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Phase Segregation in Supramolecular Polymers Based on Telechelics Synthesized via Multicomponent Reactions

structure-property relationships. The use of the Passerini and Ugi multicomponent reactions to synthesize various poly(ethylene- co -butylene) telechelics with diverse amide end-groups is reported. The thermal properties of the supramolecular polymers obtained through their solid-state assembly are investigated and their nanophase-segregation is studied, which is dictated by the end-group volume fraction and the amide–amide hydrogen bonding.


Introduction
Supramolecular polymers are a broad class of macromolecules in which the monomeric units are held together through noncovalent interactions. [1] These materials are typically based on H-bonding interactions, metal-ligand complexation, ion pairing, or pi-pi stacking, displaying a wide range of binding strengths that vary from very weak to covalent-like. [1] The possibility of addressing these reversible interactions with stimuli such as UV-light, sonication, or variations of temperature and pH makes supramolecular polymers a very attractive platform for the development of stimuli-responsive materials. [2][3][4][5] While using strongly binding supramolecular motifs such as by component choice, which is particularly useful for establishing structure/property relationships and, in the present work, to study the effect of various amide end-groups on the phase segregation behavior of low molecular weight telechelics. We thus report here the synthesis of a family of telechelics that share the same poly(ethylene-co-butylene) (PEB) backbone but display different amide derivatives as end-groups and study their phase segregation behavior. The hydrophobic, amorphous PEB backbone was chosen for its well-known propensity to phase segregate when covalently linked to polar and crystalline groups, [7,10,24] and both the Passerini and the Ugi MCRs were used to functionalize its end-groups with diverse amide derivatives. This synthetic approach was also chosen on the basis of the high crystallinity of most small molecules synthesized via Passerini and Ugi MRCs, [43,44] which were expected to provide a driving force for phase segregation.

Characterization
1 H-NMR spectra were recorded on a Bruker AVANCE DPX spectrometer operating at 300 MHz. CDCl 3 was used as solvent and the resonance signal at 7.26 ppm ( 1 H) served as reference for the chemical shift δ. Differential scanning calorimetry (DSC) studies were carried out with a Mettler-Toledo STAR system under nitrogen atmosphere using a sample mass of ≈5 mg. The glass transition temperature T g is reported as the midpoint of the step change in the heat capacity and the melting point is reported as the minimum of the endothermic peak. Small-angle X-ray (SAXS) spectra were recorded with a NanoMax-IQ camera (Rigaku Innovative Technologies, Auburn Hills, MI, USA). The samples were introduced in quartz capillaries (1 mm ∅) and kept in vacuum at −20 °C during the measurements using a Julabo F25-ME cooling/heating system. Raw data were processed according to standard procedures, and the scattering spectra (1d) are presented as a function of the momentum transfer q = 4π·λ-1·sin (θ/2), where θ is the scattering angle and λ = 0.1524 nm is the photon wavelength. Size exclusion chromatography (SEC) measurements were carried out on an Agilent Technologies 1200 system equipped with a Wyatt Optilab rEX differential refractive index (dRI) detector and a Wyatt miniDAWN TREOS multiangle laser light scattering (MALLS) detector. The column system was composed of an Agilent 5 μm MIXED-C guard column and a 5 μm MIXED-D (200−400 000 g mol −1 ) column from Agilent. Tetrahydrofuran (THF) was employed as solvent/eluent and the measurements were carried out at 30 °C with a flow rate of 1 mL min −1 . The number average molecular weight and dispersity indexes were determined against poly(styrene) standards. Atomic force microscopy (AFM) was performed on a JPK Nanowizard II with a JPK Advanced SPM control station operated in tapping mode using rectangular Al-coated silicon cantilevers (NanoAndMore GmbH, k = 40 N m −1 , f = 300 kHz) with silicon tips (tip radius < 10 nm).

Synthetic Procedures
Carboxylic acid terminated poly(ethylene-co-butylene) P1. Hydroxyl terminated telechelic poly(ethylene-co-butylene) (5.00 g, 1.6 mmol), succinic anhydride (1.61 g, 16.0 mmol, 5.0 eq./end-group), and 4-dimethylaminopyridine (0.39 g, 3.2 mmol, 1.0 eq./end-group) were placed in a 100 mL roundbottomed flask equipped with a magnetic stirrer under nitrogen atmosphere. Anhydrous chloroform (50 mL) was added and the mixture was stirred at room temperature until all reagents completely dissolved. Anhydrous pyridine (1.30 mL, 16.0 mmol, 5.0 eq./end-group) was subsequently added via a syringe and the reaction mixture was stirred for 20 h at room temperature. Chloroform was then partially removed (≈40 mL) under reduced pressure and the concentrated chloroform solution was precipitated twice in 300 mL of methanol at room temperature. The product was dissolved in 200 mL of hexane, washed sequentially with 100 mL of HCl (5%) and brine, and dried over Na 2 SO 4 . Removal of the solvent under reduced pressure afforded carboxylic acid terminated poly(ethylene-co-butylene) P1 as a colorless oil (3.2 g, 60%). 1 H-NMR (CDCl 3 , 360 MHz,): δ (ppm) = 0.83 (t, J = 6.7 Hz, CH 3 backbone), 0.95-2.25 (m, CH 2 and CH backbone), 2.57-2.74 (m, COCH 2 CH 2 CO), 4.05-4.18 (m, COOCH 2 ). The 1 H-NMR spectrum shows residual alkene signals between 5.3 and 4.6 ppm. This is due to the fact that Krasol HLBH-P is industrially produced via hydrogenation of hydroxyl terminated polybutadiene, and the hydrogenation extent is slightly batch-dependent. M n = 6.5 kDa; the molecular weight is higher than expected as a result of fractionation during the precipitation steps.

Results and Discussion
Telechelic poly(ethylene-co-butylene)s (PEBs) containing amide end-groups were synthesized from carboxylic acid terminated telechelic PEB P1 (M n = 6.5 kDa; Ð = 1.13, Scheme 1), which was obtained by base-catalyzed ring opening of succinic anhydride with hydroxyl terminated telechelic PEB. The Passerini or Ugi reaction of P1 was performed with well-established components used in previous studies [41,43,44] including aldehydes 1a-1c, isocyanides 2a-2c, and amine 3a (Scheme 1). In addition, we introduced less common components exhibiting long aliphatic moieties (C 12 -C 22 ) such as lauric aldehyde 1d, isocyanides 2e and 2f, and octadecylamine 3b. We also tested anthracene-and pyrenecarboxaldehyde; however, very low conversion of the acid end-groups was achieved using these aromatic aldehydes arguably due to their low solubility and inherent steric demand. The Passerini reactions were performed in dichloromethane and the Ugi reactions in a mixture of THF and methanol. Full conversion of the end-groups (within the limit of detection of 1 H-NMR) was achieved after 24 h by performing the reaction in highly concentrated solution and using an excess of the components (two equivalents per acid end-group). Thus, amide-functionalized telechelics P2-P12 were obtained in good yields and considered for further characterization (Scheme 1 and Table 1).
The successful synthesis of P2-P12 was proven by 1 H-NMR; Figure 1 shows the assigned spectra of P1 (for reference) as well as P7, P9, and P12 as representative examples. The spectra of all other products can be found in the Supporting Information. The synthesized telechelics display the characteristic signal of the amide NH proton at ≈6.6 ppm and all the other expected signals for each newly formed end-group, which could be unambiguously assigned. Besides, the signals of the new endgroups display the expected integrals ratio (see the Supporting Information) with respect to the α-hydrogens of the ester functional group of P1 (protons a in Figure 1), which indicates that the carboxylic acid end-groups are fully converted (within the detection limit of 1 H-NMR). The corresponding SEC-traces of P2-P12 indicate a shift to higher molecular weights while keeping a narrow molecular weight distribution (Figure 2 and the Supporting Information).
The microstructure of telechelic PEBs P2-P12 was first investigated via DSC. All polymers show the glass transition temperature of the PEB backbone at ≈−50 °C (Table 1); however, only four of them (P7, P8, P9, and P12) display an additional melting endotherm characteristic of a phase segregated structure (Table 1 and Figure 3). The melting temperatures of these polymers range from 13 to 39 °C and P8 and P9 form self-standing films at room temperature. We further analyzed these four polymers via SAXS at −20 °C, i.e., well below their melting temperatures, and in all cases observed correlation peaks indicative of a microphase segregated morphology with short range order (Figure 4). The calculated domain spacings are 9.7 nm for P7, 8.7 nm for P8, 11.6 nm for P9, and 11.2 nm for P12.
Unlike P7, P8, P9, and P12, which phase segregate as a result of end-group crystallization, all other telechelic PEBs were found to be fully amorphous regardless of any attempts   to anneal them. P7, P8, P9, and P12 have in common the presence of long aliphatic chains, which appear to promote crystallization much more efficiently than other groups such as benzyl (P4, P5, and P11) or naphthyl (P6). These observations are in sharp contrast to the crystallinity of previously reported small molecules with similar functionalities synthesized via Passerini and Ugi MRCs, [43,44] and suggest a strong influence of the molar fraction of the end-groups on their aggregation and crystallization. Indeed, the end-group molar fraction in P7, P8, P9, and P12 varies between 12 and 17 mol%, while that of the polymers that did not show microphase segregation oscillates between 7 and 10 mol%. The reduced concentration of motifs in the associated state appears to prevent their crystallization into a separate hard phase. The nanostructure of P9 was subsequently investigated by AFM. Figure 5 shows the height and phase images recorded on the surface of a sample that was melt-cast, cooled to −20 °C for 12 h (same procedure as for the SAXS analysis) and stored inside a desiccator at ambient temperature. The micrograph reveals a nonordered fibril-like nanostructure in agreement with the presence of two phases which, according to the phase image, display clearly different stiffness. These observations are in line with the DSC and SAXS data, and support our conclusion that these telechelics phase segregate into a soft PEB phase and a stiff crystalline phase. In addition, the spacing between adjacent fibrils (see the Supporting Information for profiles) is in good agreement with the domain spacing obtained by SAXS (11.6 nm).
Besides the end-group molar fraction, the absence of crystallization in some of these polymers shows that phase segregation in systems based on weakly H-bonding motifs, such as the amides studied here, requires of highly crystalline motifs or auxiliary functional groups that promote aggregation. Thus, the long alkyl chains in P7, P8, P9, and P12 appear to induce phase segregation more efficiently than the other studied substituents. However, this observation triggers the question whether phase segregation takes place solely on the basis of alkyl chain crystallization or if this process is supported by amide-amide H-bonding. This was investigated by exposing P9 (T m = 39 °C) to trifluoroacetic acid (TFA) vapors at room temperature with the aim of selectively disrupting amide-amide H-bonds of the otherwise acid-inert structure. Such treatment transformed the initially solid sample into a viscous liquid in only 10 min (Figure 6), and the thermogram (first heating scan) of the product showed a decrease of the melting endotherm from 39 to 5 °C ( Figure 6). On the second heating scan, the melting endotherm of P9 was recovered as a result of TFA evaporation ( Figure 6). These results suggest that long alkyl chains alone can promote phase segregation on the basis of their crystallinity and high molar fraction, and that hydrogen bonding does play an important role on the crystallization behavior of these amide-functionalized PEB-based telechelics. Indeed, hydrogen bonding groups such as urea and urethane have already been shown to induce the crystallization of supramolecular motifs attached to the termini of very similar telechelics [45] and, in the present case, the amide groups seem to play the same role by templating the crystallization of the alkyl chains, which results in an increase of the melting temperature.   Overall, this work widens the application scope of the Passerini and Ugi MCRs with the synthesis of functional telechelics. Especially useful is the possibility of rapidly screening very diverse amide end-groups that these MCRs offer, which provides a new insight into the structure-property relationships of this kind of materials.

Conclusions
In this work, we have used the Passerini and Ugi multicomponent reactions to obtain a library of telechelic poly(ethylene-cobutylene)s bearing amide functionalities at both termini. This synthetic approach allowed varying the composition and size of the end-groups, and studying the effect of these parameters on the phase segregation behavior of the resulting telechelics. The introduction of small amide derivatives containing alkyl, benzyl, or naphthyl groups resulted in nonphase segregated telechelics. On the other hand, the end-group functionalization with amide groups displaying long alkyl chains provided phase segregated materials, which suggests that the end-group's molar fraction and the presence of crystalline residues play an important role on the segregation of the polar, hydrogen bonding amide end-groups from the soft poly(ethylene-co-butylene) core. Two of these materials display hard phase melting temperatures over 30 °C and thus form self-standing films at room temperature. Apart from being highly thermosensitive, these materials lose their mechanical integrity when exposed to a H-bonding competitor such as TFA (vapor), but recover their properties once it evaporates.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.  Figure 6. The pictures show a film of P9 before and after exposure to TFA vapors at room temperature for 10 min. The first and second heating scans (DSC traces) of P9 after exposure to TFA vapors are also shown.