DOI: 10.1002/chem.201405488

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Self-Assembly and Gelation of Poly(aryl ether) Dendrons Containing Hydrazide Units: Factors Controlling the Formation of Helical Structures Partha Malakar and Edamana Prasad*[a]

Abstract: Self-assembly of AB2 and AB3 type low molecular weight poly(aryl ether) dendrons that contain hydrazide units were used to investigate mechanistic aspects of helical structure formation during self-assembly. The results suggest that there are three important aspects that control helical structure formation in such systems with acyl hydrazide/ hydrazone linkage: i) J-type aggregation, ii) the hydrogenbond donor/acceptor ability of the solvent, and iii) the dielectric constant of the solvent. The monomer units self-assemble to form dimer structures through hydrogen-bonding and further assembly of the hydrogen-bonded dimers leads

to macroscopic chirality in the present case. Dimer formation was confirmed by NMR spectroscopy and by mass spectrometry. The self-assembly in the system was driven by hydrogen-bonding and p–p stacking interactions. The morphology of the aggregates formed was examined by scanning electron microscopy, and the analysis suggests that aprotic solvent systems facilitate helical fibre formation, whereas introduction of protic solvents results in the formation of flat ribbons. This detailed mechanistic study suggests that the self-assembly follows a nucleation–elongation model to form helical structures, rather than the isodesmic model.

Introduction

linked with moieties such as naphthalene, anthracene, pyrene, pyridine or glucose through an acylhydrazone linkage, form low molecular weight organo and hydro gel systems in a wide range of solvents with robust material properties.[5] Aida and co-workers reported that a poly(aryl ether) dendron, functionalised with a dipeptide at the core and ester groups at the periphery, forms an organogel with helical morphology.[6] In another study, Fan and co-workers reported that a poly(aryl ether) modified by multiple ester groups at the periphery forms a gel with fibrous morphology.[7] Percec’s group recently reported that a poly(aryl ether) dendron connected with an aliphatic chain in the periphery and a dipeptide unit at the core forms a helical porous self-assembly.[8] Furthermore, many of the poly(aryl ether) dendron based gel systems have been found to be useful as metal ion sensors, anion sensors and stabilizing agents for nanoparticles.[9] Scheme 1 contains a selection of important structures of poly(aryl ether) dendrons that have been reported to form robust self-assembled systems. Although experimental results have clearly shown that hydrogen bonding and p–p stacking interactions are the driving forces for gel formation in the above systems, mechanistic details regarding the initial and sequential steps as well as the effect of noncovalent forces on the individual steps resulting in the three-dimensional structure have not been investigated, especially for the helical-type aggregation. To explore the mechanistic aspects of self-assembly systems that favour helical assembly formation, poly(aryl ether) dendrons containing acylhydrazide units were synthesised (Scheme 2). The gel systems formed by these molecules were characterised by rheological, spectroscopic and electron microscopic measurements.[10] The purpose of the present study was

Self-assembly is an attractive synthetic strategy for constructing complex supramolecular architectures from simple molecular building blocks, as exemplified in many biological systems.[1] Quite often, self-assembly in appropriate solvent medium leads to the formation of gel systems. Such supramolecular gels have potential applications in the petrochemical industry, healthcare, water purification, electronic devices, and tissue engineering.[2] Furthermore, organo or hydro gels based on low molecular weight gelators exhibit enhanced stimuli-responsive behaviour, which makes them suitable candidates for sensing and drug delivery.[3] Self-healing is another invaluable property of supramolecular gels, which may lead to the development of smart materials.[4] Whereas the significance of developing novel low molecule weight gelators (LMWGs) has been well recognised, developing appropriate design strategies for effectively utilising noncovalent forces (p–p stacking, hydrogen bonding, van der Waals and hydrophobic) to construct three-dimensional molecular networks has not evolved in great detail. Thus, serendipity still prevails over careful design in large number of reported gel systems. Poly(aryl ether) have been widely used as monomers for developing LMWGs. Previous results from our laboratory suggest that AB3 and AB2 type poly(aryl ether) dendrons, covalently [a] P. Malakar, Prof. Dr. E. Prasad Department of Chemistry Indian Institute of Technology Madras, Chennai 600036 (India) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405488. Chem. Eur. J. 2015, 21, 1 – 9

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Scheme 1. Reported structures A–D of poly(aryl ether) dendron based systems for self-assembly or gelators.

Results and Discussion

to delineate the mechanism that occurs during helical gel formation by the poly(aryl ether) dendron derivatives.[11] Experiments were designed to understand the role of solvents in promoting helical self-assembly in such dendron systems. The mechanism of self-assembly was modelled by both isodesmic and cooperative self-assembly approaches. Various techniques such as NMR, UV/Vis spectroscopy, and circular dichroism have been utilised for the investigation. The results discussed herein provide useful guidelines for the design and synthesis of helical supramolecular assembly based on poly(aryl ether) dendron derivatives.

The structures of the compounds utilised in the present investigation are shown in Scheme 2. The compounds exhibit excellent gelation properties in a wide range of solvents and solvent mixtures. Gelation properties of poly(aryl ether) hydrazide (compounds I–IV) were studied in various solvents and solvent mixtures. The critical gel concentration (CGC) values for the compounds are summarised in Table 1. Initial studies were carried out with a UV/Vis absorption spectrophotometer for compound I in an acetonitrile–water mixture, below the CGC. Figure 1 a shows the absorption spec-

Scheme 2. Structure of dendrons utilised in the present study.

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Full Paper supporting Information, Figure S2). Conversely, compounds III and IV form partial gels in selected solvent systems. SEM images of the self-assembled structures in a number of solvents such as methylcyclohexane, chloroform–hexane, toluene, benzene, 1,2-dichlorobenzene, tetrahydrofuran, chloroform–methanol, tetrahydrofuran–water and acetonitrile–water were recorded (Figure 2). It is evident that clear helical self-as-

Table 1. Critical gel concentrations (CGCs, mg mL1) of compounds I–IV in various solvents and solvent mixtures. Solvent system

I[a]

o-xylene toluene benzene chlorobenzene 1,2-dichlorobenzene nitrobenzene dichloromethane–hexane chloroform–toluene chloroform–hexane ethyl acetate–hexane tetrahydrofuran–hexane acetonitrile—H2O

G G G G G S G G G G G G

(3.5) (4) (4) (5) (6) (5, 3:7) (9, 3:7) (5, 3:7) (5, 3:7) (5, 3:7) (1, 1:1)

II[a]

III[a]

IV[a]

G (9.5) G (10) G (11) G (12.5) G (15) S G (1.5, 1:19) S G (1.5, 1:19) G (1.5, 1:19) G (1.5, 1:19) PG

S S S S S S PG S PG PG PG PG

S S S S S S G (3, 1:9) S G (3, 1:9) PG PG PG

[a] G, gel; S, solution; PG, partial gel; all the gels are opaque in nature.

Figure 1. a) UV/Vis spectra of compound I at different concentrations in acetonitrile (solid line) and in acetonitrile–water (1:23 v/v) (dotted line); b) Image of thermo reversible gel prepared from compound I in toluene.

Figure 2. SEM images of compound I in aprotic solvents: a) toluene, b) benzene, c) methylcyclohexane, d) tetrahydrofuran, e) chloroform–hexane, and protic solvent mixtures: f) chloroform–methanol, g) acetonitrile–water, and h) tetrahydrofuran–water.

tra of compound I in acetonitrile as well as in acetonitrile– water mixture (1:23) at various concentrations of I (9 to 54 mm). Gelator I exhibits two absorption bands around 215 and 260 nm in acetonitrile (solid lines in Figure 1 a). The former absorption band is assigned to p–p* transition and the latter band is due to n–p* transition. Upon addition of water, the absorption bands are redshifted by 10 nm, which indicates the formation of J-type aggregates (spectra with dotted lines in Figure 1 a). To verify whether the J aggregates break down at higher temperatures, the absorption spectra of compound I were recorded under a range of temperatures from 20 to 80 8C. A blueshift in the absorption spectra was observed at higher temperature, suggesting that the aggregates break down as temperature increases (see the Supporting Information, Figure S1). Compounds I and II form thermally reversible gels in aromatic solvents such as o-xylene, toluene, benzene, chlorobenzene and 1,2-dichlorobenzene (Figure 1 b and the

sembly is achieved in all aprotic solvent systems, whereas thick fibres are formed in the presence of protic solvents. Consistent with the above observation, when solvent mixtures are used, the tendency to undergo morphology transition from helical to fibre is increased upon increasing the aprotic solvent content. Rheology experiments were performed for compounds I and II and it was found that the storage moduli (G’) is always greater than the loss moduli (G’’) over the entire range of frequency (Figure 3). Elastic properties of the gels can be determined by taking the difference in two moduli (G’G’’ = DG). DG values of gel from compounds I and II are 22 900 and 76 000 Pa respectively at 11 mm concentration of gelator. This indicates that compound II is a better gelator than I in terms of its viscoelasticity. The strength of the gel increases with concentration of gelator. For example, as concentration of compound I in-

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Figure 3. Frequency sweep rheometry data of gels prepared in chloroform– hexane from compound I and II ([I] = [II] = 11 mm).

creases from 10 to 13 mm, the DG values increase from 14 360 to 76 600 Pa (see the Supporting Information, Figure S3). Surprisingly, the first generation compounds (compounds I and II) form robust gels in aromatic solvents, whereas the second generation compounds failed to form gels in a wide range of solvents and solvent mixtures. SEM images of compounds III and IV are given in Figure 4, and those of III and IV Figure 5. a) CD spectra of compound I (54 mm) in methylcyclohexane (blue) at 16 8C and acetonitrile–water (green) at 40 8C; b) LD spectrum of compound I in methylcyclohexane at 16 8C.

water mixture (1:23), similar to that of methylcyclohexane (Figure 5, green line). This corroborates the hypothesis that addition of water leads to aggregation in the system, as indicated by the UV/Vis spectroscopic experiments. However, the CD signal intensity decreased rapidly with time, suggesting that the chiral assembly is metastable in acetonitrile–water mixture. This is also consistent with the SEM images, which reveal that the system eventually leads to the formation of thick fibres in acetonitrile–water mixture (Figure 2 g). In addition to CD analysis, LD signals recorded from the system revealed that a significant contribution from LD exists in the present case (Figure 5 b). This suggests that the chirality originates from the macroscopic alignment of the fibres.[6b, 16b] It is likely that compound I forms hydrogen-bonded dimers through the amide group, in aprotic solvents. This is facilitated by the less competitive nature of the solvent towards hydrogen-bond formation with the compound. To verify this hypothesis, the self-assembly of compound I was monitored in toluene by NMR spectroscopy. The downfield shift of amide protons at lower temperature suggests that compound I forms intermolecular hydrogen bonds to generate the dimer (Figure 6). It is also noted that the NMR signals become broadened at temperatures below 30 8C, indicating that the initially formed dimers can further self-assemble and form oligomers. Dimer formation was further confirmed by ESI-MS spectrometry, in which, along with the monomer, dimer molecular ion peaks were obtained with significant intensity (see the Supporting Information, Figure S5).

Figure 4. SEM images of a) compound III, and b) compound IV in chloroform–hexane [103 m].

are given in Figure 4. Comparison of the images indicates that compound III forms spherical and fibre-type assemblies whereas compound IV forms mostly fibre-type self-assemblies. The propensity for self-assembly is, presumably, reduced in this case, due to the steric encumbrance and greater solubility of the second-generation dendrons. Although compound IV forms entangled fibres, the gel was not very stable. To explore the formation of helical self-assembled structures, circular dichroism (CD) studies were performed for compound I, below CGC. The CD signals of compound I were recorded in methylcyclohexane because the absorption of the solvent is such that it does not overlap with the absorption of the compound I. Compound I exhibits an intense CD signal in methylcyclohexane with signals that appear at 230 and 270 nm (Figure 5, blue line). It was also noted that the helical structures formed in methylcyclohexane are formed only below 16 8C, suggesting that the self-assembled structure is unstable at higher temperatures (see the Supporting Information, Figure S4). Whereas no CD signal was observed in acetonitrile, compound I exhibited a positive cotton effect in acetonitrile– &

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Full Paper Table 2. Kamlet–Taft parameters of solvents.[a]

The CD data indicate that solvent plays a key role in the formation of supramolecular helical structure. Stability and morphology of aggregated species can be achieved by controlling the ratio of good to poor solvent.[12] Several attempts have been made to rationalise the effect of solvents in gelation. Smith and co-workers described solvent–gelator interactions using Kamlet–Taft parameters, which include hydrogen-bond donation ability of solvents (represented by a), hydrogen-bond acceptor ability (represented by b) of solvent, and the dipolarity or dipolarisability (represented by p*) of solvents.[13] Gelator I is soluble in aromatic solvents such as benzene, toluene, and 1,2-dichlorobenzene. The gelator was also soluble in moderately polar (dichloromethane, chloroform) and polar aprotic (tetrahydrofuran (THF), acetonitrile and N,N-dimethylformamide (DMF)) solvents. It is evident by a comparison of the a and b values of known aromatic solvents that gelation by compound I was observed in solvents having a = 0 (no hydrogen-bond donor ability) and relatively low b values (poor hydrogen-bond acceptor ability). For example, solvents such as acetonitrile and DMF prevent H-bonded dimerisation due to relatively high a and b values. Thus, no gelation was observed in the solvents mentioned above. Whereas b values decrease from o-xylene to 1,2-dichlorobenzene, CGC values for compound I increase. This counterintuitive observation indicates that, in such cases, p* values (dielectric) of the solvent play a crucial role. As evident from Table 2, the p* values increase from o-xylene to 1,2-dichlorobenzene, which indicates that the solubility increases in the same order. Given that increased solubility hinders self-assembly, increased concentration of gelators (i.e., greater CGC) is necessary for self-assembly and gelation. Solubility also plays a crucial role in controlling the thermal reversibility of the system because thermal reversibility was only observed for aromatic solvents that can solubilise the poly(aryl ether) part. Gelation was achieved in mixed solvents (e.g., hexane– chloroform, hexane–ethyl acetate, or toluene–chloroform) by the addition of poor solvent such as hexane (for moderately polar solvent) or water (for polar solvent), which can facilitate

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b

p*

0.08 0.00 0.00 0.00 0.00 0.00 0.44 0.30 0.00 0.00 0.19 1.17

0.00 0.16 0.11 0.10 0.07 0.03 0.00 0.00 0.45 0.55 0.31 0.18

0.00 0.18 0.54 0.59 0.71 0.77 0.58 0.82 0.55 0.58 0.75 1.07

p–p stacking interactions, leading to one-dimensional fibre formation. Whereas addition of water enhances p–p stacking, water competes for hydrogen-bonding with the substrates and gel formation is retarded.

Solvent effect

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a

hexane o-xylene toluene benzene chlorobenzene 1,2-dichlorobenzene chloroform dichloromethane ethyl acetate tetrahydrofuran acetonitrile water

[a] a = hydrogen bond donor ability, b = hydrogen bond acceptor ability p = solvent dipolarity or dipolarisability.

Figure 6. Variable temperature partial NMR spectra of compound I in [D8]toluene from 100 8C (bottom) to 30 8C (top).

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Solvent

Mechanism of gel formation Previous study of gel systems based on poly(aryl ether) dendron derivatives indicate that they form nanofibres or helical structures in the three-dimensional gel networks. Nonetheless, the factors that control the formation of fibres or helical structures were not explicitly identified. Experimental results indicate that the mechanistic details of the self-assembly of poly(aryl ether) dendron derivatives that form helical structures are strikingly similar to those of supramolecular polymerisation.[14] In the present case, self-assembly proceeds by either “isodesmic” (involving a single equilibrium constant) or “cooperative mechanism” (involving multiple equilibrium constants). In general, a cooperative mechanism leads to helical and/or tubular helical structure formation due to enhanced monomer interaction.[15] Usually, two monomer units dimerise, and the dimers further self-assemble to yield chiral supramolecular structures as a function of temperature.[16] Wrthner and co-workers developed a “cooperative nucleation–elongation model” (involving two steps: nucleation and elongation in which elongation is a faster process than nucleation).[17] Pantos and co-workers recently reported that the isodesmic supramolecular polymerisation pathway also leads to helical nanotube formation.[18] To understand the initial processes in the supramolecular self-assembly in the present system, the absorption spectra of compound I was recorded over a range of concentrations (2– 55 mm in acetonitrile–water (1:23) mixture). The molar absorption coefficient of compound I at 270 nm was plotted as a function of concentration. A rapid sigmoidal growth curve was obtained when the molar absorption coefficient was plotted against concentration (Figure 7 a). Similarly, a plot of chemical shift values of amide protons in compound II versus temperature resulted in a rapid decay curve, as shown in Figure 7 b. The two models, the isodesmic and the nucleation and elongation model, have been utilised to explain the observed 5

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where s ¼ Kne and Equation (3):

aagg ¼ 1  amon ¼ 1 

Kcmon KcT

ð3Þ

The values of aagg were plotted against KcT for various values of s (Figure 7 a). An isodesmic model will be represented when the value of s = 1 and a cooperative model will be represented when s < 1. The best fit for our experimental data was obtained when s = 0.000001, which implies that, Kn/Ke = 0.000001; that is, the equilibrium constant for the extended aggregation process is 106 times more than that of nucleation. Thus, the present system self-assembled through a highly cooperative process in which the aggregation is much faster than the nucleation. This is further evident from the rapid change of chemical shift, d, of the amide proton in compound II as a function of temperature (Figure 7 b and the Supporting Information, Figure S6). Such sudden changes in the observed parameters invariably suggest the involvement of a cooperative mechanism in aggregation. Upon dissolving the monomer in a given solution, the initial process is likely to be a hydrogen-bonded dimer, as indicated by temperature-dependent NMR experiments. Hydrogenbonded dimer formation results in the generation of an eightmembered cyclic system (Figure 8). The dimer can further self-

Figure 7. a) Plot of fraction of aggregated molecules (aagg) vs. KcT for different values of s {the values of s are: 1 (red spheres), 0.001 (green spheres) and 0.000001 (blue sphere)}. Experimental data points are given as black spheres. b) Amide proton chemical sift plotted as a function of temperature for compound II.

results. In the isodesmic model, aggregation formation is assumed to be one dimensional, and the equilibrium constants for the dimerisation is equal to the equilibrium constants for further aggregation steps; that is, K2 = K3 = K4 = … = Ki … = K (where Ki is the equilibrium constant for formation of the ithmer).[19] The mathematical form for this model can be expressed by Equation (1): aagg ¼ 1  amon ¼ 1 

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi cmon 2 KcT þ 1  4 KcT þ 1 ¼1 cT 2ðKcT Þ2

ð1Þ

where, amon and aagg are the mole fraction of monomeric and aggregated species, respectively, cmon and cT are the concentration of monomer and the total concentration of all the species present in solution, respectively, and K is the equilibrium constant. In the nucleation–elongation model, the equilibrium constant for the nucleation is much slower than that of elongation or growth (K). In other words, the nucleation step is followed by many isodesmic elongation steps, where, Kn ¼ 6 Ke1 = Ke2 = … = Kei = … = Ke. The cooperative model is also known to be applicable to systems in which both p–p stacking as well as hydrogen-bonding are present.[17] The mathematical expressions in the case of cooperative model assembly are given by Equation (2):

KcT ¼ ð1  sÞKcmon þ &

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Figure 8. Schematic representation of helical structure formation from compound I.

assemble through p–p stacking of the benzene units and during this continuing self-assembly process the hydrogen bonds between the primary amine groups in the dimers direct a “twist” in the self-assembled structure, leading to a helical structure. The layers in the helical self-assembly were stabilised by hydrogen bonds between the primary amine groups present in each layer. The helical fibre–fibre interactions finally immobilise solvents through physical force of interaction to result in gel formation. The mechanistic aspects of the gelation in the present system are schematically represented in Figure 8.

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Full Paper Conclusion

Acknowledgements

The hydrogen-bond triggered thermo reversible supramolecular gelation properties of poly(aryl ether) hydrazide has been explored to understand the mechanism of helical structure formation during self-assembly. Low molecular weight lower generation dendritic gelators form efficient (low CGC) and stable gels. Several steps of the gelation process were identified: gelation begins with intermolecular hydrogen bonding, followed by p–p stacking of the dimer to form a helical supramolecular polymer, which further aggregates to form a gel. Gelation properties in terms of thermo reversibility and critical gel concentration were correlated with solvent parameters. The origin of the helical structures in the nanofibers was attributed to the macroscopic chirality in the self-assembled system. Monitoring the initial events of the self-assembly suggests strongly that a cooperative mechanism is operating in the present system.

The authors thank the sophisticated analytical instrument facility and the department of metallurgical and materials engineering at the Indian Institute of Technology Madras for the scanning electron microscopy facility. We thank Dr. Abhijit P. Deshpande, Department of Chemical Engineering, Indian Institute of Technology Madras for rheology experiments. We also thank Dr. Subi Jacob George, New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, India for the linear dichroism experiments. Keywords: dendrimers · gels · helical structures · selfassembly · sol–gel processes

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Experimental Section Materials Poly(aryl ether) dendron derivatives were synthesised according to a reported procedure.[5] The compounds were characterised by using NMR spectroscopy and mass spectrometry (see the Supporting Information, Figure S7–S12). Starting materials for the synthesis (3,4,5-trihydroxy benzoate, benzyl chloride, potassium carbonate, tetrabutyl ammonium iodide, hydrazine monohydrate) were purchased from Sigma–Aldrich, Merck, Fischer scientific or Spectochem. Methylcyclohexane (UV grade) from Spectochem and acetonitrile (HPLC grade) from Merck were used for the photophysical study. [D6]Dimethyl sulfoxide and [D8]toluene were purchased from Cambridge isotope laboratories.

Instruments UV spectroscopy experiments were carried out with a JASCO V660 spectrometer. Circular dichroism and variable-temperature UV experiments were performed with an Applied Photophysics Chirascan spectrometer. LD spectra were recorded with a JASCO J815 circular dichroism spectrometer. Room-temperature NMR experiments were performed with either Bruker 400 or 500 MHz instruments. Variable-temperature NMR experiments were carried out with a 500 MHz instrument. SEM images were captured with either a FEI quanta 400 or 200 instrument. Mass spectra were recorded with a Micromass Q-TOF spectrometer. Rheology experiments were carried out with an Antan Paar MCR-301 rheometer.

Gel formation Single solvent gelation was achieved by solubilising the compound at high temperature followed by cooling with sonication. Mixed solvent gelation was done by dissolving the compound in one solvent and addition of hexane or water (depending on the miscibility in the first solvent) leads to gelation; for example, gelation in chloroform–hexane was achieved by solubilising 5 mg of compound I in 300 mL chloroform followed by addition of 700 mL of hexane. Chem. Eur. J. 2015, 21, 1 – 9

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Full Paper

FULL PAPER & Helical Structures P. Malakar, E. Prasad* && – &&

How will it gel? The J-type aggregation propensity, hydrogen-bond donor/acceptor ability of solvents, as well as the polarity of the medium have been identified as the main factors that control

Chem. Eur. J. 2015, 21, 1 – 9

the self-assembly of helical structures by poly(aryl ether) based dendron derivatives with acyl hydrazone/hydrazide units.

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Self-Assembly and Gelation of Poly(aryl ether) Dendrons Containing Hydrazide Units: Factors Controlling the Formation of Helical Structures

 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Self-assembly and gelation of poly(aryl ether) dendrons containing hydrazide units: factors controlling the formation of helical structures.

Self-assembly of AB2 and AB3 type low molecular weight poly(aryl ether) dendrons that contain hydrazide units were used to investigate mechanistic asp...
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