Tina Erdmenger

Extended dissolution studies of cellulose in imidazolium based ionic liquids

Ionic liquids (ILs) have become advantageous solvents for the dissolution and homogeneous processing of cellulose in recent years. However, despite significant efforts, only a few ILs are known for their capability to efficiently dissolve cellulose. In order to overcome this limitation, we screened a wide range of potentially suitable ILs. From our studies, some remarkable results were obtained, for example, an odd–even effect was found for different alkyl side-chain lengths of the imidazolium chlorides which could not be observed for the bromides. Furthermore, 1-ethyl-3-methylimidazolium diethyl phosphate was found to be best suitable for the dissolution of cellulose; dissolution under microwave irradiation resulted in almost no color change. No degradation of cellulose could be observed. In addition, 1-ethyl-3-methylimidazolium diethyl phosphate has a low melting point which makes the viscosity of the cellulose solution lower and, thus, easier to handle.

Influence of different branched alkyl side chains on the properties of imidazolium-based ionic liquids

Several new branched ionic liquids were synthesized under microwave irradiation applying two different synthetic approaches. Different already known ionic liquids, both linear and branched, were added to this set of new ionic liquids to investigate the influence of the branching on the thermophysical properties to elucidate first structure–property relationships. Thermogravimetric analysis was utilized to investigate the decomposition behavior and differential scanning calorimetry was used to study the influence of the branching on the thermal behavior, e.g. the melting point, the glass transition temperature, the freezing point and the cold crystallization temperature. Moreover, the water uptake of selected ionic liquids was analyzed.

Microwave-assisted chemistry : a closer look at heating efficiency

Nowadays, microwave heating has evolved into a common tool for chemists based on its numerous advantages over conventional conductive heating. Surprisingly, the efficiency of microwave-assisted heating is still rather unexplored. In this contribution, we report our investigations concerning the heating efficiency of a variety of solvents including polar and apolar substances. Moreover, the effects of adding salt or passive heating elements on the microwave heating efficiency will be addressed. Finally, the heating efficiency of demineralized water is discussed at different volumes and with different microwave power levels in both monomode and multimode microwave synthesizers, demonstrating maximum average heating efficiencies of 10% for small-scale vessels (5 mL), 20% for medium-scale (50 mL), and 30% for large-scale microwave heating (400 mL).

Fast Surface Modification by Microwave Assisted Click Reactions on Silicon Substrates

Microwave irradiation has been used for the chemical modification of functional monolayers on silicon surfaces. The thermal and chemical stability of these layers was tested under microwave irradiation to investigate the possibility to use this alternative heating process for the surface functionalization of self-assembled monolayers. The quality and morphology of the monolayers before and after microwave irradiation was analyzed by surface-sensitive techniques, such as Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM), and contact angle measurements. As a model reaction, the 1,3-dipolar cycloaddition of organic azides and terminal acetylenes was tested for the chemical modification of functional azide monolayers. Low and high molar mass compounds modified with an acetylene group were successfully clicked onto the surfaces as confirmed by FTIR spectroscopy and AFM investigations. It could be verified that the reaction can be performed in reaction times of 5 min, and a comparison to conventional heating mechanisms allowed us to conclude that the elevated reaction temperatures result in the fast reaction process.

Synthesis, Microwave‐Assisted Polymerization, and Polymer Properties of Fluorinated 2‐Phenyl‐2‐oxazolines: A Systematic Study

We present a detailed systematic study of the synthesis and ability of fluorinated 2-phenyl-2-oxazolines to undergo polymerization. The synthesis of these compounds is based on a two-step procedure that gives the desired 2-oxazolines in moderate-to-good yields. All the compounds were fully characterized by IR and NMR ((1)H, (13)C, and (19)F) spectroscopy, mass spectrometry, and elemental analysis. The 2-oxazolines were subsequently used as monomers for living cationic ring-opening polymerization (CROP) with microwave irradiation as the heat source (T=140 degrees C), nitromethane as the solvent, and methyl tosylate as the initiator. The linear first-order kinetic plots of the polymerizations accompanied by a linear increase of the molecular weight with conversion and low polydispersity index (PDI) values (generally below 1.30) indicate a living polymerization mechanism. The resulting polymerization rates reflect a strong sensitivity to the quantity of fluorine substituents in general and the presence or absence of ortho-fluoro substituents of the phenyl ring in particular. All the polymers were isolated and characterized by size-exclusion chromatography and MALDI-TOF mass spectrometry. Finally, a detailed investigation of selected polymer properties was performed by using differential scanning calorimetry, thermogravimetric analysis, and contact-angle measurements, thus resulting in structure-property relationships. Whereas the thermal properties of the polymers are mostly influenced by the presence of ortho-fluoro substituents, the surface properties are mainly determined by the presence of para- and/or meta-fluoro substituents.

Simplifying the free-radical polymerization of styrene : microwave-assisted high-temperature auto polymerizations

We have investigated the combination of the thermally auto-initiated free radical polymerization of styrene and precipitation polymerization in order to develop a fast and environmentally friendly approach to produce polystyrene. To achieve high reaction temperatures in a short period of time, microwave irradiation was utilized as the heating source. Styrene was used without any purification, e.g., without distillation or column filtration. Due to the auto-initiation of styrene at high temperatures no radical initiator was required. Different water- or ethanol-to-styrene ratios were heated far beyond their boiling points and at relatively high pressures for the auto-initiated polymerization of styrene. The obtained molecular weights could be controlled by the ethanol-to-styrene ratio in the case of ethanol as the solvent although the monomer conversions were rather low under the applied conditions. Moreover, the effect of a commercially available stable free nitroxide was investigated on the control over the polymerization. It has been observed that it is possible to control the molecular weight of the polymer by changing the ratio of styrene to free nitroxide (varied from 10:1 to 400:1) and moderate polydispersity indices (PDI = 1.3 to 1.9) could be obtained. Finally, the developed polymerization processes only require a simple purification step due to the precipitation of the polystyrene in the reaction solvent.

Scaling-up the Synthesis of 1-Butyl-3-methylimidazolium Chloride under Microwave Irradiation

Ionic liquids are considered to be ‘green’ solvents on account of their non-volatility and non-flammability – which are results of their negligible vapour pressures – as well as reusability. On the basis of ecological concerns, ionic liquids seem to be an attractive alternative to conventional volatile organic solvents. In the present work, the reaction conditions for the synthesis of 1-butyl-3-methylimidazolium chloride were optimized on a small scale (~2 mL, 10 mmol) using a single-mode microwave system. The conditions obtained were subsequently transferred to various microwave reactors, both batch and continuous flow, as well as mono-mode and multi-mode, for the direct scale-up of the synthesis from 0.01 to 1.15 mol.

Microwave-assisted synthesis of imidazolium ionenes and their application as humidity absorbers†

4,4-Imidazolium ionenes were synthesized under microwave irradiation for the first time and their application as humidity absorbers (water uptake up to 97 wt%) was investigated.