Alternative routes and solvents in polymer chemistry : microwave irradiation and ionic liquids

T. Erdmenger

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The concept of sustainable chemistry represents an area of innovation, which not only preserves resources, but also stands for a progress in the chemical industry. The principle of sustainable chemistry comprises important elements in areas like environment, economy and society, dealing with the whole life of intrinsic safe chemicals and products, including their production, processing, use and disposal. One of the largest amounts of auxiliary wastes in industry is produced by the usage of solvents. Therefore, alternative reaction media are investigated in order to reduce or replace organic solvents. The most widely used green solvents in, e.g., polymer research are ionic liquids, zupercritical CO2 and water. In addition, also alternative energy sources, such as photochemistry, microwave energy, electron beam and ultrasound, are investigated in order to replace conventional heat sources for, e.g., polymer processing. The main goal of utilizing alternative energy sources is to improve the efficiency of the process by reducing the polymerization time. Ionic liquids are considered to be ‘green’ solvents on account of their non-volatility and nonflammability – which are results of their negligible vapor pressure – as well as their reusability. On the basis of ecological concerns, ionic liquids seem to be an attractive alternative to conventional volatile organic solvents. Ionic liquids with a linear alkyl side chain can be synthesized in a fast and efficient way at elevated temperatures (170 °C) by using microwave irradiation. In case of the ionic liquids with branched alkyl side chains, the synthesis could be accelerated as well, but the equilibrium was shifting towards undesired side products compared to the synthesis at conventional conditions (80 °C). In this regard, several new branched ionic liquids, e.g. 1-(1-ethylpropyl)-3-methylimidazolium iodide and 1-(1-methylbenzyl)-3- methylimidazolium chloride and their tetrafluoroborate containing analogues, were synthesized applying two different synthetic approaches. The direct scaling for the ionic liquids with linear alkyl side chain was investigated from small scale (0.01 mol) to large scale (1.15 mol). In this case, comparable results were obtained for the direct up-scaling utilizing different microwave reactors under otherwise similar reaction conditions. The results of the continuous flow experiments indicated that 1-butyl-3-methylimidazolium chloride can be synthesized with short reaction times by using continuous flow microwave systems. However, direct scaling from the batch experiments was not possible. Even when employing a residence time of 16 min, a complete conversion could not be obtained. Nonetheless, for the first time, the synthesis of ionic liquids in continuous flow reactors was achieved. In case of [C2MIM][Et2PO4], higher conversions were achieved, since the reaction proceeds in a homogeneous phase, but unfortunately only strongly colored ionic liquids could be obtained with the applied conditions, while not showing any decomposition products in the 1H NMR spectrum. In order to elucidate first structure–property relationships, the synthesized ionic liquids, both linear and branched, were investigated by thermogravimetric analysis, differential scanning calorimetry, and water uptake measurements of selected ionic liquids. The results obtained for the decomposition temperature support a SN2 (alkyl) and SN1 (aryl) decomposition pathway for branched ionic liquids with alkyl and aryl side chain, respectively, containing chloride as counter ion. In case of tetrafluoroborate containing ionic liquids a decomposition mechanism initiated by the anion seems to take place. Moreover, tetrafluoroborate containing ionic liquids and ionic liquids with linear alkyl side chains revealed lower glass transition temperatures compared to the ionic liquids with chloride anion or branched alkyl side chains, respectively. In general, the ionic liquids with an aromatic group showed the highest Tg values of all the investigated ionic liquids. In addition, the water uptake of the ionic liquids was measured and revealed a systematic dependency on the length of the alkyl side chain and on the branching. It was found that the water absorption decreased with the length of the alkyl chain and that branched alkyl chains increased the water uptake as a result of their decreased ability to self-assembly. The described results provide a better insight into the structure-property relationship of ionic liquids, allowing the fine-tuning of the chemical and physical properties. Cellulose is the most abundant natural polymer in nature and its derivative products have many important applications. However, cellulose is insoluble in water and most common organic solvents, because of its fibril structure and the pronounced presence of inter- and intermolecular hydrogen bonding. In recent years, ionic liquids were found to dissolve cellulose, but the candidates known are still limited. In order to extend the range of suitable ionic liquids, we screened known but also new tailor-made ionic liquids. In particular, the influence of different alkyl chain lengths, branched alkyl side chains and the anion on the dissolution of cellulose was investigated. A strong odd-even effect of the alkyl chain length on the solubility of cellulose in the ionic liquid was observed for imidazolium based ionic liquids with linear and branched alkyl side chains bearing chloride as counter ion. Alkyl side chains with an odd number of CH2 repeating units showed in general good dissolving properties, whereas an even number of CH2 repeating units was not able to dissolve cellulose. The difference in solubility might be explained by a different range of conformations for odd and even alkyl chains. In general, only the ionic liquids with chloride, acetate and phosphate counter anions showed good dissolving properties for cellulose. Moreover, the microwave-assisted dissolution of cellulose was investigated and optimized. Selected ionic liquids were used as solvent in the tritylation reaction. It was found, that pyridine is required to capture hydrogen chloride and that the reaction time could be reduced from 48 h (reaction in DMA/LiCl) to 3 h ([C4MIM][Cl]) in order to reach the desired DS of nearly 1.0. Unfortunately, recycling of the ionic liquid could not be achieved when pyridine was used as a base. However, this was possible when triethylamine was used as a base. New 4,4-imidazolium ionenes were synthesized under microwave irradiation. The polymerization times could be decreased from 24 to 1 h as a result of elevated temperatures above the boiling points of the applied solvents. Different approaches, such as monomer imbalance and monofunctional reagents, were applied in order to control the molar mass of the polymers. Analytical ultracentrifugation measurements indicate the formation of macrocyclic rings to a large extend (82 to 93%). Furthermore, the properties of the synthesized 4,4-imidazolium ionenes, such as thermal behavior, solubility behavior and water uptake were investigated as well. It was found that the decomposition temperatures were comparable to the values reported in literature for ammonium ionenes, while the glass transition temperatures obtained were lower compared to values reported in literature. In addition, the 4,4-imidazolium ionenes showed a high water uptake. The ability to absorb water is mainly depending on the counter ions (chloride showed a higher water uptake than bromide). The combination of the thermal auto-initiated free radical polymerization of styrene and the precipitation polymerization were investigated in order to develop a fast and environmentally friendly approach to produce polystyrene. To achieve high reaction temperatures in a short time, microwave irradiation was utilized as heating source. Styrene was used without any purification, e.g. without distillation or column filtration. First experiments were carried out using nearcritical water (water in the temperature range of 250 to 350 °C) as solvent, because the polarity and hydrogen-bonding of water are highly temperature depending. Due to the auto-initiation of styrene at high temperatures no radical initiator was required. The polymerization of styrene in near-critical water always led to polymers with comparable molar masses although different styrene concentrations were applied. In case of ethanol as solvent, the obtained molar masses could be controlled by the ethanol-to-styrene ratio although the monomer conversions were rather low under the applied conditions (1 to 13%). In order to achieve a better control over the molar mass SG-1, a commercially available stable free nitroxide, was applied to mediate the polymerization. It was found that the molar masses can be controlled by different styrene:SG-1 ratios (from 10:1 to 400:1). In this case 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. Another example of using near-critical water is the hydrolytic ring-opening polymerization of polyamide. In this thesis a polyamide 12 pre-polymer was synthesized under microwave irradiation at high temperatures and pressures, indicating that less side products are formed compared to the thermal polymerization. Since these are preliminary results, further experiments are required in order to investigate if the utilization of microwave irradiation can provide advantages over thermal heating. In general, it was shown that microwave irradiation and ionic liquids are interesting alternatives to conventional energy sources and solvents. In particular, a better understanding of the structure-property relationships of branched ionic liquids was achieved resulting in a superior knowledge about the influence of the alkyl side chains of ionic liquids on the cellulose dissolution process. New concepts (combination of thermal and precipitation polymerization, near-critical water) and polymers (imidazolium ionenes) were investigated utilizing microwave irradiation as heating source to achieve short reaction times.
Originele taal-2Engels
KwalificatieDoctor in de Filosofie
Toekennende instantie
  • Chemical Engineering and Chemistry
Begeleider(s)/adviseur
  • Schubert, Ulrich, Promotor
  • Gohy, Jean-Francois, Promotor
  • Vitz, Jürgen, Co-Promotor
Datum van toekenning1 sep 2009
Plaats van publicatieEindhoven
Uitgever
Gedrukte ISBN's978-90-386-1916-3
DOI's
StatusGepubliceerd - 2009

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