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Eingeladener Vortrag
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The dispersed iron oxide nanoparticles of ferrofluids in aqueous solution are difficult to characterize due to their protective polymer coatings. We report on the bimodal size distribution of superparamagnetic iron oxide nanoparticles found in the MRI contrast agent Resovist, which is a representative example of commercial nanoparticle-based pharmaceutical formulations. The radii of the majority of the nanoparticles (>99%) range from 4 to 13 nm (less than 1% of the particles display radii up to 21 nm). The maxima of the size distributions are at 5.0 and 9.9 nm. The analysis was performed with in situ characterization of Resovist via online coupling of asymmetrical flow field-flow fractionation (A4F) with small-angle X-ray scattering (SAXS) using a standard copper X-ray tube as a radiation source. The outlet of the A4F was directly coupled to a flow capillary on the SAXS instrument. SAXS curves of nanoparticle fractions were recorded at 1-min time intervals. We recommend using the A4F-SAXS coupling as a routine method for analysis of dispersed nanoparticles with sizes in the range of 1-100 nm. It allows a fast and quantitative comparison of different batches without the need for sample preparation.
Liquid chromatography (LC) at critical conditions of adsorption was used to separate various poly(ethylene oxides), poly(propylene oxides) and their copolymers. For the first time, the determination of the critical conditions by means of Ultra Performance Liquid Chromatography (UPLC) coupled to Electrospray Ionization Time-of-flight Mass Spectrometry (ESI-TOF MS) is reported. In contrast to established, mostly laborious routines to find suitable chromatographic separation conditions, this coupling enables a very fast adjustment of parameters. Similar to LC Matrix-assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (LC/MALDI MS) coupling, a two-dimensional analysis of homo- and copolymers regarding its functionality type and molecular weight distribution, as well as copolymer composition, can be performed simultaneously. Furthermore, there is no need for using polymer standards for the determination of critical conditions or Size Exclusion Chromatography calibration.
The localization of polymeric composition in samples prepared for matrix-assisted laser desorption/ionization (MALDI) analysis has been investigated by imaging mass spectrometry. Various matrices and solvents were used for sample spot preparation of a polybutyleneglycol (PBG 1000). It was shown that in visibly homogeneous spots, prepared using the dried droplet method, separation between matrix and polymer takes place. Moreover, using -cyano-4-hydroxycinnamic acid (CCA) as matrix and methanol as solvent molecular mass separation of the polymer homologues in the spots was detectable. In contrast to manually spotted samples, dry spray deposition results in homogeneous layers showing no separation effects.
Polycondensations of 1,6-hexane diol and sebacic acid were conducted in bulk with addition of a lanthanide triflate as acidic catalyst. With exception of promethium triflate all lanthanide triflates were studied. A particularly low molecular weight was obtained with neodym triflate and the best results with samarium triflate. With Sm(OTf)3 weight average (Mw) values up to 65 kDa (uncorrected SEC data) were achieved after optimization of the reaction conditions. Comparison of these results with those obtained from bismuth, magnesium, and zinc triflates, on the one hand, and comparison with the acidities of all catalysts, on the other, indicates that the esterification mechanism involves complexation of monomer by metal ions. Preparation of multiblock copoly(ether ester)s failed due to insufficient incorporation of poly(tetrahydrofuran) diols.
The development of dendritic metal ion carrier systems for use in a biological environment is a challenging task as the carrier system must possess multiple features (e.g. a protective shell for metal decomplexation, targeting functions, metal-intradendrimer complexes, etc.) to substitute for the function of metal proteins in processes such as copper metabolism. Thus, Cu(II) complexation by a series of poly(propyleneimine) glycodendrimers ranging up to the fifth generation that have either a dense maltose or maltotriose shell was investigated by UV/Vis spectroscopy and electron paramagnetic resonance (EPR). As a necessary step towards potential biological application, we elucidated the complexation capacity, location of the Cu(II)-dendrimer complexes and the Cu(II) coordination sphere in the dendritic environment. A generation-dependent Cu(II) complexation was found. Furthermore, analysis of the EPR spectra revealed that internal and external Cu(II) coordination and the symmetry (axial and rhombic) of the generated complexes depend on the oligosaccharide shell, dendrimer generation and the relative concentrations of Cu(II) and the dendrimers. Both axial and rhombic symmetries are generation dependent, but also distort with increasing generation number. External coordination of Cu(II) is supported by sugar groups and water molecules. Finally, a third-generation dendrimer with a maltose shell was used to explore the general complexation behaviour of the dendritic poly(propyleneimine) scaffold towards different metal ions [Cu(II), Ag(I), VO(IV), Ni(II), Eu(III) and UO2(VI)].
εCL was polymerized using the triflates of lanthanum, samarium, magnesium, aluminum, scandium, and bismuth as catalysts. Bismuth triflate proved to be extraordinarily reactive, and catalyzed polymerizations of εCL even at 20 °C. Adding DTBMP reduced the polymerization rate only slightly. Furthermore, no evidence of a cationic mechanism was found by end‐group analyses. Polymerization at 20 °C either in bulk or in solution only yielded polyesters of low or medium molecular weights. Yet addition of alcohols allowed for a proper control of molecular weight and end‐groups. Additionally, low catalyst concentrations and low temperature resulted in narrow molecular weight distributions and polylactones almost free of cyclic compounds.
Sequences and end groups of complex copolyesters were determined by fragmentation analysis by means of matrix-assisted laser desorption/ionization collision-induced dissociation tandem mass spectrometry (MALDI CID MS/MS). The complexity of the crude copolyester mixture was reduced by a chromatographic separation followed by a MALDI time-of-flight (TOF) investigation of fractions. Due to overlapping compositional and end-group information a clear assignment of end groups was very difficult. However, the fragmentation of suitable precursor ions resulted in typical fragment ion patterns and, therefore, enabled a fast and unambiguous determination of the end groups and composition of this important class of polymers.
We report on the characterization of the solution structure of poly(N-vinyl-2-pyrrolidone)s (PVP) by small-angle X-ray scattering (SAXS) and by online coupling of asymmetrical flow field-flow fractionation (A4F), SAXS and dynamic light scattering (DLS). The commercial products PVP K30 and PVP K90 with nominal molar masses of 40 × 103 and 360 × 103 g mol-1, respectively, were investigated separately and as binary mixture. Detailed information for all polymer fractions is available on the polymer contour lengths and the diffusion coefficients. Key areas of applications for the A4F-SAXS-DLS coupling are seen in comparison to static light scattering for polymers with radii of gyration smaller than 10 nm, for which only SAXS produces precise analytical results on the size of the polymers in solution.
Using three different catalysts, water-initiated polymerizations of ε-caprolactone were conducted in bulk with variation of the monomer/water ratio. The resulting CH2OH and CO2H- terminated polylactones were subjected in situ to azeotropic polycondensations. With Bi-triflate and temperatures, the polycondensations were not much successful and involved side reactions. With ZnCl2, and especially SnCl2, considerably higher molar masses were achieved. The substitution of toluene for chlorobenzene for refluxing gave better results. The polycondensations broadened the molar mass distribution of the ROP-based prepolymers, and polydispersities between 1.4–1.8 were obtained. The MALDITOF mass spectra revealed that the polycondensations significantly enhanced the fraction of rings due to efficient 'end-biting' reactions. By comparison with copolymerization experiments and Sn methoxide-initiated polymerizations, it was demonstrated that equilibration reactions, such as the formation of rings by 'back-biting,' did not occur.
Copolyesters of glycolic acid combined with adipic acid and 1,4 butanediol were synthesized, their in vitro hydrolytic degradation was studied and correlated with their structure. The hydrolytic degradation of the copolyesters was directly related with the degree of crystallinity and the diameter of the crystallites. It was found that glycolate units disturb the ordering of the butylene adipate units, which results in a decrease of the crystallinity. By comparing the hydrolysis parameters of synthesized copolyesters with those of similar aliphatic copolyesters a hydrolysis mechanism was proposed. According to this mechanism, the degradation takes place not only by the loss of end units, but also through the removal of larger segments. MALDI-TOF MS data showed that the possibility of a copolyester macromolecule forming cyclic structures correlates with the arrangement of ester groups in the macromolecules and the structure of the linear segments.
The MALDI-TOF MS analysis of copolymers very often results in complex spectra. A chromatographic separation/fractionation prior to the MALDI investigation can be advantageous since the MALDI mass spectra of fractions very often reveal well-resolved peaks of distinguishable copolymer series. For this purpose, different modes of chromatography have been applied. Chromatographic runs were transferred to MALDI targets utilizing a combined air/electrospray deposition device. Using the new MassChrom2D software, fraction-dependent 2D copolymer compositions plots were obtained providing additional information on the chromatographic mode, and enabling fast modification of conditions to increase separation.
In the present study the incorporation of iron into an organic polymer, composed of formamidine subunits [R–N=C(H)–NH–R], has been examined. The catalytic ability of the recyclable material was investigated in the iron-catalyzed formation of C–C bonds. After optimization of the reaction conditions, excellent yields and chemoselectivities were feasible.
A recently introduced procedure involving a mechanistic switch from reversible addition–fragmentation chain transfer (RAFT) polymerization to ring-opening polymerization (ROP) to form diblock copolymers is applied to synthesize ABA (star) block copolymers. The synthetic steps include the polymerization of styrene with R-group designed RAFT agents, the transformation of the thiocarbonyl thio end groups into OH functionalities, and their subsequent chain extension by ROP. The obtained linear ABA poly(ε-caprolactone)-block-poly(styrene)-block-poly(ε-caprolactone) (pCL-b-pS-b-pCL) (12 500 g mol1 ≤ Mn ≤ 33 000 g mol1) and the star-shaped poly(styrene)-block-poly(ε-caprolactone) (Mn = 36 000 g mol1) copolymers were analyzed by size exclusion chromatography (SEC), nuclear magnetic resonance (NMR), infrared (IR) spectroscopy, and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. The focus of the current study is on the detailed characterization of the ABA (star) block polymers via multidimensional chromatographic techniques specifically high performance liquid chromatography coupled to size exclusion chromatography (HPLC-SEC). In particular, we demonstrate the first time separation of poly(ε-caprolactone) (pCL) homopolymer and additionally poly(styrene) (pS) from the ABA poly(ε-caprolactone)-b-poly(styrene)-b-poly(ε-caprolactone) and star-shaped poly(styrene)-b-poly(ε-caprolactone) block copolymer utilizing critical conditions (CC) for pCL with concomitant gradient elution liquid chromatography (GELC).
In recent years mass spectrometric methods, in particular matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) time-of-flight (TOF) mass spectrometry (MS) have become indispensable tools for analyzing molecular masses and mass distributions of polymers. Moreover, these techniques offer the advantage of being able to determine simultaneously chemical functionality, and compositional and topological distributions of polymers. Especially in combination with liquid and/or gas phase separation methods, a comprehensive polymer characterization can be achieved and the difficulties of MS to analyze chemically heterogeneous polymers, copolymers, and samples with broad molecular mass distribution can be overcome.
Systems containing multiple photochromic units possibly display a synergistic interplay of individual switching events and hence potentially give rise to unprecedented photoresponsive behavior. Among such systems photoswitchable foldamers are attractive as the photoisomerization events are coupled to the helixcoil conformational transition. To gain comprehensive insight into the role of the number of switching units (statistics) as well as their specific location and relative orientation in the helix backbone, several series of foldamers have been synthesized and characterized. In these series of foldamers, the local environment of the photoswitchable units was precisely tuned as π,π-stacking interactions were enforced to occur between specific pairs, i.e. azobenzene–azobenzene, azobenzene–tolane, or phenylene–phenylene units. These particular arrangements are reflected not only in the stability of the helical conformation, but also affect the photoresponsive behavior, i.e. the rate of photoisomerization and extent of denaturation. Furthermore, determining the intramolecular spin–spin distance in a series of TEMPO-labeled foldamers with variable chain lengths and different spatial locations of the spin-labels deduced an independent verification of the photoinduced helix–coil transition by ESR spectroscopy. Quantitative analysis of the corresponding ESR spectra shows an excellent correlation of the extent of intramolecular spin–spin coupling and the intensity of the Cotton effect in CD spectroscopy. From all of these results an unusual relationship between the rate of photoisomerization and the extent of photoinduced denaturation could be unraveled, as they are not going hand-in-hand but compete with each other, i.e. the easier the individual switching event is, the harder it becomes to achieve a high degree of unfolding. This insight into the effect of microenvironment on the ease of individual switching events and the role of statistics on the resulting degree of the overall conformational transition is of general interest for the design of multi-switch architectures with improved photoresponse.
Orally ingested nanoparticles may overcome the gastrointestinal barrier, reach the circulatory system, be distributed in the organism and cause adverse health effects. However, ingested nanoparticles have to pass through different physicochemical environments, which may alter their properties before they reach the intestinal cells. In this study, silver nanoparticles are characterised physicochemically during the course of artificial digestion to simulate the biochemical processes occurring during digestion. Their cytotoxicity on intestinal cells was investigated using the Caco-2 cell model. Using field-flow fractionation combined with dynamic light scattering and small-angle X-ray scattering, the authors found that particles only partially aggregate as a result of the digestive process. Cell viabilities were determined by means of CellTiter-Blue® assay, 4',6-diamidino-2-phenylindole-staining and real-time impedance. These measurements reveal small differences between digested and undigested particles (1–100 µg/ml or 1–69 particles/cell). The findings suggest that silver nanoparticles may indeed overcome the gastrointestinal juices in their particulate form without forming large quantities of aggregates. Consequently, the authors presume that the particles can reach the intestinal epithelial cells after ingestion with only a slight reduction in their cytotoxic potential. The study indicates that it is important to determine the impact of body fluids on the nanoparticles of interest to provide a reliable interpretation of their nano-specific cytotoxicity testing in vivo and in vitro.
L-lactide (LA) was polymerized in toluene by means of neat tin(II) 2-ethylhexanoate (SnOct2). Concentration, time and temperature were varied. The isothermally crystallized polyLAs (PLA) were characterized in the virgin state with regard to topology, molar mass, melting temperature (Tm), crystal modification, high or low Tm morphology, crystallinity and crystal thickness. Even a small amount of solvent favored cyclization relative to polymerization in bulk, so that cyclic polylactides were obtained at 115 ◦C and even at 95 ◦C. At all temperatures the α-modification of PLA was obtained along with crystallinities up to 90%. With 6 M solution the high Tm morphology with Tm’s > 190 ◦C was obtained at 115 ◦C. The crystal thickness of crystallites grown from solution at 115 ◦C was on the average 10–20% higher than that of PLA polymerized in bulk. At a polymerization temperature of 75 ◦C cyclization was incomplete and fewer perfect crystallites were formed. A new hypothesis for the crystal growth of cyclic polyLAs is proposed.
For the development of selective and sensitive chemical sensors, we have developed a new family of poly(etherphosphoramide) polymers. These polymers were obtained with satisfactory yields by nucleophilic aromatic polycondensation using isosorbide as green resources, and bisphenol A with two novel difluoro phosphinothioic amide monomers. Unprecedented, the thiophosphorylated aminoheterocycles monomers, functionalized with two heterocyclic amine, N-methylpiperazine and morpholine were successfully obtained by nucleophilic substitution reaction of P(S)–Cl compound. The resulting polymers were characterized by different analytical techniques (NMR, MALDI–ToF MS, GPC, DSC, and ATG). The resulting partially green polymers, having tertiary phosphine sulfide with P–N side chain functionalities along the main chain of polymers are the sensitive film at the surface of a gold electrode for the impedimetric detection of Cd, Ni, Pb and Hg. The bio-based poly(etherphosphoramide) functionalized with N-methylpiperazine modified sensor showed better analytical performance
than petrochemical based polymers for the detection of Ni2+. A detection limit of 50 pM was obtained which is very low compared to the previously published electrochemical sensors for nickel detection.
MALDI-ToF mass spectrometry for the characterization of biobased polymers suitable for depollution
(2022)
In this work, bio-based polymers were synthesized and designed as polymers phases for adsorption of a variety of pollutants from aqueous solutions. The obtained adsorption results showed that the integration of bio-sourced synthons generate attractive hydrophilicity, and wettability of the resulting adsorbent phase. Such polymers phases enable stronger interactions with the organic and inorganic pollutants in water.
A wide variety of different bio-based polymers structures (e. g. poly(ethersulfones, poly(etherphosphines or poly(etherpyridines) with different functional groups were successfully designed in order to adapt chemical structure to different pollutants type and matrice nature.
In order to characterize and to determine the correlation between polymer structures and nature of interaction with pollutants, different analytical technics (NMR, GPC, ATG, DSC, Mass spectrometry) were successfully used. Amongst them, MALDI TOF mass spectrometry plays a superior role since this technique enables the simultaneous determination of the polymer structure, polymer end groups and molecular weight.
This work focusses on the characterization of synthesized polymers. The chemical structure of the repeating unit, the nature of the end groups, as well as the molecular weight, which are all important for the specific interactions of the polymer with the pollutants were clearly determined and suggestions to improve these interactions by changing the synthetic pathway could be given.
L-Lactide was polymerized in bulk at 140 °C with three different cyclic tin catalysts and the time was varied from 1 d up to 14 d. The MALDI TOF spectra confirmed the formation of cyclic polylactides (PLAs) and displayed a characteristic change of peak intensity distribution with formation of a “saw tooth pattern”. This pattern confirms a previous hypothesis that cyclic PLAs tend to form crystallites with extended ring conformation and relatively smooth surface. This type of crystallites is formed under thermodynamic control by transesterification on the surface of the crystallites. In this way PLAs with extraordinarily high melting temperatures (Tm's up to 200.6 °C) and extraordinarily high melting enthalpy were obtained (ΔHm's up to 105 J g−1). These ΔHm values require a revision of the maximum ΔHm value calculated in the literature for ideal PLA crystals.
Ring-expansion polymerizations (REPs) catalyzed by two cyclic tin catalysts(2-stanna-1.3-dioxa-4,5,6,7-dibenzazepine [SnBiph] and
2,2-dibutyl-2-stanna-1,3-dithiolane [DSTL) are performed at 140 °C in bulk.
Small amounts (4 vol%) of chlorobenzene or other solvents are added to facilitate transesterification reactions (ring–ring equilibration) in the solid poly(l-lactide)s. In the mass range up to m/z 13 000 crystalline PLAs displaying a so-called saw-tooth pattern in the MALDI-TOF mass spectra are obtained indicating the formation of extended-ring crystals. The characteristics of extended-ring crystallites and folded-ring crystallites are discussed. Furthermore, extremely high melting temperatures (Tm’s up to 201.2 °C) and melting enthalpies (𝚫Hm’s up to 106 J g−1)) are found confirming that 𝚫Hmmax, the 𝚫Hm of a perfect crystal, is around or above 115 J g−1 in contrast to literature data.
This article reviews the usefulness of tin(II) and tin(IV) salts and compounds as catalysts for the polymerization of lactides. The text is subdivided into nine parts mainly reflecting different polymerization strategies, such as ring-opening polymerization (ROP), ring-expansion polymerization (REP), ROP combined with simultaneous polycondensation (ROPPOC), various catalysts with unknown polymerization mechanisms, and polycondensation of lactic acid. Since the toxicity of tin salts and compounds is a matter of concern and frequently mentioned in numerous publications, the first section deals with facts instead of myths about the toxicity of tin salts and compounds.
Formation Mechanism of a Nano-Ring of Bismuth Cations and Mono-Lacunary Keggin-Type Phosphomolybdate
(2022)
A new hetero-bimetallic polyoxometalate (POM) nano-ring was synthesized in a one-pot procedure. The structure consists of tetrameric units containing four bismuth-substituted monolacunary Keggin anions including distorted [BiO8] cubes. The nano-ring is formed via self-assembly from metal precursors in aqueous acidic medium. The compound (NH4)16[(BiPMo11O39)4] ⋅ 22 H2O; (P4Bi4Mo44) was characterized by single-crystal X-ray diffraction, extended X-ray absorption fine structure spectroscopy (EXAFS), Raman spectroscopy, matrix-assisted laser desorption/ionisation-time of flight mass spectrometry (MALDI-TOF), and thermogravimetry/differential scanning calorimetry mass spectrometry (TG-DSC-MS). The formation of the nano-ring in solution was studied by time-resolved in situ small- and wide-angle X-ray scattering (SAXS/WAXS) and in situ EXAFS measurements at the Mo−K and the Bi−L3 edge indicating a two-step process consisting of condensation of Mo-anions and formation of Bi−Mo-units followed by a rapid self-assembly to yield the final tetrameric ring structure.
In contrast to other cyclic tin bisphenoxides, polymerizations of glycolide and l-lactide with the spirocyclic tin(IV) bis-1,1′-bisnapthoxide yield linear chains having a 1,1′-bisnapthol end group and no cycles. In the case of l-lactide, LA/Cat ratio and temperature are varied and at 160 °C or below, all polylactides mainly consist of even-numbered chains. A total predominance of even-numbered chains is also found for homopolymerization of glycolide, or the copolymerization of glycolide and l-lactide, when conducted <120 °C.
Linear chains having a bisnaphthol end group are again the main reaction products of ring-expansion polymerizations (REP) of 𝝐-caprolactone, but above 150 °C cycles are also formed.
The polycondensations of adipic acid and 1,10-decanediol catalyzed by toluene sulfonic acid (TSA) were reinvestigated using MALDI TOF mass spectrometry and NMR spectroscopy. Unexpected reactions of TSA were detected along with incomplete conversion of the monomers. Furthermore, transesterification reactions of end-capped poly(1,10-decanediol adipate) and end-capped poly(ε-caprolactone) catalyzed by TSA were studied. Despite the quite different (ionic) reaction mechanisms, it was found that for polycondensations performed in bulk intermolecular transesterification is more efficient than the intramolecular “back-biting”; this scenario was not considered in the Jacobson–Stockmayer theory of reversible polycondensations.
These results also confirm that the Jacobson–Stockmayer explanation of reversible polycondensations solely on the basis of ring chain equilibration is not only devoid of any experimental evidence, but also in contradiction to the results elaborated in this work.
Ring-opening polymerizations (ROP) of l-lactide (LA) are conducted in bulk at 130, 160, or 180 °C and are initiated with two different alcohols at 160 °C. The lactide/initiator ratio (LA/In) is varied from 50/1 (20/1 at 180 °C) to 900/1 and the lactide/catalyst ratio (LA/Cat) between 2000/1 and 8000/1. At all temperatures a nearly perfect control of number average molecular weight (Mn) via the LA/In ratio is feasible up to LA/In = 200/1, but at higher ratios the Mn value lags behind the theoretical values and the discrepancy increases with higher LA/Cat ratios. Variation of the LA/Cat ratio influences the formation of cycles but does not significantly influence Mn, when the LA/In ratio is kept constant. The formation of cycles is favored by lower In/Cat ratios and is the main reason for the unsatisfactory control of Mn at high LA/In ratios. The results also suggest that the cycles are mainly or exclusively formed by end-to-end cyclization and not, as believed previously, by back-biting.
Twice recrystallized L-lactide was polymerized with a dozen of different tin or bismuth catalysts in bulk at 160°C for 24 h and was annealed at 150°C afterwards. In two cases Tm values above 197.0°C were obtained. The parameters causing a scattering of the DSC data were studied and discussed. The samples prepared with SnCl2, 2,2-dibutyl-2-stanna-1,3-ditholane (DSTL) or cyclic tin(II) bisphenyldioxide (SnBiph) were subject to annealing programs with variation of time and temperatures, revealing that the Tm´s did not increase. However, an increase of Hm was achieved with maximum values in the range of 93-96 J g-1 corresponding to crystallinities off around 90%. Further studies were performed with once recrystallized L-lactide. Again, those samples directly crystallized from the polymerization process showed the highest Tm values. These data were compared with the equilibrium Tm0 and Hm0 data calculated by several research groups for perfect crystallites. A Tm0 of 213+/-2°C and a Hm0 of 106 J g-1 show the best agreement with the experimental data. The consequences of annealing for the thickness growth of crystallites are discussed on the basis of SAXS measurements. Finally, a comparison of cyclic and linear poly L-lactide)s is discussed.
Ring-opening polymerizations (ROPs) of L-lactide are performed in bulk at 130°C with tin(II) 2-ethylhexanoate as catalyst and various phenols of differentacidity as initiators. Crystalline polylactides having phenyl ester end groups are isolated, which are almost free of cyclics. The dispersities and molecular weights are higher than those obtained from alcohol-initiated ROPs under identical conditions. Polymerizations at 160°C yield higher molecular weights than expected from the monomer/initiator ratio and a considerable fraction of cycles. The fraction of cycles increases with higher reactivity of the ester end group indicating that the cycles are formed by end-to-end cyclization.
The main objective of this work was to design new advanced sorbent phases, alternating copolymers, derived from isosorbide and 2,6-difluorpyridine, to be used for the removal of aromatic organic pollutants present in water at low concentrations. Six different monomers, dianhydrohexitols isomers and bisphenol derivatives, weresynthesized in order to make it possible to study their hydrophilic and hydrophobiceffect on the sorption efficiency of the resulting polymeric phases. Before this study, we have confirmed the chemicals structures, molecular weights, and thermal properties of the obtained polymeric phases. Sorption results show a higher adsorption efficiency of P6 co-poly(ether-pyridine) based on bisphenol substituted with pyridineunits, for all tested pollutants, hydrophobic and hydrophilic ones, due to its less compact structure. Two aromatic organic pollutants, p-hydroxybenzoic acid and toluicacid, were selected as sorbates to study the adsorption characteristic, kinetics and isotherms of the copoly(ether pyridine) P6. Langmuir model led to a better fitting of the sorption isotherms; the sorption of toluic acid is easier than of that p-hydroxybenzoic acid. Comparing 1/n values for benzoic acid was two time lower for P6 compared to that for biochar and for crosslinked methacrylate resin, showing a higher efficiency
L-Lactide was polymerized with tin(II)acetate, tin(II)2-ethyl hexanoate, diphenyltin dichloride and dibutyltin bis(pentafluorophenoxide) at 130 °C in bulk. When an alcohol was added as initiator, linear chains free of cycles were formed having a degree of polymerization (DP) according to the lactide/initiator (LA/In) ratio.
Analogous polymerizations in the absence of an initiator yielded high molar mass cyclic polylactides.
Quite similar results were obtained when ε-caprolactone was polymerized with or without initiator.
Several transesterification experiments were conducted at 130 °C, either with polylactide or poly(ε-caprolactone) indicating that several transesterification mechanisms are operating under conditions that do not include formation of cycles by back-biting. Furthermore, reversible polycondensations (revPOCs) with low or moderate conversions were found that did not involve any kind of cyclization. Therefore, These results demonstrate the existence of revPOCs, which do neither obey the theory of irreversible polycondensation as defined by Flory nor the hypothesis of revPOCs as defined by Jacobson and Stockmayer. A new concept encompassing any kind of revPOCs is formulated in the form of a “polycondensation triangle”.
One series of BiSub-catalyzed ring-opening polymerizations (ROPs) is per-formed at 160 °C for 3 days with addition of difunctional cocatalysts to find out, if poly(l-lactide) crystallizes directly from the reaction mixture. An analogous series is performed with monofunctional cocatalysts. High Tm crystal-lites (Tm > 190 °C) are obtained from all bifunctional cocatalysts, but not from all monofunctional ones. It is shown by means of SAXS measurements that the high Tm values are mainly a consequence of a transesterification–homogenization process across the lamellar surfaces resulting in thickness and smoothing of the surfaces. An unusual enthalpy-driven modification of the molecular weight distribution is found for samples that have crystallized during the polymerization. A third series of ROPs is performed at 170 °C for 2 h followed by annealing at 120 °C (2 h) to induce crystallization. Complete transformation of the resulting low Tm crystallites (Tm < 180 °C) into the high Tm crystallites by annealing at 170 °C for 1 d is not achieved, despite variation of the cocatalyst.
Die MALDI-TOF Massenspektrometrie stellt eine wichtige Methode zur simultanen Bestimmung von Molmassen, Molmassenverteilungen und Endgruppen von synthetischen Polymeren dar. In dem Vortrag wird gezeigt, welche zusätzlichen Informationen über die Art der Polymerisation, der Nebenreaktionen und der Topologie der gebildeten hochmolekularen Polylaktid-Homo- und Copolymere im Verlauf ihrer Synthese erhalten werden können.
Polymerizations of L-lactide catalyzed either by neat SnCl2 or by SnCl2 + difunctional cocatalysts were conducted in bulk at 180, 160 and 140 °C with variation of the Lac/Cat ratio and time. With neat SnCl2 poly(L-lactide) having weight average molecular weights (uncorrected Mw’s) up to 190 000 g mol−1 were obtained mainly consisting of linear chains. Addition of salicylic acid or 1,1-bisphenol yielded a higher fraction of cyclic polylactides but lower molecular weights. Furthermore, SnCl2 was compared with Bu2SnCl2 and various other metal chlorides and the best results were obtained with SnCl2. With ethyl L-lactate as initiator SnCl2-catalyzed ROPs were performed at 120 °C and the lac/initiator ratio was varied. All these experiments were conducted under conditions allowing for comparison with ROPs catalyzed with neat Sn(II)-2-ethyhexanoate. Such a comparison was also performed with ε-caprolactone as monomer.
L-Lactide was polymerized by means of neat SnOct2 with variation of LA/Cat ratio, temperature and time. The resulting cyclic polylactides crystallized spontaneously at 160 °C or below, but needed nucleation via mechanical stress at 165 or 170 °C. All the crystalline polylactides obtained directly from ROP above 120 °C had melting temperatures (Tm) above 189 °C (up to 194.5 °C). SnOct2 also enabled transformation of low Tm poly(L-lactide)s (Tm <180 °C) into the high Tm m1odification by annealing, due to the impact of transesterification reactions in the interphase between the crystallites. The influence of crystallization temperature and annealing time on the crystal thickness was studied via SAXS measurements. A comparison with the crystallization and annealing experiments reported by Pennings and coworkers and Tsuji and Ikada is discussed, and a satisfactory agreement has been found, because those authors also studied polyLA samples containing SnOct2 in its active form. It is also demonstrated in this work that the high Tm modification cannot be obtained when the catalyst is removed or poisoned as it is true for commercial polylactides.
SnOct2 (Sn(II) 2-ethylhexanoate) catalyzed ROPs of L-lactide were performed in bulk with eight different alcohols as initiators. The time was varied between 1 h and 24 h for all initiators. For two initiators the temperature was also lowered to 115 ◦C. Even-numbered chains were predominantly formed in all polymerizations at short times, but the rate of transesterification (e.g. even/odd equilibration) and the molecular weight distribution were found to depend significantly on the nature of the initiator. Observed transesterification reactions also continued in solid poly (L-lactide), and with the most active initiator, almost total equilibration was achieved even at 130 ◦C. This means that all chains including those of the crystallites were involved in transesterification reactions proceeding across the flat surfaces of the crystallites. The more or less equilibrated crystalline polylactides were characterized by DSC and SAXS measurements with regard to their melting temperature (Tm), crystallinity and crystal thickness.
In this work, four triazole-based poly(ether-pyridine)s polymers were synthesized and used as an adsorbent for the removal of phenolic compounds from aqueous solutions. For this purpose, new fluoromonomers containing 1,2,3-triazole units were prepared by the Cu(I)-catalyzed 1,3-dipolar cycloaddition reaction and then used for the elaboration of novel poly(ether-pyridine-triazole)s (PEPTs) by direct polyconden-sation with isosorbide and bisphenol A. Chemical structure of fluorinated pyridinicmonomers as well as resulting polymers was confirmed by 1H and 19F NMR spectroscopic methods. The thermal behavior of the obtained PEPTs was characterized using differential scanning calorimetry and thermogravimetric analysis. Results of sorption showed that polymers can be effectively used as a sorbent for the removal of polarorganic pollutants. The isosorbide-based poly(ether-pyridine-triazole) which contains hydrophilic hydroxyl groups as pendants chains (P4) exhibited the highest sorption efficiencies (78%–100% after 1 h). In order to explain the results an adsorption mech-anism mainly based on π–π interactions and hydrogen bonding with the pendent groups is proposed.
In the framework of the development of green analytical chemistry, a silica gel (SG) coated with a semi-penetrating network based on the partially biosourced poly(ethersulfone) is studied for a greener extraction process of aromatic organic pollutants. An optimized composition of the semi-penetrating network (80% of the linear polymer (LP): isosorbide-based poly(ethersulfone) and 20% cross-linking agent (XP) type bismaleimide) leads to a total adsorption of the selected aromatic pollutants, whatever their hydrophilicity. Adsorption characteristic, kinetics and isotherms of the SG-semi-INP LP80/XP20 for p-hydroxybenzoic acid and for toluic acid were studied. Langmuir model led to a better ftting of the adsorption isotherms; the adsorption of toluic acid is easier than that of p-hydroxybenzoic acid. 1/n values of benzoic acid was lower for SGsemi-INP LP80/XP20 compared to biochar and to cross-linked methacrylate resin, showing a higher adsorption efciency.
Alcohol-initiated ring-opening polymerizations (ROPs) of L-Lactide (LA) were conducted in bulk at 130 °C catalyzed by Zn acetate (ZnAc2), lactate (ZnLac2), or caprylate (ZnCap2). 11-undecenol (UND), 1-hydroxymethylnaphtalene (HMN), and 4-nitrobenzylalcohol (4NB) were used as alcohols. Whereas variation of the alcohols had little effect, the usefulness of the catalysts increased in the order: acetate < lactate < caprylate. Hence, further alcohol-initiated polymerizations were performed with ZnCap2 alone and with variation of the lactide/catalyst (LA/Cat) ratio. With increasing LA/Cat ratio higher fractions of cyclic poly(L-lactide) (PLA) were found, so that the measured degree of polymerization (DP) is considerably lower than the theoretical value (i.e., 2 x LA/alcohol). With neat ZnCap2 cyclic PLAs were the largely prevailing reaction products. For these cyclic PLAs weight average molar masses (Mw) up to 134,000 were obtained and an optical purity around 99% was indicated by 13C NMR spectroscopy and DSC measurements even after 48 h at 150 or 160 °C.
Alcohol-initiated ring-opening polymerizations (ROP) of L-lactide (LA) were studied in solution at 70 °C, whereupon the nature of the alcohol, the LA/initiator ratio, the LA/SnOct2 ratio and the time were varied. In contrast to literature, neat SnOct2 is catalytically active in THF and several aromatic but donor solvents, such as 1,3-dioxolane, dimethylformamide (DMF) or N-methyl pyrrolidone (NMP), strongly reduce the activity of SnOct2. In agreement with literature, no cycles were formed by neat SnOct2 at 70 °C in toluene, whereas almost complete cyclization occurs at 115 °C. This finding is attributed to strongly reduced mobility of the initially formed linear chains having one Sn-O-CH and one anhydride end group. Due to better solvation and enhanced mobility cyclization occurs in THF at 70 °C.
Polyesters of bisphenol-A, bisphenol-P, catechol, and sebacic acid are prepared and different synthetic methods are compared. The diphenols are condensed with sebacoyl chloride either in dichloromethane/pyridine or in refl uxing chlorobenzene without HCl-acceptor. Further- more, bisphenol-A acetate is polycondensed with sebacic acid in bulk. All experiments are worked up so that fractionation is avoided. The extent of cyclization is estimated by matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) and the molar mass distribution by size exclusion chromatography (SEC). Polycondensations in solution yield larger fractions of cyclics and higher dispersities (up to 11). Polycondensations in bulk give lower fractions of cycles and dispersities from 4.6 to 6.3 for high molar mass polyesters or 2.8 to 3.5 for low molar mass products. Characteristic curves describing the dependence of the dispersity on the initial monomer concentration are elaborated.
Pollen represent one major cause for human allergic diseases. Currently the characterization and identification of pollen is time-consuming since it mainly relies on the microscopic determination of the genus-specific pollen morphology. A variety of new analytical approaches, like Raman - and fluorescence spectroscopy have been proposed in order to develop fast and reliable pollen identification. Recently matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was initially applied for the rapid investigation of such complex biological samples. Taxonomic differences and relations of single pollen grains could be identified.
Both commercially available lyophilized pollen and fresh pollen acquired from biological samples collected in parks and in the Botanical Garden Berlin in the years 2013-2015 were investigated. The samples were prepared by formic acid extraction in the gas phase and spotted with HCCA matrix. A variety of new approaches, like a newly developed MALDI target with micrometer sample spots sizes, were tested for their suitability. The obtained mass spectral data were investigated by principal component analysis (PCA).
The applicability of MALDI-TOF mass spectrometry for the classification of pollen according to their taxonomic relationships was proven. Specifically, chemical differences in the mass spectra at the levels of plant order, genus and in many cases even of species could be identified. Based on these results, further investigations have been undertaken to optimize the sample preparation for the classification of single pollen grains in mixtures of pollens. A novel MALDI-target design was developed to enhance the phenotypic information of pollens in their mass spectra. The combination of mass spectral patterns and multivariate statistics provide a powerful tool for the investigation of structural correlations within mixtures. The results can be used to improve the reconstruction of taxonomic relations of single species in various mixtures and might be useful for the development of a fast routine method to identify pollen based on mass spectrometry.
Poly(ester urethane)s derived from lactide, isosorbide, terephthalic acidm abd various diisocyanates
(2014)
Isosorbide-initiated oligomerizations of ʟ-lactide were preformed in bulk using SnCl2 as catalyst. The resulting telechelic OH-terminated oligoesters were in situ subjected to simultaneous polycondensation and polyaddition with mixtures of terephthaloyl chloride and diisocyanates. Most polymerizations were conducted with 4,4'-diisocyanatodiphenyl methane and 2,4-diisocyanato toluene. The consequences of excess diisocyanate and four different catalysts were studied. The isosorbide/lactide ratio and the terephthalic acid/diisocyanate ratio were varied. Number average molecular weights up to 15 kDa with polydispersities around 3–5 were obtained. Depending on the chemical structure of the copolyester and on the feed ratio, incorporation of urethane groups may reduce or enhance the glass-transition temperature, but the thermal stability decreases dramatically regardless of composition.
RATIONALE
Polymer sample spots for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) prepared by the dried-droplet method often reveal ring formation accompanied by possible segregation of matrix and sample molecules as well as of the polymer homologs itself. Since the majority of sample spots are prepared by this simple and fast method, a matrix or sample preparation method that excludes such segregation has to be found.
METHODS
Three different ionic liquid matrices based on conventionally used aromatic compounds for MALDI-TOF MS were prepared. The formation of ionic liquids was proven by 1H NMR spectroscopy. MALDI-Imaging mass spectrometry was applied to monitor the homogeneity.
RESULTS
Our results show a superior sample spot homogeneity using ionic liquid matrices. Spots could be sampled several times without visible differences in the mass spectra. A frequently observed loss of matrix in the mass spectrometer vacuum was not observed. The necessary laser irradiance was reduced, which resulted in less polymer fragmentation.
CONCLUSIONS
Ionic liquid matrices can be used to overcome segregation, a typical drawback of conventional MALDI dried-droplet preparations. Homogeneous sample spots are easy to prepare, stable in the MS vacuum and, thereby, improve the reproducibility of MALDI.
Polyelectrolytes such as PAA and its salts are widely used, but are notoriously difficult to characterize due to their polyelectrolyte properties and broad molecular mass distributions. In this paper, we report on a new PAA analysis by combining asymmetrical flow field-flow fractionation and an advanced SAXS technique using an acoustic levitator to minimize background scattering. The proof-of-principle is demonstrated with a mixture of three standard PAAs with different molecular masses. Detailed information on the PAA fractions is available on radii of gyration, polymer contour lengths, and coil conformation. Our method is expected to be applicable for a wide range of water-soluble synthetic and natural polymers and ideal for molecular masses of 5 × 103–2 × 105 g · mol-1.
PVP-block-PVAc block copolymers were synthesized by controlled radical polymerization applying a RAFT/MADIX system and were investigated by HPLC and by coupling of chromatography to FT-IR spectroscopy and MALDI-TOF MS. Chromatographic methods (LACCC and gradient techniques) were developed that allowed a separation of block copolymers according to their repeating units. The results of the spectroscopic and spectrometric analysis clearly showed transfer between radicals and process solvent. With the use of hyphenated techniques differences between main and side products were detected. In agreement with previously published results, obtained by NMR, SEC, static light scattering and MALDI-TOF MS, our data proved a non-ideal RAFT polymerization.
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.
New amphiphilic block copolymers consisting of N-vinyl pyrrolidone and vinyl acetate were synthesized via controlled radical polymerization using a reversible addition/fragmentation chain transfer (RAFT)/macromolecular design via the interchange of xanthates (MADIX) system. The synthesis was carried out in 1,4-dioxane as process solvent. In order to get conclusions on the mechanism of the polymerization the molecular structure of formed copolymers was analysed by means of different analytical techniques. 13C NMR spectroscopy was used for the determination of the monomer ratios. End groups were analysed by means of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. This technique was also used to determine possible fragmentations of the RAFT end groups. By means of a combination of size exclusion chromatography, 13C NMR and static light scattering molar mass distributions and absolute molar masses could be analysed. The results clearly show a non-ideal RAFT mechanism.
Molecular masses and end groups of polystyrene (PS) formed in a novel container-less polymerization strategy, based on levitated droplets in an acoustic trap, were determined by Gel Permeation Chromatography (GPC) and Matrix-assisted Laser Desorption/Ionization Time of Flight Mass spectrometry (MALDI-TOF MS).
We report on a hyphenated polymer analysis method consisting of asymmetrical flow field-flow fractionation (A4F) coupled online with small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS). A mixture of six poly(styrene sulfonate)s with molar masses in the range of 6.5 × 103 to 1.0 × 106 g mol-1 was used as a model system for polyelectrolytes in aqueous solutions with a broad molar mass distribution. A complete polymer separation and analysis was performed in 60 min. Detailed information for all polymer fractions are available on i) the radii of gyration, which were determined from the SAXS data interpretation in terms of the Debye model (Gaussian chains), and ii) the diffusion coefficients (from DLS). We recommend using the A4F-SAXS-DLS coupling as a possible new reference method for the detailed analysis of complex polymer mixtures. Advantages of the use of SAXS are seen in comparison to static light scattering for polymers with radii of gyration smaller then 15 nm, for which only SAXS produces precise analytical results on the size of the polymers in solution.
Four classes of unsaturated copolyesters of L-lactide were prepared either from isosorbide or bis(hydroxymethyl)tricyclodecane in combination with fumaric acid or from 1,4-butenediol or 1,4- butynediol with terephthalic acid. All syntheses were performed in such a way that lactide was oligomerized with a diol as the initiator and the resulting oligomers were polycondensed with a dicarboxylic acid dichloride either in a one-pot synthesis or in a two-step procedure. For most copolyesters the SEC measurements gave weight average molecular weights in the range of 30–60 kg mol⁻1 and dispersities in the range of 4.2–6.2. The MALDI-TOF mass spectra displayed a high content of cycles and indicated an irreversible kinetic course of all polycondensations. Glass-transition temperatures (Tg) above 90 °C were only found for two copolyesters of isosorbide. Addition of bromine to copolyesters of 1,4-butenediol yielded flame retarding biodegradable polymers.
A reversed phase high performance liquid chromatography coupled to an inductively coupled Plasma mass spectrometer (HPLC–ICP-MS) approach in combination with isotope dilution analysis (IDA) for the separation and parallel quantification of nanostructured and ionic silver (Ag) is presented. The main Focus of this work was the determination of the ionic Ag concentration. For a sufficient stabilization of the Ions without dissolving the nanoparticles (NPs), the eluent had to be initially optimized. The determined Ag ion concentration was in a good agreement with results obtained using ultrafiltration. Further, the mechanism of the NP separation in the HPLC column was investigated. Typical size exclusion effects were found by comparing results from columns with different pore sizes. Since the recovery rates decreased with increasing Ag NP size and large Ag NPs did not elute from the column, additional interactions of the particles with the stationary phase were assumed. Our results reveal that the presented method is not only applicable to Ag NPs, but also to gold and polystyrene NPs. Finally, IDA-HPLC-ICP-MS experiments in single particle mode were performed to determine the particle cut-off size. The comparison with conventional spICP-MS experiments resulted in a similar diameter and particle size distribution.
The protein corona, which forms on the nanoparticle's surface in most biological media, determines the nanoparticles physicochemical characteristics. The formation of the protein corona has a significant impact on the biodistribution and clearance of nanoparticles in vivo. Therefore, the ability to influence the formation of the protein corona is essential to most biomedical applications, including drug delivery and imaging. In this study, we investigate the protein adsorption on nanoparticles with a hydrodynamic radius of 30 nm and a coating of thermoresponsive poly(2-isopropyl-2-oxazoline) in serum. Using multiangle dynamic light scattering (DLS) we demonstrate that heating of the nanoparticles above their phase separation temperature induces the formation of agglomerates, with a hydrodynamic radius of 1 µm. In serum, noticeably stronger agglomeration occurs at lower temperatures compared to serum-free conditions. Cryogenic transmission electron microscopy (cryo-TEM) revealed a high packing density of agglomerates when serum was not present. In contrast, in the presence of serum, agglomerated nanoparticles were loosely packed, indicating that proteins are intercalated between them. Moreover, an increase in protein content is observed upon heating, confirming that protein adsorption is induced by the alteration of the surface during phase separation. After cooling and switching the surface back, most of the agglomerates were dissolved and the main fraction returned to the original size of approximately 30 nm as shown by asymmetrical flow-field flow fractionation (AF-FFF) and DLS. Furthermore, the amounts of adsorbed proteins are similar before and after heating the nanoparticles to above their phase-separation temperature. Overall, our results demonstrate that the thermoresponsivity of the polymer coating enables turning the corona formation on nanoparticles on and off in situ. As the local heating of body areas can be easily done in vivo, the thermoresponsive coating could potentially be used to induce the agglomeration of nanoparticles and proteins and the accumulation of nanoparticles in a targeted body region.
Thin coatings of poly(acrylic acid) (PAA) and poly(hydroxyethylmethacrylate) (PHEMA) were deposited onto carbon fibers by means of the electrospray ionization (ESI) technique in ambient air. These high-molecular weight polymer layers were used as adhesion promoters in carbon fiberepoxy resin composites. Within the ESI process, the carbon fibers were completely enwrapped with polymer in the upper 10 plies of a carbon fiber roving. As identified with scanning electron microscopy also shadowed fibers in a bundle as well as backsides of fiber rovings were pinhole-free coated with polymers (electrophoretic effect'). Under the conditions used, the layers have a granular structure. Residual solvent was absent in the deposit. PAA and PHEMA films did not show any changes in composition and structure in comparison with the original polymers as analyzed by X-ray photo-electron spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Single-fiber pullout tests of coated fibers embedded in epoxy resin showed significantly increased interfacial shear strength. It is assumed that chemical bonds between carbon fiber poly(acrylic acid) and epoxy resin contribute significantly to the improved interactions.
Rationale
The ionization of polystyrenes in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is typically achieved by the use of silver salts. Since silver salts can cause severe problems, such as cluster formation, fragmentation of polymer chains and end group cleavage, their substitution by alkali salts is highly desirable.
Methods
The influence of various cations (Ag+, Cs+ and Rb+) on the MALDI process of polystyrene (PS) mixtures and high mass polystyrenes was examined. The sample preparation was kept as straightforward as possible. Consequently, no recrystallization or other cleaning procedures were applied.
Results
The investigation of a polystyrene mixture showed that higher molecular polystyrenes could be more easily ionized using caesium, rather than rubidium or silver salts. In combination with the use of DCTB as matrix a high-mass polymer analysis could be achieved, which was demonstrated by the detection of a 1.1 MDa PS.
Conclusions
A fast, simple and robust MALDI sample preparation method for the analysis of ultra-high molecular weight polystyrenes based on the use of DCTB and caesium salts has been presented. The suitability of the presented method has been validated by using different mass spectrometers and detectors.
In this work, sample losses of silver nanoparticles (Ag NPs) in asymmetrical flow field-flow fractionation (AF4) have been systematically investigated with the main focus on instrumental conditions like focusing and cross-flow parameters as well as sample concentration and buffer composition. Special attention was drawn to the AF4 membrane. For monitoring possible silver depositions on the membrane, imaging laser ablation coupled to inductively coupled plasma mass spectrometry (LA-ICP-MS) was used. Our results show that the sample residue on the membrane was below 0.6% of the total injected amount and therefore could be almost completely avoided at low sample concentrations and optimized conditions. By investigation of the AF4 flows using inductively coupled plasma mass spectrometry (ICP-MS), we found the recovery rate in the detector flow under optimized conditions to be nearly 90%, while the cross-flow, slot-outlet flow and purge flow showed negligible amounts of under 0.5%. The analysis of an aqueous ionic Ag standard solution resulted in recovery rates of over 6% and the ionic Ag content in the sample was found to be nearly 8%. Therefore, we were able to indicate the ionic Ag content as the most important source of sample loss in this study.
Poly(alkylene isophthalate)s were prepared by different methods, either in solution or in bulk. The SEC measurements were evaluated in such a way that all oligomers were included. In solution (monomer conc. 0.10.7 mol/L) large fractions of rings were formed and high dispersities (up to 12) were obtained, which disagree with theoretical predictions. Polycondensations in bulk did neither generate cyclics by 'back-biting' nor by end-to-end cyclization, when the maximum temperature was limited to 210 °C. The dispersities of these perfectly linear polyesters were again higher than the theoretical values. Regardless of the synthetic method monomeric cycles were never observed. Furthermore, SEC measurements performed in tetrahydrofuran and in chloroform and SEC measurements performed in three different institutes were compared. Finally, SEC measurements of five samples were performed with universal calibration and a correction factor of 0.71 ± 0.02 was found for normal calibration with polystyrene.
Characterization of randomly branched polymers utilizing liquid chromatography and mass spectrometry
(2014)
Branching in polymers is an important way to modify the materials properties, however the characterization of random branching in polymeric materials is a challenge in polymer analysis. In this work, liquid adsorption chromatography methods are developed for a commercially available hyperbranched polyester (Boltorn™). This Chromatographie techniques was then coupled to offline MALDI-TOF MS analysis, a first in the analysis of randomly branched polymers. The coupling of these two techniques provides superior MALDI-TOF spectra, enabling the easy identification of structural subdistributons based on theoretical molecular weight. Detailed analysis of the MALDI-TOF spectra shows that these Chromatographie conditions separate cyclic Boltorn polymers (with no core molecule) from non-cyclic polymers (with core molecule), and these are the only two architectures observed. MALDI MS also confirms that the Chromatographie Separation mode is adsorption, but further analysis is needed to determine if there is a Separation by degree of branching.
A reversed phase high performance liquid chromatography coupled to an inductively coupled plasma mass spectrometer (HPLC-ICP-MS) approach in combination with isotope dilution analysis (IDA) for the separation and parallel quantification of nanostructured and ionic silver (Ag) is presented. The main focus of this work was the determination of the ionic Ag concentration. For a sufficient stabilization of the ions without dissolving the nanoparticles (NPs), the eluent had to be initially optimized. The determined Ag ion concentration was in a good agreement with results obtained using ultrafiltration. Further, the mechanism of the NP separation in the HPLC column was investigated. Typical size exclusion effects were found by comparing results from columns with different pore sizes. Since the recovery rates decreased with increasing Ag NP size and large Ag NPs did not elute from the column, additional interactions of the particles with the stationary phase were assumed. Our results reveal that the presented method is not only applicable to Ag NPs, but also to gold and polystyrene NPs. Finally, IDA-HPLC-ICP-MS experiments in single particle mode were performed to determine the particle cut-off size. The comparison with conventional spICP-MS experiments resulted in a similar diameter and particle size distribution.
Designing artificial macromolecules with absolute sequence order represents a considerable challenge. Here we report an advanced light-induced avenue to monodisperse sequencedefined functional linear macromolecules up to decamers via a unique photochemical approach. The versatility of the synthetic strategy—combining sequential and modular concepts—enables the synthesis of perfect macromolecules varying in chemical constitution and topology. Specific functions are placed at arbitrary positions along the chain via the successive addition of monomer units and blocks, leading to a library of functional homopolymers, alternating copolymers and block copolymers. The in-depth characterization of each sequence-defined chain confirms the precision nature of the macromolecules. Decoding of the functional information contained in the molecular structure is achieved via tandem mass spectrometry without recourse to their synthetic history, showing that the sequence information can be read.We submit that the presented photochemical strategy is a viable and advanced concept for coding individual monomer units along a macromolecular chain.
Meso-Lactide was polymerized in bulk at 60, 80, and 100 °C by means of three different types of catalysts: dibutyltinsulfides (2,2-dibutyl-2-stanna-1,3-dithiolane and 2,20-dibutyl-2-stanna-1,3-dithiane), dibutyltin derivatives of substituted cate-chols (BuCa, CyCa, and BzCa), and dibutyltin derivatives of2,2 dihydroxybiphenyl (SnBi) and 2,2-dihydroxy-1,10-binaphthyl(SnNa. Only the latter two catalysts were active at 60 °C. The architecture of the resulting polylactides depends very much on the structure of the catalyst and on the temperature. At the lowest temperature (60 °C), SnBi and SnNa mainly yielded even-numbered linear chains, but SnNa also yielded even-numbered cycles at 100 °C and short reaction times. In contrast,BuCa, CyCa, and BzCa mainly yielded odd-numbered cycles, although the same catalysts yielded even-numbered linear chains when benzylalcohol was added.
We will report on a new kind of ring-expansion polymerization (REP) of cyclic esters based of three classes of catalysts.
In contrast to REP described previously these new polymerizations are characterized by a self-extrusion of the catalysts resulting in cyclic homopolyesters as the only reaction product. These REP of L-lactide were performed in bulk at temperatures between 100 and 180°C without racemization. A simplified scheme of a REP catalyzed by BuCa or SnBi is presented below. With meso-lactide temperatures down to 60°C were realized. Depending on catalyst, time and temperature with equal quantities of even and odd-numbered cycles a strong predominance of either even or odd-numbered cycles was found.
Eine neue Art einer Ring-Expansion Polymerization (REP) von zyklischen Polylaktiden mittels neuer Katalysatoren wird präsentiert.
MALDI-TOF Massenspektrometrie und andere analytische Techniken wurden zur Aufklärung des Mechanismus eingesetzt. Dabei zeigte sich, dass im Gegensatz zu anderen REP, ausschließlich zyklische Polymere gebildet wurden. Diese stellen neue Kandidatenmaterialien für zukünftige neue CRM dar.
Two new catalysts (SnNa and SnBi) were prepared from dibutyltin oxide and 2,2′-dihydroxybiphenyl or2,2′dihydroxy(1,1′-binaphtyl). These catalysts enabled rapid polymerizations of L-lactide at 160 or 180 °C in bulk, whereby almost exclusively cyclic polylactides were formed. These polymerizations were free of racemization and yielded pol(L-lactide)s having weight average molecular weights (Mw's) up to 140 000 g mol−1. The Mw's varied little with the Lac/Cat ratio as expected for a ring expansion polymerization (REP). Polymerizations performed in bulk at 140, 120 and 102 °C yielded cyclic polylactides with lower molecular weights. At 102 °C a strong predominance of even-numbered cycles was found with SnNa as catalyst. SnNa can also catalyze alcohol-initiated ROPs yielding linear poly(L-lactide) free of cyclics.
Unprecedented Ionization Processes in Mass SpectrometryProvide Missing Link between ESI and MALDI
(2018)
In the field of mass spectrometry,producing intact, highly-charged protein ions from surfaces is a conundrum with significant potential payoff in application areas ranging from bio-medical to clinical research. Here, we report on the ability to form intact, highly-charged protein ions on high vacuum time-of-flight mass spectrometers in the linear and reflectron modes achievable using experimental conditions that allow effective matrix removal from both the sample surfaces and from the charged clusters formed by the laser Ablation event. The charge states are the highest reported on high vacuum mass spectrometers, yet they remain at only around athird of the highest charge obtained using laser ablation with a suitable matrix at atmospheric pressure. Other than physical instrument modifications, the key to forming abundant and stable highly-charged ions appears to be the volatility of the matrix used. Cumulative results suggest mechanistic links between the ionization process reported here and traditional ionization methods of electrospray ionization and matrix-assisted laser desorp-tion/ionization.
Multivariate analysis of MALDI imaging mass spectrometry data of mixtures of single pollen grains
(2018)
Mixtures of pollen grains of three different species (Corylus avellana, Alnus cordata, and Pinus sylvestris) were investigated by matrixassisted laser desorption/ionization time-of-flight imaging mass spectrometry (MALDI-TOF imaging MS). The amount of pollen grains was reduced stepwise from > 10 to single pollen grains. For sample pretreatment, we modified a previously applied approach, where any additional extraction steps were omitted.
Our results show that characteristic pollen MALDI mass spectra can be obtained from a single pollen grain, which is the prerequisite for a reliable pollen classification in practical applications. MALDI imaging of laterally resolved pollen grains provides additional information by reducing the complexity of the MS spectra of mixtures, where frequently peak discrimination is observed. Combined with multivariate statistical analyses, such as principal component analysis (PCA), our approach offers the chance for a fast and reliable identification of individual pollen grains by mass spectrometry.
The effects of weathering exposure on unfilled and filled thermoplastic polyurethanes (TPU) materials are described as performed under different humidity conditions. For this purpose, a weathering device was used with UV-A 340 nm lamps at a constant temperature of 40 °C. The effects of environmental (UV and humidity condition) degradation on the frictional properties of TPU materials are presented along with surface analyses to characterize the chemistry of the degradative process. Photooxidative degradation of unfilled polymer leads to deterioration of physical and mechanical properties, which affects its tribological behavior significantly. Due to crosslinking, the stiffness of the material increases, reducing drastically the friction coefficient of unfilled TPUs. The frictional behavior of glass fiber reinforced TPU is less affected by radiation.
The structure and composition of polycarbonate polydimethylsiloxane copolymer (PC-co-PDMS) was investigated by applying various analytical approaches including chromatographic separation methods, spectrometric, and spectroscopic detection techniques. In particular, size exclusion chromatography (SEC) and liquid adsorption chromatography operating at different conditions (e.g. using gradient solvent systems) were used to achieve separations according to molar mass and functionality distribution. The coupling of both techniques resulted in fingerprint two-dimensional plots, which could be used to easily compare different copolymer batches. Matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry was applied for structural investigations. The different ionization behavior of both comonomers, however, strongly limited the applicability of this technique. In contrast to that, Fourier-transform Infrared (FTIR) spectroscopy could be used to quantify the amount of PDMS in the copolymer at different points in the chromatogram. The resulting methodology was capable of distinguishing PC-co-PDMS copolymer from PC homopolymer chains present in the material.
Two sets of polystyrene nanoparticles (PSNPs) with comparable core sizes but different carboxyl group densities were made and separated using asymmetric flow field flow fractionation (AF4), capillary electrophoresis (CE), and the off-line hyphenation of both methods. Our results revealed the significant potential of two-dimensional off-line AF4-CE hyphenation to improve the separation and demonstrated for the first time, the applicability of CE to determine the functional group density of nanoparticles (NPs). Compared to the result acquired with conductometric titration, the result obtained with synthesized 100 nm sized PSNPs revealed only a slight deviation of 1.7%. Commercial 100 nm sized PSNPs yielded a deviation of 4.6 %. For 60 nm sized PSNPs, a larger deviation of 10.6 % between both methods was observed, which is attributed to the lower separation resolution.
L-lactide was polymerized in bulk at 120, 140, 160 and 180°C with neat tin(II) 2-ethylhexanoate (SnOct2) as catalyst. At 180°C the Lac/Cat ratio was varied from 25/1 up to 8 000/1 and at 160°C from 25/1 up to 6 000/1. The vast majority of the resulting polylactides consist of cycles in combination with a small fraction of linear chains having one octanoate and one COOH end group. The linear chains almost vanished at high Lac/Cat ratios, as evidenced by MALDI-TOF mass spectrometry and measurements of intrinsic viscosities and dn/dc values. At Lac/Cat ratios <1000/1 the number average molar masses (Mn) are far higher than expected for stoichiometic initiation, and above 400/1 the molar masses vary relatively little with the Lac/Cat ratio. At 180° slight discoloration even at short times and degradation of the molar masses were observed, but at 160°C or below colorless products with weight average molar masses (Mw) up to 310 000 g mol-1 were obtained. The formation of high molar mass cyclic polylactides is explained by a ROPPOC (Ring-Opening Polymerizatiom with simultaneous Polycondensation) mechanism with intermediate formation of linear chains having one Sn-O-CH end group and one mixed anhydride end group. Additional experiments with tin(II)acetate as catalyst confirm this interpretation. These findings together with the detection of several transesterification mechanisms confirm previous critique of the Jacobson-Stockmayer theory.
A new concept called “Ring-Opening Polymerization (ROP) combined with simultaneous POlyCondensation” (ROPPOC) is presented and discussed. This synthetic strategy is based on the intermediate formation of chains having two end groups that can react with each other. The ROPPOC syntheses are subdivided into three groups according to the nature of the chain ends: two ionic end groups, one ionic and one covalent chain end and a combination of two reactive covalent end groups may be involved, depending on the catalyst. The usefulness for the preparation of cyclic polymers is discussed with a review of numerous previously published examples. These examples concern to following classes of cyclic polymers: polypeptides, polyamides, polyesters, including polycarbonates, and cyclic polysiloxanes. It is demonstrated, that the results of certain ROPPOC syntheses are in contradiction to the Jacobson-Stockmayer theory. Finally, the usefulness of ROPPOCs for the detection of polydisperse catenanes is discussed.
In addition to molar mass distribution (MMD) synthetic polymers often exhibit an additional chemical heterogeneity distribution, expressed by different end groups and other structural variations (e.g. tacticity, copolymer composition etc.). Ionization in MALDI MS is always strongly affected by such chemical properties. For example, the abundance of cyclics in MALDI TOF mass spectra is frequently reported to greatly exceed that of linears. Thus, the MALDI ionization behavior of various neat end-capped linear poly(L-lactide)s and one cyclic poly(L-lactide) was investigated and compared with that of blends of both structures. Moreover, the influence of the cationizing salt was investigated too.
Neat compounds and various blends of cyclic and linear species were prepared and studied using two MALDI TOF mass spectrometers under identical conditions with regard to sample preparation and instrumental conditions, except for the laser power and the salt used for cationization. Polymer samples were additionally characterized by NMR and SEC.
The steady increase of the laser intensity caused an exponential increase of the peak intensities of both linear and cyclic polylactides.The response of linear polylactides (in the investigated molecular mass range), whether as neat polymer or in blends with other linear polylactides was almost similar. This clearly supports our assumption that ionization in MALDI is probably unaffected by the end group structure.The variation of the laser power shows only little effect on the intensity ratio of linear-to linear and cyclic-to-linear polylactides in blends. Whereas neat linear polylactides at all laser intensities have a significantly higher abundance than neat cyclics, in mixtures of both an overestimation of cyclic species in MALDI TOF mass spectra of polylactides was found. However, this is far less distinct than frequently reported for other polymers.Concluding, peak suppression of linear polymers in mixtures of both architectures can be excluded, which also means, that polylactides showing only peaks of cyclic compounds in their MALDI - TOF mass spectra do not contain a significant fraction of linear analogues.
Our study is the first systematic comparison of the MALDI ionization of neat and blended cyclic and linear polylactides.
The preparation and characterization of poly(ionic liquid)s (PILs) bearing a polystyrene backbone via reversible addition fragmentation chain transfer (RAFT) polymerization and their photolithographic patterning on silicon wafers is reported. The controlled radical polymerization of the styrenic ionic liquid (IL) monomers ([BVBIM]X, X = Cl− or Tf2N−) by RAFT polymerization is investigated in detail. We provide a general synthetic tool to access this class of PILs with controlled molecular weight and relatively narrow molecular weight distribution (2000 g mol−1 ≤ Mn ≤ 10 000 g mol−1 with dispersities between 1.4 and 1.3 for p([BVBIM]Cl); 2100 g mol−1 ≤ MP ≤ 14 000 g mol−1 for p([BVBIM]Tf2N)). More importantly, we provide an in-depth characterization of the PILs and demonstrate a detailed mass spectrometric analysis via matrix-assisted laser desorption ionization (MALDI) as well as – for the first time for PILs – electrospray ionization mass spectrometry (ESI-MS). Importantly, p([BVBIM]Cl) and p([DMVBIM]Tf2N) were photochemically patterned on silicon wafers. Therefore, a RAFT agent carrying a photoactive group based on ortho-quinodimethane chemistry – more precisely photoenol chemistry – was photochemically linked for subsequent controlled radical polymerization of [BVBIM]Cl and [DMVBIM]Tf2N. The successful spatially-resolved photografting is evidenced by surface-sensitive characterization methods such as X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The presented method allows for the functionalization of diverse surfaces with poly(ionic liquid)s.
Trimesoyl chloride is polycondensed with various alpha,omega'-alkanediols in dichloromethane at different concentrations using equifunctional feed ratios. As evidenced by MALDI-TOF (matrix assisted laser desorption/ionization-time of flight) mass spectrometry the soluble reaction products mainly consist of perfect multicyclic oligomers and polymers. The solphase extracted from the gels also consists of perfect multicycles. SEC (size exclusion chromatography) measurements show that both soluble reaction products and extracted solphases also contain a high molar mass fraction of perfect and nonperfect multicycles extending up to masses beyond 10 5 g mol-1. When the polycondensation is stopped after a few minutes perfect multicycles are already detectable in the reaction mixture along with functional (multi)cyclic oligomers. These results prove that at initial monomer concentrations < 0.2 mol L-1 networks and large multicyclic polymers are synthesized from functional cyclic oligomers formed in early stages of the polycondensation and not from hyperbranched polymers. This interpretation is presented as 'egg-first theory' and compared with the 'hen-first theory' of Stockmayer and Flory.
The analytical utility of a new and simple to use ionization method, matrix-assisted ionization (MAI), coupled with ion mobility spectrometry (IMS) and mass spectrometry (MS) is used to characterize a 2-armed europium(III)-containing poly(ethylene glycol) (Eu-PEG) complex directly from a crude sample. MAI was used with the matrix 1,2-dicyanobenzene, which affords low chemical background relative to matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI). MAI provides high ion abundance of desired products in comparison to ESI and MALDI. Inductively coupled plasma-MS measurements were used to estimate a maximum of 10% of the crude sample by mass was the 2-arm Eu-PEG complex, supporting evidence of selective ionization of Eu-PEG complexes using the new MAI matrix, 1,2-dicyanobenzene. Multiply charged ions formed in MAI enhance the IMS gas-phase separation, especially relative to the singly charged ions observed with MALDI. Individual components are cleanly separated and readily identified, allowing characterization of the 2-arm Eu-PEG conjugate from a mixture of the 1-arm Eu-PEG complex and unreacted starting materials. Size-exclusion chromatography, liquid chromatography at critical conditions, MALDI-MS, ESI-MS, and ESI-IMS-MS had difficulties with this analysis, or failed.
The segregation in dried droplet MALDI sample spots was analyzed with regard to the matrix-to-sample ratio using optical microscopy, MALDI imaging mass spectrometry (MALDI MSI) and IR imaging spectroscopy. In this context, different polymer/matrix/solvent systems usually applied in the analysis of synthetic polymers were investigated. The use of typical matrix concentrations (10 mg mL-1) in almost every case resulted in ring patterns, whereas higher concentrated matrix solutions always led to homogeneous sample spot layers. The data revealed that segregation is predominantly caused by matrix transport in the drying droplet, whereas polymer segregation seems to be only secondary.
Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been suggested as a promising tool for the investigation of pollen, but the usefulness of this approach for classification and identification of pollen species has to be proven by an application to samples of varying taxonomic relations.
Polysarcosine (M-n = 3650-20 000 g mol(-1), D similar to 1.1) was synthesized from the air and moisture stable N-phenoxycarbonyl-N-methylglycine. Polymerization was achieved by in situ transformation of the urethane precursor into the corresponding N-methylglycine-N-carboxyanhydride, when in the presence of a non-nucleophilic tertiary amine base and a primary amine initiator.
Numerous new tin catalysts that enable the synthesis of cyclic polylactides with broad variation in their molecular mass were recently developed. The abundance of cyclics in matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra is, however, frequently reported to greatly exceed that of linears. Thus, the MALDI ionization behavior of various end-capped linear poly(L-lactide)s and one cyclic poly(L-lactide) was investigated and compared.
Neat compounds and various blends of cyclic and linear species were prepared and studied under identical conditions with regard to sample preparation and instrumental condition, except for the laser power. For this purpose, two different MALDI-TOF mass spectrometers were applied.
Our results reveal that cyclics indeed show a slightly better ionization in MALDI, although their ionization as a neat compound seems to be less effective than that of linear polylactides. The ionization of most linear polylactides investigated does not depend on the end group structure. However, linear polylactides containing 12-bromododecyl end groups reveal an unexpected saturation effect that is not caused by fragmentation of the polymer or the end group, or by electronic saturation of the detector digitizer. Furthermore, polylactides with a 2-bromoethyl end group did not show such a saturation effect.
An overestimation of cyclic species in MALDI-TOF mass spectra of poly(L-lactide)s must be considered, but the commonly assumed peak suppression of linear polymers in mixtures of both structures can be excluded.
Cyclic poly(L-lactide)s were prepared by ring-opening polymerization combined with simultaneous polycondensation (ROPPOC) in bulk at 160 ° with dibutyltin bis(4-cyanophenoxide) as catalyst. It is demonstrated by MALDI TOF mass spectrometry and 1H NMR end group analyses that cycles are formed by endto-end cyclization in addition to “back-biting” transesterification. Formation of high molar mass cyclic poly L-lactide)s by means of several more reactive ROPPOC catalysts presented previously and in new experiments is discussed. These experimental results, together with theoretical arguments, prove that part of the Jacobson–Stockmayer theory is wrong. The critical monomer concentration, above which end-toend cyclization is seemingly impossible, does not exist and reversible like irreversible polycondensations can theoretically proceed up to 100% conversion, so that finally all reaction products will necessarily adopt a cyclic architecture.
Ring–Ring Equilibration in Solid, Even-Numbered Cyclic Poly(l-lactide)s and their Stereocomplexes
(2020)
Even-numbered cyclic poly(d-lactide) and poly(l-lactide) are prepared by ringexpansion polymerization. The cyclic pol(l-lactide) is annealed either at 120 or at 160 °C for several days. The progress of transesterification in the solid state is monitored by the formation of odd-numbered cycles via matrix-assisted laser desorption/ionization-time of flight mass spectrometry. The changes of the crystallinity are monitored by differential scanning calorimetry, wideand small-angle x-ray scattering (WAXS and SAXS) measurements. Despite total even-odd equilibration at 160 °C, the crystallinity of poly(l-lactide) is not reduced. Furthermore, the crystallinity of the stereocomplexes of both cyclic polylactides do not decrease or vanish, as expected, when a blocky or random stereosequence is formed by transesterification. This conclusion is confirmed by 13C NMR spectroscopy. These measurements demonstrate that transesterification is a ring–ring equilibration involving the loops on the surfaces of the lamellar crystallites thereby improving crystallinity and 3D packing of crystallites without significant broadening of the molecular weight distribution.
Matrix‐assisted ionization (MAI) mass spectrometry does not require voltages, a laser beam, or added heat to initiate ionization, but it is strongly dependent on the choice of matrix and the vacuum conditions. High charge state distributions of nonvolatile analyte ions produced by MAI suggest that the ionization mechanism may be similar to that of electrospray ionization (ESI), but different from matrix‐assisted laser desorption/ionization (MALDI). While significant information is available for MAI using mass spectrometers operating at atmospheric and intermediate pressure, little is known about the mechanism at high vacuum.
Eleven MAI matrices were studied on a high‐vacuum time‐of‐flight (TOF) mass spectrometer using a 266 nm pulsed laser beam under otherwise typical MALDI conditions. Detailed comparisons with the commonly used MALDI matrices and theoretical prediction were made for 3‐nitrobenzonitrile (3‐NBN), which is the only MAI matrix that works well in high vacuum when irradiated with a laser.
Screening of MAI matrices with good absorption at 266 nm but with various degrees of volatility and laser energies suggests that volatility and absorption at the laser wavelength may be necessary, but not sufficient, criteria to explain the formation of multiply charged analyte ions. 3‐NBN produces intact, highly charged ions of nonvolatile analytes in high‐vacuum TOF with the use of a laser, demonstrating that ESI‐like ions can be produced in high vacuum. Theoretical calculations and mass spectra suggest that thermally induced proton transfer, which is the major ionization mechanism in MALDI, is not important with the 3‐NBN matrix at 266 nm laser wavelength. 3‐NBN:analyte crystal morphology is, however, important in ion generation in high vacuum.
The 3‐NBN MAI matrix produces intact, highly charged ions of nonvolatile compounds in high‐vacuum TOF mass spectrometers with the aid of ablation and/or heating by laser irradiation, and shows a different ionization mechanism from that of typical MALDI matrices.
SnOct 2-Catalyzed Syntheses of Cyclic Poly (l-lactide) s with Catechol as Low-Toxic Co-catalyst
(2019)
Polymerizations of l-lactide in bulk at 160 or 180 °C were performed with 1/1 mixtures of catechol (CA) or 4-tert-butylcatechol (BuCA) and tin(II)-2-ethylhexanoate (SnOct2) as catalysts and a variation of the Lac/Cat ratio. Weight average molar masses (Mw) up to 170,000 g mol−1 were obtained with CA and up to 120,000 g mol−1 with BuCA. The cyclic structure of the resulting poly(l-lactide)s was proven by MALDI-TOF mass spectrometry and by comparison of their hydrodynamic volumes with those of commercial linear poly(l-lactide)s. The predominance of even-numbered cycles increased with lower temperatures and shorter polymerization times. This fnding indicates that the cyclic architecture is the results of a ring-expansion polymerization mechanism. Addition of silylated BuCA as co-catalyst was less favorable than addition of free BuCA.
L-Lactide was polymerized in bulk with tin(II)2-ethylhexanoate SnOct2) as catalyst and salicylic acid as cocatalyst. The Lac/Cat ratio, Cocat/Cat ratio, temperature and time were varied. Increasing Cocat/Cat ratios reduced both,polymerization rate and molecular weight. However,under optimized conditions high molar mass (Mw up to 178,000), colorless, cyclic polylactides were formed in a short time. A few polymerizations performed at 160 and 180°C with the combination of SnOct2 and silylated salicylic acid gave similar results. Neat tin II) salicylate was prepared from SnOct2 and used for REPs of L-lactide in bulk, but the results were not better than those obtained from combinations of SnOct2 and salicylic acid. Furthermore, dibutyltin salicylate was synthesized and used as catalyst for polymerizations of L-lactide in bulk at temperatures varying from 102 to 160°C. Cyclic polylactides with Mw’s up to 40,000 were the main reaction products. At 100–102°C a predominance of odd-numbered cycles was found proving a REP mechanism.