Supramolecular Polymer Based on Hydrogen Bonding - Essay Example

Synthesis and characterization of a series of 3,5-bis(trifluoromethyl)phenyl)-3-phenylurea derivatives serving as starting materials for the creation of supramolecular polymers employing the self-assembly process has been conducted. Supramolecular polymers are different from conventional polymers and are based on non-covalent reversible interactions. One of the main features of such polymers is low, controllable viscosity which can be adjusted through the introduction of monomeric units possessing the ability to interact with the chain stoppers. The main purpose of the work was to isolate small receptor molecules which contained AADD hydrogen bonding in their structure. This will lead to the development of inexpensive procedure affording urea based monomers acting the terminals of the chains. According to H1 NMR both 2-(4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido) phenyl)acetic acid and 4-(4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido)phenyl)butanoic acid were impossible to purify using recrystallization. However, the same method of purification was successful in case of using 4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido)benzoic acid as a substrate for purificalion. This purified product was taken on to the next step where the transformation from benzoic acid derivative into two electron acceptor group was conducted introducing tetrabutylammonium hydroxide. Subsequent polymer formation was achieved through self-assembly process between two electron donating group of urea and two electron accepting group of the previously synthesized benzoic acid salt. 2) Conclusions The research provided an account on the supramolecular chemistry of a set of 1-(3,5-bis(trifluoromethyl)phenyl)-3-phenylurea derivatives. It was motivated by the extraordinary chemistry manifested by supramolecular polymers based on hydrogen bonding. The work pointed out to the substantial differences between the obtained substrates. First of all, 2-(4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido)phenyl)acetic acid and 4-(4-(3-(3,5-bis (trifluoromethyl)phenyl)ureido)phenyl)butanoic acid contained impurities which were impossible to remove using recrystallization. In both cases two purification attempts were made, both unsuccessful. On the other hand the same technique was applicable in purifying 4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido)benzoic acid. This fact leads to the conclusion that purification method should be changed in case of the first two derivatives should the synthesis be repeated. For example, flash column chromatography would be an attractive alternative. To conclude, an inexpensive and efficient method that allowed the production of urea based monomers was developed. The reaction affords excellent yields and the process is easy to carry out. Because tetrabutylammonium hydroxide and inorganic bases can be compared in their basic properties, it was possible to obtain the salt of the required monomer in an excellent 70% yield. The work is partly successful and due to time limits it was not possible to repeat each step again. It raised important question which will form the bases of the future work. 3) Future work First of all, in future, it is important to find efficient methods of purification products achieved in experiment one and two. By doing that it will be possible, comparing the yield, draw conclusions about formation of the intermediates. The produced intermediates will be transformed into supramolecular polymer substrates chain-stopping properties of which will be investigated. As it is seen from the first three experiments the only difference between substrates is the number of carbon atoms in the chain next to the benzene ring. For this reason it will be possible to draw conclusions between viscosity of the formed suprapolymer and the number of carbon atoms. Solutions of the mentioned polymers are known to have high viscosity and their rheological behaviour is different in case of different polymers. For example, it was established that polymers containing ureidopyrimidinone units are different from those based on benzene. Other properties, for instance temperature range should also be investigated As it was previously stated supramolecular polymers are different from ordinary ones. Reversible nature of bonds in the former type of polymers imposes unique behaviour. For example, they can switch between rings and linear chains. Due to this unique behaviour, the mechanism of supramolecular polymer formation should be analysed. (Wright and Cumming, 1969). 4) Results and discussion 4.1. Experiments 1 and 2 Our studies of 2-(4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido)phenyl)acetic (1) and 4-(4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido)phenyl)butanoic acid (2) formation was motivated by the desire to obtain the corresponding salts which could be used as building blocks in the synthesis of supramolecular polymers. Figure 1: 2-(4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido)phenyl)acetic (1) and 4-(4-(3-(3,5-bis (trifluoromethyl)phenyl)ureido)phenyl)butanoic acid (2). In the first experiment the reaction between 2-(4-aminophenyl)acetic acid (3) and 1-isocyanato-3,5-bis(trifluoromethyl)benzene (4) afforded the formation of crude 2-(4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido)phenyl)acetic acid (1) in an excellent 91.58% yield(Figure 2). Figure 2: Experiment 1. The reaction was straightforward due to high reactivity of the isocyano group towards the amino group. In the second experiment the reaction between 4-(4-aminophenyl)butanoic acid (5) and 1-isocyanato-3,5-bis(trifluoromethyl)benzene (4) afforded the isolation of crude 4-(4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido)phenyl)butanoic acid (2) in an excellent 98.2% yield (Figure 3). Figure 3: Experiment 2. Although, according to proton NMR in both cases the formation of the desired products was successful, purification difficulties due to high similarity between the compounds in the crude mixture did not allow the products to be employed in the step of formation the corresponding salts. For the same reason there was no point in characterising the products using carbon NMR and IR, though the compounds are new, are not described in literature and require full characterisation. Normal formation of linear chains is possible only if the monomers are bifunctional. For this reason the biggest amount of monomeric building blocks possess two functional groups in their structure. The presence of monofunctional monomers is unwanted because they damage the desired structure by forming extra chain ends and reduce the polymerisation degree. Such compounds are also called “chain stoppers” and usually are hard to remove. Impurities have the ability to perform exactly the same function which leads to the disruptive formation of supramolecular polymerion. These are the main reasons which excluded products (1) and (2) from converting them to the corresponding salts and studying the mechanism of supramolecular polymer formation. 4.1. Experiments 3 and 4 Discovered in early 20th century supramolecular polymers have presented a number of advantages over traditional macromolecules. One of their unique ability is self-healing i.e. the ability to reversible assemble themselves through hydrogen bonding, Van der Waals and electrostatic interactions. Because the mentioned forces are significantly weaker then covalent bonds supramolecular polymers are dynamically more flexible, less stable thermodynamically and more liable kinetically compared to traditional polymers. (DeGreef and Meijer, 2008; Zigon and Ambrozic, 2003; Ciferi and Dekker, 2000) In contrast to the first two experiments, synthesis three and four were successful and are worth discussing in more detail. The formation of the desired product (7) can be illustrated on the following diagram. Figure 4: Formation of 4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido)benzoic acid (7). The reaction follows the same pattern as in the previous two experiments and involves condensation between 4-aminobenzoic acid (6) and 1-isocyanato-3,5-bis (trifluoromethyl)benzene (4) afforded the formation of 2-(4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido)phenyl)acetic acid (1) in an excellent 91.58% yield(Figure 4). After recrystallisation the product was collected as white crystals. The yield should not be compared to the achieved for compounds (1) and (2) due to impure nature of the products. The compound was new and its structure was confirmed by NMR and IR. Main patterns of the IR spectra are presented in the following table: Table 1: Major IR peaks of 4-(3-(3,5-bis(trifluoromethyl)phenyl)ureido)benzoic acid (7). The presence of urea group was confirmed by observing a characteristic absorption in the region that corresponds to N-H group (3283.9 cm-1, 3339 cm-1). In each case two peaks close to V-shape were observed. Presence of the carboxyl group was also established by pointing to the strong sharp peak at 1660.8 cm-1. Both aromatic C=C region (1590 cm-1) and C-F bonds (1250-1100 cm-1) were observed on the spectrum. Talking about proton NMR, the peaks can be represented in the following table: Signal, ppm Splitting Pattern Assigned Protons 9.48 Singlet A 9.35 Singlet B 8.19-8.10 Multiplet C 7.89 Doublet (J=8.8 Hz) D 7.61 Doublet (J=8.9 Hz) E 7.65 Doublet (J=0.7 Hz) F 12.63 Singlet G Table 2: Proton NMR peaks corresponding to of 4-(3-(3,5-bis(trifluoromethyl)phenyl) ureido)benzoic acid (7). The signals can be assigned to each proton and presented on the following figure: Figure 5: Proton NMR signals assignment First of all, it is clear that HG is the most deshielded one. Also, there are no protons nearby. For this reason, it appeared as a singlet at 12.63 ppm. Next, moving from right to left. There are two sets by two of identical protons, HD and HE at 7.89 and 7.61 ppm. Theoretically, these protons should appear as two dublets looking like a quadruplet in the “roof” form. Initially, HE will be observed and then HD because in the former case there is an electron donating NH adjacent group which will provide extra shielding. Protons HA and HB appeared as two singlets. This is completely reasonable and expected because there are no protons they could couple with and two benzene rings are different. Also, they are slightly different due to different nature of the attached benzene rings. Ring “A” is less electron rich then ring “B” therefore, HA will be more deshielded then proton HB. CF3 is a powerful electron withdrawing group, there are two of them. Obviously, the effect cannot be compared with the influence of one carboxylic group. In the ring “A” a very interesting effect of W-coupling is observed (Partly highlighted in bold). Such presumption is in perfect correlation with the coupling constant for such interaction. (0 Read More