Double Dynamic Supramolecular Polymers of Covalent Oligo

Article pubs.acs.org/Macromolecules

Double Dynamic Supramolecular Polymers of Covalent OligoDynamers Gael̈ Schaeffer,† Eric Buhler,‡ Sauveur J. Candau,† and Jean-Marie Lehn*,† †

Laboratoire de Chimie Supramoléculaire, Institut de Science et d’Ingénierie Supramoléculaires (ISIS), Université de Strasbourg, 8 allée Gaspard Monge, Strasbourg 67000, France ‡ Laboratoire Matière et Systèmes Complexes (MSC) UMR 7057, Université Paris Diderot-Paris 7, Bâtiment Condorcet, 75205 Paris cedex 13, France S Supporting Information *

ABSTRACT: Double-dynamic polymers, incorporating both molecular and supramolecular dynamic features (“double dynamers”) have been generated, where these functions are present in a nonstoichiometric ratio in the main chain of the polymer. It has been achieved by (1) the formation of covalent oligo-dynamers in which the monomers are connected by reversible covalent interactions and (2) the association of these oligomers through supramolecular interactions (hydrogen bonding). This procedure leads to the formation of an entity that can be seen as a supramolecular polymer of molecular oligo-dynamers. It thus combines two types of dynamic processes that do not simply alternate in the polymeric chain but may be incorporated in various ratios. These non-alternating double dynamic polymers have been generated by sequential construction and the different steps have been characterized by NMR spectroscopy, mass spectrometry and light scattering.

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acylhydrazone units in the main chain (double dynamer),64 where the two dynamic functions can be addressed selectively. Other examples have shown that it is possible to prepare a supramolecular polymer by using two different types of noncovalent interactions, such as hydrogen bonds and metal ion coordination,65−68 which can also be used to control the sequence in a block copolymer.69 More recently, the orthogonality of these two types of interactions has been demonstrated in dendritic systems.70 Different combinations of dynamic functions have also been implemented to form dynamers. A system combining hydrogen bonds and electrostatic interactions has been developed, showing that these two types of interactions can be used simultaneously. 71,72 Coulombic interactions have also been combined with coordination chemistry to create multiply stranded polymeric entities.73 Host-guest interactions have been utilized together with metal-ligand interactions to yield linear74 or cross-linked polymers.75 Similarly, the formation of polymeric chains has been achieved by combining hydrogen bonding with hostguest76 or with pi-stacking interactions.77,78 Supra-macromolecular species have also been obtained by using two different types of similar dynamic functions, e.g., two types of hydrogen bonding units79,80 (in some cases, geometrically orthogonal) or two different metal-ligand coordination interactions.81 More recently, two dynamic functions have

INTRODUCTION The use of dynamic connections between monomers leads to the generation of dynamic polymers, termed, in short, dynamers.1−16 These entities exhibit reversible formation and component exchange due to the lability of the connections in the polymeric chain. They present the ability to undergo modification of their constitution via incorporation/decorporation and exchange of components as well as the capability to adapt their length in response to a specific stimulus or to changes in the environment. A range of novel properties, including self-healing17−20 may be expected to become accessible. The dynamic character may result from the use of either supramolecular or molecular (reversible covalent) junctions between the monomers. Several types of noncovalent interactions, such as hydrogen bonds,19,21−31 metal−ligand interactions32−36 or other types of “weak interactions”37−46 have been used to generate supramolecular polymers.47 Similarly, reversible covalent polymers have been created by using various reactions, such as transesterification,48−51 transetherification,52 Diels−Alder reactions,20,53,54 boronate ester formation55 or imine-like condensations.3−13,56−62 Diversity and complexity in dynamers may be provided through the use of multiple dynamic processes in the main chain of a polymer.63 It gives access to entities that display potentially the properties of all the functional groups used in the polymer and thus are responsive to a larger number of stimuli. It has been achieved by combining supramolecular and molecular dynamicities to form a polymer. Such dynamers have been obtained with, for example, hydrogen bonds and © 2013 American Chemical Society

Received: March 1, 2013 Revised: June 13, 2013 Published: July 1, 2013 5664

dx.doi.org/10.1021/ma400449u | Macromolecules 2013, 46, 5664−5671

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been introduced in the main chain to control the degradation rate of the polymer.62 It is also worth mentioning that different types of dynamic units have been used for selective grafting of lateral side chains.82 Except for some dendritic,70 multiple strand,73 cross-linked,75 and laterally stacked systems,80 in all the other examples mentioned previously the sequence of the dynamic functions in the chain is similar. This is a direct consequence of the design of the species. The bifunctional monomers are designed so as to present one of the dynamic functionalities at each end. Thus, the polymerization creates an entity where the dynamic units alternate regularly along the chain. The use of different dynamicities in a polymer increases the functional complexity of the entity (more properties accessible and system responsive to more stimuli) but the structural complexity is not enhanced. In the work presented here we describe a novel polymeric system based on nonstoichiometric molecular and supramolecular dynamic links, i.e. links that do not simply alternate in the main chain. This feature has been achieved through the use of a sequential construction of the final architecture. (1) Oligomers containing dynamic covalent bonds (oligo-dynamers) were first prepared by taking advantage of quantitative acylhydrazone formation in organic solvents under conditions where the two monomers were not present in equimolar amounts. (2) The resulting oligo-dynamers were then functionalized on their unreacted termini with groups containing hydrogen bonding sites, and (3) finally, these monomers were polymerized by supramolecular polyassociation with a monomer presenting complementary hydrogen bonding groups. The final entity can be seen as a supramolecular polymer (due to hydrogen bonds) of covalent oligo-dynamers (containing reversible acylhydrazone connections). Thus, it is a double dynamer where the dynamic units are not simply alternating in the polymer; i.e., they may be present in a variable ratio in the chain (Scheme 1). In this way, the double

Scheme 2. Molecular (Top) and Schematic (Bottom) Representation of (a) the Bis(hydrazide) Bis-HYD and (b) the Bis(aldehyde) Bis-ALD Building Blocks Used as Monomers

the capacity to provide soluble supramolecular polymers,21,23,25 were chosen as the central part of the monomers (Scheme 2). The bis-hydrazide monomer (Bis-HYD) was synthesized according to a previously described procedure,83 whereas the bis-aldehyde (Bis-ALD) was prepared by modifying a known compound (see Supporting Information). In order to generate the desired aldehyde-terminated oligoacylhydrazones Oligo-ALD, the two difunctional monomers were mixed in unequal amounts, in 5/4 Bis-ALD/Bis-HYD ratio. The formation of a specific oligo-dynamer (see also below) is shown in Scheme 3. Acylhydrazone formation is known to be quantitative in organic solvents, so nonstoichiometric conditions, nBis‑ALD = nBis‑HYD + 1, lead to the formation of an oligomer terminated by unreacted aldehyde units, which may be used for further functionalization. Scheme 3 shows the simplest possible outcome of the reaction, which in fact (see ahead) produces a mixture of oligomers. The size of the blocks can be adjusted by varying the stoichiometry of their components. It is known that the properties of block copolymers are highly dependent on the composition and the arrangement of the blocks in the chain.84 In the present case, changing the size of the blocks in the system leads to drastic changes in the solubility of the final supramolecular polymer (in typical organic solvents). For example in C2D2Cl4, if the blocks are relatively long (Bis-ALD/ Bis-HYD = 10/9), the final polymer tends to form large aggregates (approximately 500 nm), whereas when the blocks are shorter (Bis-ALD/Bis-HYD = 5/4), the solubility of the final supramolecular polymer is enhanced. If the blocks are too short (Bis-ALD/Bis-HYD = 3/2), the polar end-capping groups begin to influence the solubility to such an extent that the final polymer becomes insoluble. Therefore, the fine-tuning of the system presented here (Bis-ALD/Bis-HYD = 5/4) allowed for the formation of a highly soluble final entity that could be characterized by static and dynamic light scattering techniques. The choice of a ratio 5/4 for the two monomers was also made in order to obtain an oligomer large enough not to be considered as a small molecule, but with the −CHO chain ends still easily observable by NMR spectroscopy. The NMR spectra of the reactants and of the resulting mixture of oligomeric products Oligo-ALD are shown in Figure 1. The 1H NMR spectrum of a 5 mM solution of Bis-HYD in C2D2Cl4 (Figure 1, top) is in good agreement with that reported in the literature.83 The spectrum of Bis-ALD (Figure 1, middle) agrees with the structure. The required amount of Bis-HYD was added to a 5 mM solution of Bis-ALD in C2D2Cl4 to give a molar ratio Bis-ALD:Bis-HYD = 5:4 and the mixture was heated at 50 °C overnight. The resulting solution

Scheme 1. Schematic Representation of a Molecular and Supramolecular Double Dynamer Where the Two Dynamic Functions Do Not Alternate Regularly in the Chain and May Be Introduced at Will in Various Proportions through the Build-up Strategy

dynamicity not only increases the functional complexity of the polymeric entity, but also its structural one, paving the way toward dynamic macromolecular engineering.

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RESULTS AND DISCUSSION For the purpose of the present work, the first step was the synthesis of the covalent dynamic building blocks Bis-HYD and Bis-ALD (Scheme 2), followed by their condensation to form aldehyde-terminated oligo-acylhydrazones Oligo-ALD (Scheme 3). In view of the poor solubility of polyacylhydrazones in organic solvents,3 tartaric acid derivatives functionalized with long lateral alkyl chains, which have already shown 5665


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Scheme 3. Schematic (Top) and Molecular (Bottom) Representation of the Formation of Oligo-Dynamers (Oligo-ALD) by Condensation of 5 equiv of Bis-ALD and 4 equiv of Bis-HYD

was allowed to cool to room temperature and its 1H NMR spectrum was recorded (Figure 1, bottom). The marked broadening of most of the signals, with respect to the monomers, is indication for the formation of a larger entity, corresponding to oligomers, Oligo-ALD. The signals due to Bis-HYD (in particular the two peaks corresponding to the NH and NH2 protonsrespectively around 7.9 and 4.2 ppm) are broadened beyond detection and the peaks corresponding to the protons a, b, and c (assigned in Scheme 3) have decreased relatively by approximately 80%, in good agreement with the fact that the end groups of the oligomers are aldehyde units that are similar to those present in the monomer Bis-ALD. The singlet at approximately 8.2 ppm has the correct integral to be assigned to the CH resonance of the acylhydrazone links. The width and loss of resolvable multiplicity of the signals of the CH2 protons of the lateral alkyl chains (between 3.4 and 4 ppm) are also congruent with the formation of oligomeric species (linear or cyclic).

Figure 1. 400 MHz 1H NMR spectra of Bis-HYD (top), Bis-ALD (middle) and a mixture of 5 eq. of Bis-ALD and 4 eq. of Bis-HYD (bottom) at 5 mM in C2D2Cl4 . The peaks marked a, b and c are assigned to the aldehyde and two aromatic protons as identified in Scheme 3. 5666


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solvents. Inasmuch as these molecules must possess end-groups suitable for further polymerization reactions, they can be described as telechelic oligomers, a property that has been put to use through reactions to introduce H-bonding entities. The functionalization of the telechelic oligo-dynamers OligoALD with hydrogen bonding subunits was achieved by their condensation with HB-HYD, a hydrazide derived from a sextuple H-bonding receptor subunit HB.85,86 Two equivalents of HB-HYD were added to a solution in C2D2Cl4 of OligoALD (containing aldehyde groups at both ends), and after one night at 50 °C the solution was cooled to room temperature. A simplified representation of the reaction expected to take place with any given oligomer is shown in Scheme 4. The reaction was monitored by NMR (Figure 3). The peaks corresponding to the CHO protons a and to the aromatic doublets b and c were absent in the product mixture, indicating complete reaction of both aldehyde groups of each oligomer. The compound HB-HYD is insoluble in C2D2Cl4 but after one night in presence of Oligo-ALD at 50 °C, a clear solution was obtained even after allowing the sample to return to room temperature. Two new peaks at 8.2 and 8.8 ppm are assigned to the NH amide protons of the H-bonding receptor group and the increased complexity of the aromatic region is as expected due to the grafting of the HB-HYD units. The H-bonding functionalized Oligo-HB mixture was analyzed by MALDI-TOF mass spectrometry (Figure 4). Again, two different calibrations were required in order to cover as wide a mass range as possible. While a progression of peaks separated by a constant unit as in the spectrum of the precursor oligomer mixture was not observed, it was possible to assign most of the major peaks to adducts of species that could be present in the media. Of the bifunctional entities present, AB2C2 was dominant but adducts of A2B3C2 and A3B4C2 were also detectable. The absence of a repetitive pattern could be explained by the fact that the added HB-HYD can not only condense with the terminal aldehyde unit but can also react by exchange reactions with the acylhydrazone units within the oligomeric chains. Nonetheless, no free −CHO proton signals were observed in the NMR spectrum of the reaction mixture, so all aldehyde units must have retained their involvement in the acylhydrazone entities. Adding HB-HYD possibly increased the polydispersity of the oligomers but led only to bifunctional entities. The formation of large, compact 3D aggregates around 300 nm in hydrodynamic radius was established by light scattering measurements for Oligo-HB solutions in C2D2Cl4, thus showing that Oligo-HB itself can assemble. The polydispersity index (PDI) obtained using the cumulant analysis (see Supporting Information for details) is equal to 0.34, a large value characteristic of polydisperse aggregates. The size distribution obtained by applying the Contin method to our data corroborates this trend and is presented in the Supporting Information (see Figure SI-5). The functionalized oligomers Oligo-HB were converted into supramolecular polymers by connecting them through the ditopic hydrogen bonding linker Bis-CYAN (Scheme 5). This Bis-CYAN component has been widely used in the past and is known to generate supramolecular polymers (SPs) by interaction with entities containing (at least) two HB receptor subunits, complementary to the CYAN group.23,25 One equivalent of this bifunctional compound was added to a solution containing one equivalent of functionalized oligomers Oligo-HB in C2D2Cl4. The anticipated polymerization to give

While the formation of cyclic species is possible, these would contain equimolar quantities of bis-aldehyde and bis-hydrazide, such that their formation would lead to an increase of free BisALD in the reaction mixture, which would in turn be expected to react with the linear oligomers also present in the solution, thus forming shorter oligomers. In fact, all of the species observed in the MALDI-TOF mass spectrum of the OligoALD solution (see Figure 2) show an AnBn+1 stoichiometry,

Figure 2. MALDI−TOF experiment on a solution of Oligo-ALD in C2D2Cl4, calibrated for small masses (peptide calibration standard 2 from Bruker; matrix, HCCA) (top) or bigger masses (calibration, PEG 6000 from Fluka; matrix, DHB) (bottom).

indicating that the species present are predominantly linear. Moreover, the connecting groups between the monomers in linear and cyclic oligomers are acylhydrazones that can react further, thus allowing for the ring-opening of cyclic species that might have formed and leading to the final polymer (which has been characterized by light scattering experiments). For clarity, the compounds Bis-HYD and Bis-ALD are named respectively A and B. The results in the upper part of Figure 2 were obtained by calibrating the instrument with the peptide calibration standard 2 from Bruker, the matrix used being HCCA (α-cyano-4-hydroxycinnamic acid). The spectrum shown in the lower part was obtained by using PEG 6000 from Fluka as a calibration standard; DHB (2,5-dihydroxybenzoic acid) was used as a matrix. Ignoring what appears to be a minor error due to the use of different calibration standards for the two mass ranges, this spectrum shows typical polymer characteristics, in that there are major peaks separated by a constant amount of 1173 Da. This difference is consistent with each species differing by an AB(−H2O) unit. Condensation of one molecule of A and one molecule of B with the loss of one molecule of water (molecular weight: 694.9 + 514.8 −18 = 1191.7) gives a unit which has to be connected to the pre-existing oligomer by losing a molecule of water (1191.7 − 18 = 1173.7). The first observable peak at 717 Da corresponds to the sodium ion adduct of compound B. By successive addition of one repeat unit, the species [AB2 + Na]+, [A2B3 + Na]+, [A3B4 + Na]+, [A4B5 + Na]+ and [A5B6 + Na]+ result. Given the use of an excess of B over A, the dominant species should be “AxBx+1”. Clearly, a mixture of oligomers appears to have been formed, although it is possible that the use of the MALDI TOF method, coupled to the fragility of the oligomers, may have led to a biased detection of only the lighter and smaller species. Nonetheless, the DOSY NMR measurements show that there is not a large range in the size of the oligomers, so that both spectroscopic studies may be taken to indicate that the particular reaction provided a mixture of a relatively narrow range of small oligomers, presenting high solubility in apolar 5667


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Scheme 4. Schematic (Top) and Structural (Bottom) Representation of the Functionalization of the Oligo-ALD by 2 equiv of HB-HYD (in C2D2Cl4)

SPs through H-bond formation was monitored by 1H NMR spectroscopy (Figure 5). The compound Bis-CYAN contains the same tartaric acid core as the two monomers Bis-ALD and Bis-HYD used to generate the oligomers Oligo-ALD and subsequently OligoHB. Therefore, the addition of 1 equiv of Bis-CYAN to 1 equiv of Oligo-HB in C2D2Cl4 did not lead to drastic changes in the 1 H NMR spectrum, as the signals were mostly overlapping. Just one equivalent of Bis-CYAN was added, of course, with respect to 4 equiv of Bis-HYD and 5 equiv of Bis-ALD. The peak at 9.1 ppm (due to the NH proton of the cyanuric acid in Bis-CYAN) was broadened due to the involvement in supramolecular interactions. Overall, peak widths were not markedly affected

Figure 3. 400 MHz 1H NMR spectra in C2D2Cl4 of Oligo-ALD before (top) and after (bottom) functionalization by 2 equiv of HB-HYD to give Oligo-HB.

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Figure 4. MALDI−TOF experiment on a solution of Oligo-HB in C2D2Cl4, calibration for small (bottom) or bigger (top) masses. For simplicity, the compounds Bis-HYD, Bis-ALD, and HB-HYD are respectively denoted A, B, and C.

Figure 6. SLS scattered intensity as a function of q for a C2D2Cl4 solution containing an equivalent mixture of Oligo-HB and Bis-CYAN at 6 g/L. The linear correlation between 1/R(q) and q2 in the low q range is shown in the inset.

Scheme 5. Molecular (Left) and Schematic (Right) Representations of the Compound Bis-CYAN

molecular weight much larger than that obtained for this system (see next paragraph). The low SLS q data were fitted to a classical Guinier expression (1/R(q) = 1/R(0) × (1 + q2RG2/3)), which provides the average radius of gyration RG and the zero-wave vector scattering intensity, R(q = 0). The best linear fit (see inset of Figure 6) to the data, gives RG,apparent = 208 nm at 6 g/L. Neglecting the excluded volume interactions, the extrapolation to zero-q of the scattered intensity provides a direct measure of the apparent weight-average molecular weight of the supramolecular polymers, Mw (see Supporting Information). The use of the value of the refractive index increment determined on similar systems,25,27 gives Mw,apparent = 925200 g/mol at 6 g/L. The normalized scattered electric field autocorrelation function, g(1)(q,t), obtained by DLS is clearly monomodal as illustrated by Figure 7. The relaxation time, which varies as q−2, provides the apparent cooperative diffusion coefficient of the supramolecular polymers. The Stokes-Einstein relation (see Supporting Information) gives an apparent hydrodynamic radius of RH = 137 nm. The polydispersity index (PDI), k2/ k12, was obtained using the classical cumulant analysis (see

Figure 5. 400 MHz 1H NMR spectrum of: (top) a solution of OligoHB; (middle) a solution of Bis-CYAN; (bottom) an equimolar mixture of Oligo-HB and Bis-CYAN generating the supramolecular polymers SPs (all in C2D2Cl4).

by the reaction with Bis-CYAN, indicating that the reactant oligomers were already of low mobility. To probe both the size and conformations of the supramolecular polymers generated, a series of static (SLS) and dynamic (DLS) light scattering experiments was conducted on a C2D2Cl4 solution containing an equivalent mixture of Oligo-HB and Bis-CYAN. Figure 6 displays the SLS scattering pattern for such a mixture. The scattering profile exhibits: (i) a Guinier regime at low q associated with the finite mass and size of the scattered objects; and (ii) a regime at higher q in which the q dependence of the scattered intensity (Rayleigh ratio of the polymers R(q)) can be described by a power law with an exponent close to −2 and consistent with Gaussian coils. Here a great care is needed because the q−2 scaling is observed for 0.012 ≤ q (Å−1) ≤ 0.027, and usually a larger q-range of scaling law is needed to determine precisely the fractal dimension of the scattering particle. However, more compact structures displaying for instance a q−3 law would be characterized by a

Figure 7. Scattered electric field autocorrelation function, g(1)(q,t), at θ = 90°, for the supramolecular polymers SPs formed in a C2D2Cl4 solution containing an equivalent mixture of Oligo-HB and Bis-CYAN at 6 g/L. The normalized distribution of the scattered intensity as a function of the size, obtained with the Contin method, is shown in the inset. 5669


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Supporting Information), where k1 and k2 represent the first and the second cumulant, respectively. For an equivalent mixture of Oligo-HB and Bis-CYAN a PDI of 0.17 was obtained, a value characteristic of rather polydisperse polymers. The results obtained by applying the Contin procedure to our data and presented in the inset of Figure 7 corroborate the trend observed using the cumulant method and give RH,apparent = 129 ± 10 nm at 6 g/L. The stability with time of the SPs mixtures has been checked using DLS also, and no evolution in RH was observed over several months. Important information on the conformation of the polymers can be provided by the ratio RG/RH. Neglecting virial effects, the value of the ratio is close to 1.5 in the whole range of q, a value calculated in particular for Gaussian coils. Together with the q−2 dependence measured in the scattering vector regime q > RG−1, these results are in favor of the formation of noncompact structures, such as Gaussian supramolecular polymers. The polymers formed were also characterized by variable temperature NMR (Supporting Information). The observed changes in the NMR shifts were not drastic but varying the temperature from −20 to +90 °C changed the signal breadth remarkably. Note that even at −20 °C the solution remained perfectly clear; i.e., no sign of large scale aggregation or precipitation was visible.

ACKNOWLEDGMENTS G.S. thanks the Ministere de la Recherche for a doctoral fellowship. This research was supported by the University of Strasbourg and the CNRS (UMR 7006) as well as by the ANR 2010 BLAN-717-1 Project and the ERC Advanced Grant SUPRADAPT.

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CONCLUSION In the present work, we have shown that the use of a sequential construction strategy for a polymeric entity allowed for the generation of a double dynamer type supramolecular polymeric system that combines two dynamic links (molecular/covalent and supramolecular/noncovalent) that do not simply alternate in the macromolecular main chain, conferring thus two types of dynamicities, whose relative proportions may be controlled through the sequence of steps. A non-equimolar ratio of reagents in a quantitative reaction (acylhydrazone formation) was used to create telechelic dynamic oligomers (of a size that can be tuned by adjusting the ratio of the reagents). These entities were then functionalized by units capable of undergoing noncovalent interactions, here hydrogen bonds. The last step was the generation of the supramolecular polymeric entity(ies) by connecting these oligo-dynamers via the complementary partner for establishing the hydrogen bonding interactions. The exploration and implementation of such strategies paves the way for the use of multiple (orthogonal) dynamic functions toward the engineering of complex dynamic macromolecular structures of combined molecular and supramolecular nature. ASSOCIATED CONTENT

S Supporting Information *

Synthesis details of all the compounds, DOSY and variable temperature NMR, and details on light scattering experiments. 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: (J.-M.L.) [email protected]. Notes

The authors declare no competing financial interest. 5670


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Double Dynamic Supramolecular Polymers of Covalent Oligo

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