Tunable Self-Assembled Elastomers Using Triply Hydrogen-Bonded

May 16, 2012 - Triple hydrogen bonding between novel amidoisocytosine (AIC) and ureidoimidazole (UIM) motifs is used to promote assembly of the materi...
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Tunable Self-Assembled Elastomers Using Triply Hydrogen-Bonded Arrays Adam Gooch,† Chinemelum Nedolisa,‡ Kelly A. Houton,† Christopher I. Lindsay,§ Alberto Saiani,‡ and Andrew J. Wilson*,†,⊥ †

School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom The School of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom § Huntsman Polyurethanes, Everslaan 45, 3078 Everberg, Belgium ⊥ Astbury Centre for Structural Molecular Biology, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom ‡

S Supporting Information *

ABSTRACT: This article describes the synthesis and surpamolecular assembly of polyurethane-based elastomers. Triple hydrogen bonding between novel amidoisocytosine (AIC) and ureidoimidazole (UIM) motifs is used to promote assembly of the material. The material comprises an amorphous phase derived from a telechelic diol and a hard crystalline phase that comprises the supramolecular end groups. The use of a heterocomplementary hydrogen bonding interaction results in two unique features: (1) assembly of the elastomer occurs only in the presence of both components, and (2) different feed ratios used during synthesis allow the materials properties to be tuned as the stoichiometries of the components found in the amorphous and crystalline phases of the material are varied. The approach hence offers supramolecular control over materials properties and results in materials that can be melted and therefore processed at lower temperature compared to standard covalent elastomers.

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INTRODUCTION Supramolecular polymers1−6 represent dynamic systems that utilize noncovalent interactions to assemble easy-to-make molecules into architectures with the physical properties characteristic of traditional covalent polymers.4 Furthermore, supramolecular polymers typically exhibit emergent and stimuli-responsive properties (not commonly expressed by covalent polymers) that can be modulated through environmental variation (e.g., temperature and light).7−9 Such properties make supramolecular polymers an intriguing class of materials that have found application as adhesives,10 coatings,11 scaffolds for tissue engineering,12,13 and healable materials.7,14,15 In solution, the degree of polymerization (DP) is highly dependent upon the affinity (Ka) between (macro)monomer building blocks.16 Ka may be altered by changes in environment which consequently have a dramatic effect upon DP and therefore mechanical properties. Material properties in the bulk state, howeverwhere lateral interactions17−19 play a more prominent roleare also highly dependent upon the crystallinity and morphology of the polymer20−22 which tempers the requirement for high Ka’s between supramolecular monomers. Among the plethora of noncovalent interactions that have been used in supramolecular polymerization, selfcomplementary arrays of hydrogen bonds have been extensively used.23−26 Heterocomplementary building blocks have also been used to construct architectures27−34 that are inaccessible using self-complementary motifs and afford additional supra© 2012 American Chemical Society

molecular control in that both components are required for materials assembly.20,35 Herein we describe a self-assembled elastomer based on association between heterocomplementary triple-hydrogen bonding motifs (Figure 1). Several groups have described elastomers assembled through self-complementary hydrogen bonding between the termini of chain extended polyurethanes21,22,36,37 and through bis-urea cross-linking between polymer chains;38,39 however, the architecture we describe is unique in exhibiting the typical morphology and properties of elastomeric materials only when both the macromonomer and linker components are present, demonstrating a requirement for selective and orthogonal three-point hydrogen bonding to occur in the presence of other competing noncovalent forces. Beyond the additional supramolecular control afforded using two-component heterocomplementary building blocks, the materials behavior may be fine-tuned by using supramolecular chain extender units obtained by altering the stoichiometries of starting material feed during synthesis; as the hard phase is assembled exclusively from supramolecular building blocks, different feed ratios used during synthesis give rise to different stoichiometries of the supramolecular building blocks found in the hard phase (Figure 1). Received: January 17, 2012 Revised: May 3, 2012 Published: May 16, 2012 4723

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Figure 1. Schematic illustrating the assembly of an elastomer mediated by formation of a triply hydrogen-bonded heterodimer. (a) Three-point hydrogen bonding interaction of AIC (pink) and UIM (green) motifs together with control dialkylurea derivative (blue). (b) Reaction of telechelic diol with MDI and then amine leading to UIM provides a statistical mixture of chain elongated telechelic (denoted m) and capped MDI (denoted n). Addition of n + 1 equivalents of ditopic AIC (diamidoisocytosine or DAC) results in hydrogen-bond-mediated assembly of an elastomer in which lateral urea/urethane interactions of the DAC and capped MDI form the hard block while the chain extended telechelic forms the soft block; the proportion of each block is determined by the proportion of chain extended telechelic relative to capped MDI, which in turn is controlled by component stoichiometry during synthesis. Use of diamidopyridine (BAP) which is incapable of hydrogen bonding with UIM prevents self-assembly (c) Use of an amine that does not lead to a triply hydrogen bonding motif prevents self-assembly. (d) Structures of compounds used in this work.

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Synthesis of octanedioic acid bis-[(6-pentanoylamino-pyridin-2-yl)amide], 6 (BAP) is described in the Supporting Information.40 Methods. General Procedure for the Synthesis of Macromonomer Units. The required amount of 4,4′-methylenediphenyl diisocyanate (MDI) 2 was dissolved in dimethylacetamide (DMAC) (5 mL g−1) under a nitrogen atmosphere over 1 h. A solution of poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (polyol) 1 dissolved in DMAC (30 mL g−1) was added dropwise to the stirred MDI 2 solution over 30 min at 87 °C. The reaction was heated at 87 °C for a further 1.5 h. A solution of the required amine dissolved in DMAC (10 mL g−1) was then added dropwise over 30 min, and the reaction mixture was heated at 87 °C for a further 16 h. After this time, the required amount of DAC 5 was added in one portion and stirred for a further 16 h. The reaction mixture was then allowed to cool to room temperature, and the solvent was removed by vacuum distillation to provide the target material. TGA analysis confirmed the presence of residual DMAC within these samples which was removed by film casting prior to materials analysis (Figure ESI8). Synthesis of Supramolecular Polymer 4c.5 (NCO/OH = 4.00). The general procedure for the synthesis of macromolecules was followed using diol 1 (2.00 g, 1.00 mmol), MDI 2 (1.00 g, 4.00 mmol), 5,6dimethyl-2-aminobenzimidazole (0.970 g, 6.00 mmol), and DAC 5 (1.165 g, 3.00 mmol) to provide the title mixture as a hard, brown rubbery material. 1H NMR (300 MHz, DMSO-d6) δ: 11.80 (2H, br s, DAC-NH), 11.60 (2H, br s, DAC-NH), 7.47−7.32 (12H, MDI-ArH), 7.20−7.06 (10H, MDI-ArH), 6.89 (6H, s, diUIM-ArH), 5.91 (2H, s, DAC ArH), 4.25−4.05 (6H, m, MDI CH2), 3.70−3.31 (44H, m, blockpoly(ethylene) CH2, CH2, block-poly(propylene oxide) CH2), 2.25 (16H, s, diUIM-CH3), 2.13 (6H, s, DAC-CH3), 1.21−1.00 (31H, m, block-poly(propylene oxide) CH3). 13C NMR (100 MHz, DMSO-d6) δ: 176.5, 169.5, 157.2, 15.4, 128.8, 118.2, 113.9, 107.0, 74.6−74.4 (m), 72.4−72.2 (m), 69.7, 38.8, 37.4, 35.8, 34.4, 27.9, 24.1, 21.4, 19.8, 19.7, 17.2. IR (solid state) νmax/cm−1 = 3354, 2970, 2929, 1664, 1509.

EXPERIMENTAL SECTION

General Points. All reagents were purchased from commercial sources and were used without further purification. All nonaqueous procedures were performed under an inert atmosphere of nitrogen or argon. Triethylamine was distilled from calcium hydride and stored over potassium hydroxide pellets prior to use. Poly(ethylene glycol)block-poly(propylene glycol)-block-poly(ethylene glycol) (polyol) 1 was stored over 4 Å molecular sieves under a nitrogen atmosphere and dried by heating at 60 °C under reduced pressure for 6 h before each use. Thin layer chromatography was carried out on Merck silica gel 60F254 precoated aluminum foil sheets and visualized using UV light (254 nm). Flash column chromatography was carried out at medium pressure using slurry packed Fluka silica gel (SiO2), 35−70 μm, 60 Å, with the specified eluent. Melting points were determined using Griffin D5 variable temperature apparatus and are uncorrected. NMR spectra were obtained using Bruker DMX500, Bruker DMX400, or Bruker AMD300 spectrometers operating at 500, 400, or 300 MHz, respectively, for 1H spectra and at 125, 100, and 75 MHz, respectively, for 13C NMR as stated. All spectral data reported were acquired at 295 K. Chemical shifts (δ) are quoted in part per million (ppm), using the residual solvent peak as an internal standard (e.g., δH 7.26 and δC 77.00 for CDCl3). Coupling constants (J) are reported in hertz to an accuracy of 0.1 Hz. The abbreviations used to denote 1H NMR multiplicity are as follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br, broad; app, apparent. Signal assignment was aided by analysis of DEPT, COSY, NOESY, ROESY, HMBC, HSQC, and variable-temperature 1H NMR experiments where necessary. IR spectra were obtained using a Perkin-Elmer FTIR spectrometer. High-resolution mass spectra were recorded by the University of Leeds mass spectrometry service using electrospray ionization (ESI), and high-resolution mass spectra were recorded on a Bruker Daltonics micrOTOF spectrometer using electrospray ionization (ESI). Synthesis of N 1 ,N 8 -bis(6-methyl-4-oxo-1,4-dihydropyrimidin-2-yl)octanediamide, 5 (DAC), was recently described elsewhere.40 4724

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Figure 2. (a) Synthesis of supramolecular building blocks 4a−c, 5, and 6 for elastomer assembly (notation and color coding as for Figure 1) (b). (b) Photographs of 1, 3, 4c(NCO:OH, 2:1), and 4c(NCO:OH, 2:1).5 samples. atmosphere, and a scanning rate of 10 °C min−1 was used. The data were analyzed using Universal Analysis 2000 software provided with the instrument.

Dynamic Mechanical Thermal Analysis (DMTA). Measurements were performed using a TA Q800 DMTA Instruments equipped with a liquid nitrogen cooling accessory. Single cantilever mode was used for the characterization of the samples within a temperature range of −100 to 170 °C. The test frequency and amplitude used were 1 Hz and 10 μm, respectively. Wide-Angle X-ray Scattering (WAXS). Measurements were performed on a Philips X’Pert APD (PW 3710) instrument equipped with a copper anode source (generator settings: 40 mA, 50 kV). Scattering pattern were collected in the angular range: 5°−70° (2θ). The scan type, step time, and step size used were continuous, 10 s, and 0.07° (2θ), respectively. Small-Angle X-ray Scattering (SAXS). Measurements were performed using a Hecus S3-MICRO X-ray instrument equipped with a 2D Pilatus detector. The scan time was 1000 s. Thermogravimetric Analysis (TGA). Measurements were performed using a TA Instruments TGA Q500. Experiments were performed using the high-resolution dynamic mode, from room temperature to 600 °C. Samples were placed in platinum pans, and experiments were performed under a nitrogen atmosphere. Differential Scanning Calorimetry (DSC). Measurements were performed on a DSC Q100 from TA Instruments equipped with an autosampler and an RCS cooling system. A total of 5−10 mg of sample was placed in a standard aluminum pan that was not hermetically sealed. Experiments were performed under a nitrogen

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RESULTS AND DISCUSSION Synthesis and Assembly. Our group recently introduced a series of conformer independent hydrogen bonding motifs.41−44 Of these, the triply hydrogen-bonded heterodimer comprising a DDA ureidoimidazole (UIM) motif and its complementary AAD amidoisocytosine (AIC) motif exhibits stability Ka ∼ 33 × 104 (±16 × 104) M−1 in CDCl3.41,43,44 This stability is not ordinarily sufficient to form linear supramolecular polymers in dilute solution; however, we considered this sufficient in the bulk state where lateral interactions play a more important role and saw polyurethane elastomers as an ideal target to illustrate the utility of these motifs. The synthesis and assembly of the supramolecular polymers is outlined in Figure 2a. Isocyanate end-capped “prepolymer” 3 is obtained from hydroxyl-terminated telechelic poly(ethylene glycol)−poly(propylene glycol)−poly(ethylene glycol) (Mw ∼ 2000) 1 and 4,4′-methylenediphenyl diisocyanate (MDI) 2. 1 was selected due to its amorphous nature and commercial availability. IR spectroscopy confirmed the success of each stage 4725

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capped polyurethane 3. Further reaction with amine to give 4c affords a viscous oil, whereas the final polymer 4c(NCO:OH, 2:1).5 is a solid. Furthermore, following isolation of 4c, redissolution in DMAC with DAC 5 followed by removal of solvent afforded a similar material to that which is obtained directly, indicating that materials properties derive from the noncovalent chemistry of both components. The material obtained through these procedures is brown/orange in color; while a small amount of degradation may occur to the end group as has been noted for other related heterocycles, the starting aminobenzimidazole is similarly colored. The chemical stability of these architectures will therefore be the subject of future study. Analytical confirmation of hydrogen-bondmediated assembly using IR spectroscopy is complicated by the presence of multiple hydrogen bonding functionalities in all samplesfor instance, the urethane and urea groups each engage in hydrogen bonding. Nonetheless, distinct differences are observed in the IR spectra of 5, 4c(NCO:OH, 2:1), and 4c(NCO:OH, 2:1).5of particular note is the appearance of a new peak at 1667 cm−1 in the supramolecular polymer (see Figure ESI4). Control samples do not provide materials with comparable properties (see Figure ESI5). Addition of DAC 5 to 1 results in formation of translucent paste 1.5, while addition of DAC 5 to either the aniline- or the di-n-butylamine-capped PU (4a and 4b, respectively) fails to provide a solid material capable of supporting its own weight. Both controls 4a and 4b exhibit visual changes in materials properties and might be ascribed to urethane/urea interactions and/or π−π interactions of the MDI (and aniline unit). Similarly, these controls differentiate materials properties that arise because of supramolecular effects from those that arise due to covalent chain extension of polyol 1 by MDI 2. In addition, no materials properties were observed for the combination 4c(NCO:OH, 2:1).6; this represents a different control whereby H-bonding complementarity is switched off in the supramolecular chain extender component rather that the functionalized telechelic (as for 4a and 4b). Finally, our earlier work on simple ditopic AIC and UIM building blocks points to the ability of these motifs to form self-assembled architecturesin those experiments solubility prevented analysis above 1 mM; however, very stable cyclic dimers were characterized at this concentration.40 We would anticipate that as the concentration increases, oligomers would start to be observed consistent with the ring− chain equilibrium.46 Thus, to obtain the material with elastomeric properties, all components 4c and 5 are necessary pointing to three-point hydrogen bonding as a key contributor of the materials properties. Materials Characterization. The final materials were solvent casted as very thin films (Figure ESI6) to ensure full drying (TGA analysis of the samples before and after processing confirms the removal of residual DMAC; Figure ESI8) and then compression-molded at 120 °C as 18 × 2 × 2 cm specimens (Figure ESI7) for characterization via DMTA, SAXS, WAXS, and DSC. In Figure 3, the DMTA curves obtained for 4c(NCO:OH, 4:1).5 sample are presented. As can be seen, a typical elastomeric mechanical response was obtained with a tan δ < 1, showing that the material behaves like an elastomeric solid albeit with a relatively low G′ (∼100 MPa at 25 °C) compared to standard polyurethane elastomers. The DMTA results also suggest the presence in the sample of a phase-separated morphology typical of elastomeric materials. Two glass transitions at ∼−45 °C (TgSP, soft phase) and ∼70 °C (TgHP, hard phase) were observed. This result was in good

of the procedure (Figure ESI1). This procedure was repeated for a range of NCO to OH ratio’s as illustrated in Table 1. Table 1. Structure, Composition, and Properties of Polymers 1 and 4a−c in the Presence or Absence of 5 amine 1.5 4a.5 4b.5 4c 4c.5 4c.6 4c.5

none aniline di-n-butylamine 5,6-dimethyl-2aminobenzimidazole 5,6-dimethyl-2aminobenzimidazole 5,6-dimethyl-2aminobenzimidazole 5,6-dimethyl-2aminobenzimidazole

NCO:OH ratioa

DACb 5

0:1 2:1 2:1 2:1

yes yes yes no

2:1

yes

2:1

BAP

4:1

yes

supramolecular polymer no no no no yes

c

no yes

a

The NCO:OH ratio refers to the ratio of functional groups present in the starting telechelic 1 and MDI 2 (e.g., for NCO:OH ratio, 2 equiv of MDI 2 are added to 1 equiv of telechelic 1, giving 4 isocyanates and 2 alcohol functional groups). bDAC refers to diamidoisocytosine. c BAP refers to bis(diamido)pyridine.

Prepolymer 3 was capped with a range of amines including 6,7-dimethyl-2-aminobenzimidazole to obtain the UIM array, aniline, and di-n-butylamine. The latter two were synthesized to act as negative controls lacking the ability to engage in multipoint hydrogen bonding, while 5,6-dimethyl-2-aminobenzimidazole was used in preference to the 2-amino-tertbutylimidazole, which we used in earlier work41,43,44 because of its commercial availability. Again, IR spectroscopy confirms complete reaction of prepolymer 3, as evidenced by the absence of NCO peaks between 2270−2100 cm−1 upon addition of amine to give 4a−c (Figure ESI1). The ditopic nature of each component dictates that the polyurethanes 4 are comprised of a statistical mixture of capped and chain extended polyol 1 obtained alongside MDI 2 capped at both termini with the target amine. For instance at an NCO to OH ratio of 2:1, only 50% of polyol 1 becomes capped at both OH groups with MDI, whereas this rises to 75% at an NCO to OH ratio of 4:1 (see Figures ESI2 and ESI3 for a detailed statistical analysis). The corresponding ditopic hydrogen bonding linker units diamidoisocytosine (DAC) 5 and bis(diamido)pyridine (BAP) 6 were synthesized in one or two steps, respectively. BAP was synthesized as a control compound; the DAD presentation of H-bonding groups is not matched for H bonding with UIM motifs (as in 4c). Addition of linker 5 or 6 prior to solvent removal affords polymers 1.5, 4a−c.5, and 4c.6. The materials properties that we describe below derive from different stoichiometries of diol/MDI which leads to different stoichiometries of the components found in the crystalline and amorphous phases of the material. This differs from the commonly observed differences in materials properties that are observed when the stoichiometry of complementary hydrogen bonding motifs is not matched, resulting in chain capping;29,45 it should be noted that in the experiments described here we were careful to match the stoichiometry of the DAC motif and the UIM motifs, and variation of this parameter will form the basis of future work. Visual inspection of the product obtained at each stage of the synthetic procedure confirms the formation of a supramolecular material (Figure 2b). The starting PEG− PPG−PEG diol 1 is a viscous liquid, as is the resultant MDI 4726

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noted that all the other control samples could not be processed to give solid specimens for DMTA analysis, further confirming the key role played by the linear hydrogen bonding arrays in these supramolecular materials. Significantly, the 4c(NCO:OH, 2:1).5 sample could be molded but could not be used for DMTA analysis as it was too fragile, suggesting that this sample has poorer mechanical properties compared to 4c(NCO:OH, 4:1).5 although DSC, SAXS, and WAXS suggest a similar phase morphology (Figure 5). Indeed, two glass transitions suggesting the presence of a

Figure 3. DMTA curves obtained for 4c(NCO:OH, 4:1).5 sample: storage modulus (top) and tan δ (bottom) vs temperature.

agreement with DSC experiments performed on the same sample (Figure 4a) showing two glass transitions at similar

Figure 5. (a) DSC, (b) SAXS, and (c) WAXS curves obtained for 4c(NCO:OH, 2:1).5 sample.

phase separated morphology were observed by DSC at ∼−45 °C (TgSP) and ∼−100 °C (TgHP) for 4c(NCO:OH, 2:1).5. While TgSP was observed at similar temperatures, TgHP was observed at higher temperature for 4c(NCO:OH, 2:1).5 compared to 4c(NCO:OH, 4:1).5 illustrating the different nature of the hard segments present in the two samples, i.e., supramolecular chain extended hard segment in sample 4c(NCO:OH, 4:1).5. A single scattering peak was also observed in SAXS at 10.4 nm for 4c(NCO:OH, 2:1).5. The increase in the interdomain distance for this sample is consistent with the fact that this sample contains less hard segments (in volume fraction) compared to the 4c(NCO:OH, 4:1).5. Indeed, for similar phase-separated morphologies the interdomain distance is expected to decrease with increasing hard segment content. Low levels of crystallinity (lower than for 4c(NCO:OH, 4:1).5) were also detected by WAXS. The difference in properties between these two samples clearly shows the key role played by supramolecular chain extension in these materials for the control of the mechanical properties. It is well-known that the extension of the hard segment in polyurethanes leads to a higher degree of phase separation and usually a higher degree of crystallinity of the hard phase, resulting in improved mechanical properties.50−52

Figure 4. (a) DSC, (b) SAXS, and (c) WAXS curves obtained for 4c(NCO:OH, 4:1).5 sample.

temperatures. The presence of a phase-separated morphology was confirmed by SAXS. A single scattering peak corresponding to an interdomain distance of 7.4 nm was observed (Figure 4b).47−49 In addition, low levels of crystallinity were also detected in the material via WAXS, which is again typical for polyurethane elastomers (Figure 4c) and indicates the presence of lateral interactions between hard segments.47−49 It should be 4727

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(8) Masiero, S.; Lena, S.; Pieraccini, S.; Spada, G. P. Angew. Chem., Int. Ed. 2008, 47, 3184. (9) Burnworth, M.; Tang, L.; Kumpfer, J. R.; Duncan, A. J.; Beyer, F. L.; Fiore, G. L.; Rowan, S. J.; Weder, C. Nature 2011, 472, 334. (10) Eling, B.; Lindsay, C. I. In US. Pat. Appl. Publ., 2007; Vol. US 2007/0149751 A1. (11) Loontjens, J. A.; Jansen, J. E. G. A.; Plum, B. J. M. In US. Pat. Appl. Publ., 2000; Vol. US 6,683,151 B1. (12) Dankers, P. Y. W.; Meijer, E. W. Bull. Chem. Soc. Jpn. 2007, 80, 2047. (13) Dankers, P. Y. W.; Harmsen, M. C.; Brouwer, L. A.; Van Luyn, M. J. A.; Meijer, E. W. Nat. Mater. 2005, 4, 568. (14) Burattini, S.; Greenland, B. W.; Merino, D. H.; Weng, W.; Seppala, J.; Colquhoun, H. M.; Hayes, W.; Mackay, M. E.; Hamley, I. W.; Rowan, S. J. J. Am. Chem. Soc. 2010, 132, 12051. (15) Burattini, S.; Greenland, B. W.; Chappell, D.; Colquhoun, H. M.; Hayes, W. Chem. Soc. Rev. 2010, 39, 1973. (16) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. Rev. 2001, 101, 4071. (17) Yount, W. C.; Loveless, D. M.; Craig, S. L. Angew. Chem., Int. Ed. 2005, 44, 2746. (18) Sivakova, S.; Bohnsack, D. A.; Mackay, M. E.; Suwanmala, P.; Rowan, S. J. J. Am. Chem. Soc. 2005, 127, 18202. (19) Kautz, H.; van Beek, D. J. M.; Sijbesma, R. P.; Meijer, E. W. Macromolecules 2006, 39, 4265. (20) Binder, W. H.; Bernstorff, S.; Kluger, C.; Petraru, L.; Kunz, M. J. Adv. Mater. 2005, 17, 2824. (21) Woodward, P. J.; Hermida Merino, D.; Greenland, B. W.; Hamley, I. W.; Light, Z.; Slark, A. T.; Hayes, W. Macromolecules 2010, 43, 2512. (22) Woodward, P.; Merino, D. H.; Hamley, I. W.; Slark, A. T.; Hayes, W. Aust. J. Chem. 2009, 62, 790. (23) Folmer, B. J. B.; Sijbesma, R. P.; Versteegen, R. M.; Rijt, J. A. J. v. d.; Meijer, E. W. Adv. Mater. 2000, 12, 874. (24) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirschberg, J. H. K. K.; Lange, R. F. M.; Lowe, J. K. L.; Meijer, E. W. Science 1997, 278, 1601. (25) Yamaguchi, K.; Lizotte, J. R.; Hercules, D. M.; Vergne, M. J.; Long, T. E. J. Am. Chem. Soc. 2002, 124, 8599. (26) Lafitte, V. G. H.; Aliev, A. E.; Horton, P. N.; Hursthouse, M. B.; Bala, K.; Golding, P.; Hailes, H. C. J. Am. Chem. Soc. 2006, 128, 6544. (27) Thibault, R. J.; Hotchkiss, P. J.; Gray, M.; Rotello, V. M. J. Am. Chem. Soc. 2003, 125, 11249. (28) Yang, X.; Hua, F.; Yamato, K.; Ruckenstein, E.; Gong, B.; Kim, W.; Ryu, C. Y. Angew. Chem., Int. Ed. 2004, 43, 6471. (29) Ligthart, G. B. W. L.; Ohkawa, H.; Sijbesma, R. P.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 810. (30) Scherman, O. A.; Ligthart, G. B. W. L.; Sijbesma, R. P.; Meijer, E. W. Angew. Chem., Int. Ed. 2006, 45, 2072. (31) Scherman, O. A.; Ligthart, G. B. W. L.; Ohkawa, H.; Sijbesma, R. P.; Meijer, E. W. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 11850. (32) Park, T.; Zimmerman, S. C. J. Am. Chem. Soc. 2006, 128, 13986. (33) Park, T.; Zimmerman, S. C. J. Am. Chem. Soc. 2006, 128, 11582. (34) Kolomiets, E.; Buhler, E.; Candau, S. J.; Lehn, J. M. Macromolecules 2006, 39, 1173. (35) Hirst, A. R.; Smith, D. K. Chem.Eur. J. 2005, 11, 5496. (36) Lillya, C. P.; Baker, R. J.; Hutte, S.; Winter, H. H.; Lin, Y. G.; Shi, J.; Dickinson, L. C.; Chien, J. C. W. Macromolecules 1992, 25, 2076. (37) Woodward, P.; Clarke, A.; Greenland, B. W.; Hermida Merino, D.; Yates, L.; Slark, A. T.; Miravet, J. F.; Hayes, W. Soft Matter 2009, 5, 2000. (38) Versteegen, R. M.; Kleppinger, R.; Sijbesma, R. P.; Meijer, E. W. Macromolecules 2005, 39, 772. (39) Courtois, J.; Baroudi, I.; Nouvel, N.; Degrandi, E.; Pensec, S.; Ducouret, G.; Chanéac, C.; Bouteiller, L.; Creton, C. Adv. Funct. Mater. 2010, 20, 1803. (40) Gooch, A.; Barrett, S.; Fisher, J.; Lindsay, C. I.; Wilson, A. J. Org. Biomol. Chem. 2011, 9, 5938.

As highlighted previously, the statistical nature of the synthesis leads to a heterogeneous mixture of covalently capped and covalently chain extended polyol 1, and it is therefore noteworthy that the sample 4c(NCO:OH, 4:1).5 is composed of a higher proportion of lower molecular weight constituents. Hence, the properties clearly derive from the supramolecular interactions which are present: noncovalent chain extension through three-point hydrogen bonding interactions between the terminal UIM and AIC motifs resulting in extended hard segments. This, in turn, leads to better phase separation and a higher degree of crystallinity, resulting in superior mechanical properties for 4c(NCO:OH, 4:1).5.

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CONCLUSIONS In conclusion, we have shown that elastomers can be assembled using easy to access triply hydrogen-bonded heterodimers of moderate stability and that materials properties can be modified through component stoichiometry. Uniquely, materials assembly is dependent on the presence of two heterocomplementary components which must interact in the presence of a multitude of additional competitive noncovalent forces, giving additional supramolecular control in comparison to materials assembled through homocomplementary motifs. These results clearly demonstrate that heterocomplementary hydrogen bonding motifs can be used to create a new family of elastomeric materials which can be processed using standard protocols at relatively low temperatures in comparison to standard covalent elastomers. The material properties of elastomers are advantageous in a myriad of applications ranging from adhesives to fibers and foams,53 and our own future studies will focus on exploiting this approach for the optimization and development of easily processed functional materials.

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ASSOCIATED CONTENT

S Supporting Information *

Details of experimental procedures, small molecule/polymer characterization, materials characterization, and chain-extension statistical calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Huntsman Polyurethanes and EPSRC. REFERENCES

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