salts and the target plate material. Judging from their standard electrode potentials, these redox reactions would not normally occur, i.e.,they require an additional energy input, strongly suggesting that the observed redox reactions are laser-induced. Furthermore, copper granules were found to successfully sequester PS from a tetrahydrofuran(THF) solution, consistent with the view complex formation with the copper target plate can take place prior to the MALDI-MS measurement. Elsevier B.V. 2017 International Journal of Mass Spectrometry 416 80 89 10.1016/j.ijms.2016.10.007 2017-07-27