Dendronized Polymers with Silver and Mercury ... - ACS Publications

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Dendronized Polymers with Silver and Mercury Cations Recognition: Complexation Studies and Polyelectrolyte Behavior Jérôme Roeser,†,‡,# Benoît Heinrich,† Cyril Bourgogne,†,∥ Michel Rawiso,§ Sylvia Michel,‡ Véronique Hubscher-Bruder,‡ Françoise Arnaud-Neu,*,‡ and Stéphane Méry*,† †

Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), Université de Strasbourg, CNRS UMR 7504, 23 rue du Loess, BP43, 67034 Strasbourg Cedex 02, France ‡ Institut Pluridisciplinaire Hubert Curien (IPHC), Université de Strasbourg, CNRS UMR 7178, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France § Institut Charles Sadron (ICS), CNRS UPR 22, Université de Strasbourg, 23 rue du Loess, BP 84047, 67034 Strasbourg Cedex 02, France S Supporting Information *

ABSTRACT: Metal binding properties of a series of wormlike dendronized polymers bearing oxathiaether-based dendrons are reported. Extensive characterization of the complexation properties toward a large range of metal cations showed a high and selective affinity of the polymers for Ag+ and Hg2+ cations. Its origin is explained by the presence of specific M···S and M···O interactions (M = Ag+ or Hg2+) within a cage structure formed by the dendritic moiety. The stoichiometry of the complexation is found to be affected by the degree of steric constraints in the dendronized materials. The effect of Ag+ complexation leads to the appearance of polyelectrolyte/ charged colloid properties which were intensively studied by SANS. A significant result is the absence of major modification of the (spherocylinder) shape of the polymers upon Ag+ sequestration which confirms the above mentioned complexation scenario. Another outstanding result of Ag+ complexation is the Coulombic stabilization of the charged denpols that drastically affects their thermoresponsive properties (sharp elevation of LCST), indicating possible chemosensing applications.

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bioscience in particular.7 Because of all these intrinsic features, dendronized polymers constitute promising novel functional and hierarchically structured materials that should find applications in a wide range of domains including catalysis,6a,8 biomedicine,9 sensing,10 or nanoscience.11 Highly selective systems or processes are of major interest in the development of new materials. For instance, selective systems able to finely discriminate metal ions can find applications in waste treatment or sensing. Polymer-supported catalytically active metals are also of both academic and industrial interest.12 Because of the cavity that can be formed within the dendritic side chains, dendronized polymers (and dendrimers in general) are ideal candidates for host−guest applications.13 It is worth noting that for a given generation a more congested dendritic environment can be reached more easily with dendronized polymers (worm shaped) than with classical dendrimers (globular shaped), which should be of interest in some aspects of complexation.

INTRODUCTION Since the first synthesis of dendronized polymers (also called denpols) two decades ago, research on this new type of macromolecular architecture was mainly focused on synthetic optimization.1 This tremendous work leads now to the largest synthetic structure with a molecular precision ever achieved.2 A high number of structures with different types of functional groups are nowadays described, and the chemical tools to succeed in their challenging synthesis start now to be under control and understood.1−4 Naturally, interest in this field starts to focus more on application of these original macromolecules in order to elaborate new functional materials. Applications of this class of macromolecules can be brought by several of their intrinsic features. Probably the most promising one is the fact that a high number of functional groups can be incorporated within a single molecule in a very controlled manner (i.e., at every repeat unit).2,5 The presence of an interior cavity within the dendritic shell leading to a special chemical and shielded environment is also of interest for applications.6 Finally, the typical worm-shape morphology of dendronized polymers, whose flexibility can be varied according to the dendrons’ degree of crowding, constitutes another point of interest, in © 2013 American Chemical Society

Received: February 18, 2013 Revised: August 7, 2013 Published: August 29, 2013 7075

dx.doi.org/10.1021/ma400348v | Macromolecules 2013, 46, 7075−7085

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Figure 1. Molecular structure of the bis- and tris-dendronized polymers (and corresponding monomers) with peripheral ethyleneoxy chains (EO2 and EO3). The degree of polymerization ranges between 6 and 82 (see Table 1).

So far, only a few studies of metal complexation by dendronized polymers have been reported in the literature. Some reports describe changes in the polymer self-organization by swelling effects upon Li+ binding by oligo(ethyleneoxy) (EO) chains used as spacers14 or placed at the dendron periphery.15 Polymer swelling was also studied upon binding of K+ by polymers’ crown ether side chains.16 In other studies, complexation was used to build polymer backbone17 or dendritic side chains.18 Finally, a dendronized polymer with metal ion sensing properties was reported.19 Actually, metal complexation studies by dendritic materials have mainly been performed on globular-shaped dendrimers (such as PAMAM or PPI derivatives). Surprisingly, crowding effect on metal binding (e.g., selectivity, complex stoichiometry, complex stability, etc.) has not been much investigated, and contradictory results were reported. For instance, a limitation in Cu2+ chelation was observed in PAMAM only beyond the fifth generation,20 although the crucial pH parameter was not considered.21 In another study, a metal selectivity was observed with increasing generation of PAMAM derivatives.22 Contradictory effects were also observed in 1,2,3-triazole-linked dendrimers depending on the metal cations or oxo anions considered.23 Finally, no steric effect was noticed in the phosphine−gold complexation at the peripheral branches of dendrimers up to the 10th generation.24 All these results show that general statements about the influence of chelating unit crowding on metal complexation are difficult to make because steric effects are not the only parameters to consider but also (i) electrostatic repulsion between charged species, (ii) a possible change in the stoichiometry of the complexes, (iii) a change in the geometry of the binding chelates, and (iv) a possible chain backfolding.

In other respects, ion-binding polymers are naturally charged and therefore are polyelectrolytes. Because of the morphology of dendronized polymers that present a thickness with an outer surface, a quite different polyelectrolyte behavior should be observed from what is found for classical linear polymers, considered as having zero “thickness”. In dendronized polymers and depending on the material, the charges may be localized not only at the vicinity of the backbone but also within the dendritic shell or at the periphery. In any case, counterions should hardly penetrate into the dendritic shell but should rather be distributed away from the outer surface. It follows that specific polyelectrolyte behavior is expected to occur in dendronized polymers, in particular in the morphology change upon complexation and in the ion location. In this regard, one recent investigation by light scattering experiment showed a significant dendronized polymer swelling by Li+-binding of the crown ether side chains.25 The authors claimed that the chain expansion was induced by steric repulsion but not involving electrostatic effects. We recently reported the synthesis of a series of dendronized polymers bearing peripheral oligo(ethyleneoxy) (EO) chains.26 Interestingly, the fine-tuning of the EO side-chain density could finely modify their water solubility, leading to a new series of thermoresponsive dendronized polymers.27 These denpols were prepared by a polymer-analogous functionalization with short EO chains by an efficient thiol−ene coupling procedure.5 The obtained dendrons are thus structurally quite similar to systems known to have interesting complexing properties (mixed S/O systems, i.e., oxathiacrown ethers28) which lead us to start investigating their affinity with metals, with a look on the crowding-induced effect, in particular. 7076


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content of Ag+ were performed by successive additions of 10 μL aliquots of 31.25 × 10−3 mol L−1 of AgNO3 solution (0.2 equiv) in deionized water into 250 μL of a polymer (PG1bis-EO3 (DP = 24)) solution 6.24 × 10−3 mol L−1 in deionized water. The polymer dilution effect (going from 10.0 g L−1 in the neat polymer solution up to 7.1 g L−1 after addition of 2.0 equiv of Ag+) was neglected in regards to the huge temperature jumps observed.26 The solubility of the Ag+-denpol complexes in organic solvents was checked (in the dark) by mixing ≈2 mg of freshly prepared Ag+− denpol complex in 1 mL of solvent. The (dried) complex was initially obtained from the lyophilization of a water solution of PG1bis-EO3 (DP = 37) containing Ag+ (2 equiv per monomer repeat unit) that was mechanically stirred at low temperature ( 85) were observed for the tris-dendronized polymers, which have the highest density of grafted chains. Finally, the length of the EO side chains does not play a major role in the extraction properties as similar selectivities and extraction efficiencies are observed for both chain lengths (EO2 and EO3).

Figure 3. Extraction percentages (%E) of Ag+ picrate (CAg = 2.5 × 10−4 M) from water into dichloromethane by the dendronized polymers and their monomer analogues. The values are taken for PG1bis and PG1tris polymers of DP = 12 and 6, respectively. 7078


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Figure 4. (a) Microcalorimetric titration of 2.5 mL of PG1bis-OE2 (2.63 × 10−4 M) by addition of 17 × 15 μL of 1.32 × 10−2 M AgBF4 in methanol, at 25 °C. Δt = 11 min between two injections. (b) Experimental (■) and calculated (line) corrected heat Q vs the silver to monomer unit molar ratio assuming the formation of Ag2L species.

Table 1. Thermodynamic Parameters of Ag+ Complexes with the Dendronized Polymers and Their Monomer Analogues in Methanol (T = 25 °C)a ligand G1bis PG1bis

G1tris PG1tris a

chain EO2 EO3 EO2 EO3 EO2 EO3 EO2 EO3 EO2 EO3 EO2 EO3 EO2 EO3

DP

12 12 26 26 56 56 82 82

06 06

complex (Ag:L) 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 3:1 3:1 2:1 2:1

log β2 7.7 7.3 8.1 7.9 9.1 7.7 8.1 7.9 8.4 8.6

± ± ± ± ± ± ± ± ± ±

log β3

0.1 0.2 0.2 0.2 0.2 0.5 0.1 0.2 0.2 0.2 11.6 ± 0.1 11.2 ± 0.1

7.7 ± 0.5 7.1 ± 0.5

−ΔGc 44.6 42.3 46 4 52 44 46.4 45 49 49.4 67.1 64.6 44 41

± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.6 0.8 1 1 1 3 0.7 1 1 0.2 0.6 0.6 2 2

−ΔHc 144 166 188 177 123 181 177 161 160 126 215 252 223 257

± ± ± ± ± ± ± ± ± ± ± ± ± ±

10 35 10 8 2 32 5 7 10 9 1 13 9 15

TΔSc −100 −124 −142 −132 −70 −136 −130 −116 −112 −77 −147 −188 −178 −216

± ± ± ± ± ± ± ± ± ± ± ± ± ±

11 36 11 9 3 35 6 9 11 9 2 14 11 17

ΔGc, ΔHc, and TΔSc are expressed in kJ mol−1.

leading to similar fits (Figures S7 and S8). The origin of this ambiguity could be the tendency of some of these compounds to precipitate upon addition of the mercury salt. However, the interpretation is clearly consistent with rather strong complexation, the thermodynamic parameters being of the same order of magnitude as for Ag+. The stability of the Ag2L complexes formed with the EO2 and EO3 bis-monomers (7.7 and 7.3 log units, respectively) is consistent with that found for Ag2L complexes with thiacrown ethers35 and oxathiacrown ethers.36 The small increase of the complex stability with polymers (ranging from 8.1 to 9.1 and from 7.7 to 8.6 log units for the PG1bis-EO2 and -EO3, respectively) is in agreement with a better organization of the complexing chains around the polymeric core, as compared to the monomers where these chains have more degrees of freedom. This can thus be seen as a small stabilization effect due to preorganization, in a similar way but to a much lesser extent, to the macrocyclic effect, where the organization of complexing chains into a macrocycle increases drastically the stability of the complexes formed.37 In this PG1bis series, no dependence of log β values with DP is observed, however. An influence of the number of oxygen atoms in the side chain was also observed as the complexes formed are less stable with the EO3 than with the EO2 covered polymers, a result which is consistent with the low affinity of Ag+ for hard donor atoms.38 This last result reflects also that with the longer EO3 chains the Ag+ has to penetrate more

the formation of Ag2L complexes, independently of the degree of polymerization and the number of EO units in the side chains. The formation of stable Ag2L species was also observed with the monomer analogues, indicating that the complexation properties are related to the structure of the monomers itself. Thus, these fits are consistent with the model describing the stoichiometric complexation of one cation per dendron in each repeat unit (vide infra, Figure 6). Among the tris-dendronized series, the monomers, following the same tendency as their bis-dendronized analogues to complex one cation per dendron, were thus able, due to the presence of a supplementary dendron, to form Ag3L complexes (Figure S6). However, the influence of congestion appeared for the corresponding polymers PG1tris, where the three coordination sites could not be occupied anymore and only Ag2L complexes could form (Table 1). This result shows that the very high chain density in PG1tris polymers hinders the incorporation of more than 2 Ag+ per monomer unit. This limitation can be explained by steric constraints, i.e., the lack of free volume (congestion), and/or by electrostatic repulsion effects between charged Ag+ particles. It must be noted that the large exothermic effects observed for the complexation of Hg2+ by the polymers PG1bis-EO2 and PG1tris-EO2 and their related monomers could not be clearly interpreted in terms of either Hg2L or Hg3L complex formation, the consideration of both kinds of complexes 7079


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Molecular modeling provided further insight into the complex structure. Simulations showed that the silver cation was inserted in a cavity close to the three sulfur atoms forming a dendron. Ag···S bond distances were determined to be 2.51 Å, which lies within the typical range of values described in the literature obtained by XRD for complexes of Ag+ with oxathiacrown ethers (2.47−2.52 Å).34f,i,j Embedding of Ag+ was completed by a folding over of the three chains with supplementary interaction of Ag+ with the two first oxygen atoms of the chains, but at longer distances (Ag···O1 and Ag··· O2 at 3.81 and 3.95 Å, respectively). A striking result of the modeling consists in the distance of about 5 Å between sulfur atoms engaged in the silver complex, which is similar to the natural lateral distance between molten chains and implies that the complexation occurs without significant volume change. Moreover, silver cations are substantially smaller than this distance (diameter ≈ 2.58 Å) and should therefore be able to diffuse in the EO outer shell without hindrance, explaining the negligible impact of the EO chain length observed during extraction experiments. This model structure moreover clarifies the influence of steric effect, since three thioether units have to be brought in close contact for complexation, that is logically more difficult with monomers and short polymers, as chains have to coil to cover the larger shell area. Geometrical relations are different for the more voluminous counterions (≥5 Å dry bowl diameter for nitrate, estimated from salt densities) that could not enter the denpol shell without spreading EO branches.

deeply within the dendritic shell to attain the S binding atoms, which indeed is sterically and energetically more unfavorable. The complexes formed with the PG1tris polymers (log β2 = 7.7 and 7.1 for the EO2 and EO3 grafted polymers, respectively) are less stable than with the PG1bis polymers, which seem to provide the best conformation of complexing units, most probably due to an optimum conformation of the dendritic moieties around the polymeric backbone. The values of the thermodynamic parameters of complexation (Table 1) show the predominance of the enthalpy contributions. These especially high enthalpy changes (−ΔHc = 120−190 kJ mol−1), consistent with the literature data concerning oxathiaethers,28b,34b,36 are certainly related to the strong interaction between Ag+ and the soft sulfur atoms. The very unfavorable entropy changes can be interpreted in terms of reorganization of the dendrons during complexation. Structure of the Denpol−Cation Complex. 1H NMR has been used as a complementary technique to characterize the complexation of the dendronized monomer G1bis-EO3 with a number of soft metal cations (i.e., Ag+, Hg2+, Cd2+, Pb2+, and Cu2+). These cations were selected for their availability as nitrate salts and their solubility in methanol (i.e., the same solvent as in ITC experiments). The spectrum of the monomer was recorded in deuterated methanol in the presence of 2 equiv of metal cation and compared to the spectrum of the pure monomer (Figure 5). Changes in the spectra were only

Figure 6. Lower energy conformation for Ag+ interacting with a dendron. In the frame are the calculated distances from Ag+ to the sulfur and oxygen atoms of the dendritic chains.

To conclude on that part, it is worth emphasizing that, among the many different Ag+- and Hg2+-binding systems reported in the literature, a very few of them are able to significantly show such a strong affinity as compared to other soft cations such as Cu2+, Cd2+, or Pb2+.28c,34i,39 Solubility of the Ag + −Denpol Complexes. The solubility of the denpols should drastically be affected by the ionic sequestration. Notably, a determinant criterion is the destiny of the counterions. If the counterions could enter in the depth of the shell, drawn by ionic attraction of the silver cations, the outer shell surface would remain unchanged with respect to the neat denpols and the solubility in organic solvents should be same. Alternatively, the counterions located close to the surface of the denpol might modify this surface and the solubility in organic solvents. The solubility in water is on the contrary determined by the ability of the complexed denpol to release the counterions in the solvent. The solvation of the

Figure 5. Shifts in 1H NMR spectra of monomer G1bis-EO3 in deuterated methanol after the addition of 2 mol equiv of Ag+, Hg2+, Cd2+, Cu2+, and Pb2+ nitrate (NO3−) salts.

obtained in presence of Ag+ and Hg2+. The significant chemical shifts changes observed for the protons in the neighborhood of the S atom (Hh to k) indicated that the silver and mercury cations were incorporated in this area and were stabilized by the sulfur atoms. 1H NMR thus confirmed that the metal complexes are formed only in presence of Ag+ and Hg2+ and involve the thioether units in the depth of the shell. 7080


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work that mainly deals with the original and promising properties of the water solvated denpol−silver complexes. Among these original properties, one should not forget the LCST shift itself, occurring for silver, and without any response for other noncomplexing cations. In a certain way, these materials constitute a new type of thermoresponsive chemosensor specific for Ag+, where the thermoresponsive part of the polymer presents itself complexing properties. Thermoresponsive polymers with absorption capacity for other metal ions have already been reported in the literature, a very few of them being ion specific, however.40 They are usually derived from known thermoresponsive polymers (e.g., PNIPAAm) by incorporation of metal chelating sites, such as crown ether units41,42 or others.40,41,43,44 The content of chelating units is usually low, leading in most cases to moderate variations of LCST with addition of metal cation.40a,41,42a,43a,44 An increase of the LCST is generally observed upon complexation, although a decrease may be observed in the case of multivalent ions as they form some electrostatic bridges in between macromolecules.41,43 Finally, the original Ag+ thermal sensitivity observed in our PG1bis-EO3 polymers comes from the delicate balance between the incorporated silver charge fraction and the thermoresponsive properties of the denpol provided by its hydrophilic EO coverage. Morphology of the Ag+−Denpol Complexes. In a previous paper,26 the core−shell structure of the polymer series PG1bis-EOx was mainly investigated by small-angle neutron scattering (SANS) in solution. Scattering curves could be satisfactorily described by a simple spherocylinder model. The dense packing at the core−shell interface could explain the similar diameter of about 40 Å for all polymers, whatever the DP values (Figure 8). Moreover, the same spherocylinder geometry was found for polymer solutions in tetrahydrofuran or in water (below the LCST).

counterions would then confer a polyelectrolyte character to the denpol complexes that would then be soluble and homogeneously dispersed in water. Among the various neat EO-based dendronized materials, the PG1bis-EO3 polymer series show limited and strongly temperature-dependent water solubility:26 at low temperature, polymers are soluble but form small aggregates that grow with temperature until precipitation, at the so-called “lower critical solution temperature” (LCST). Polymers of this series are therefore ideal systems to appreciate water-solubility changes induced by complexation, and one of them (DP = 24) was selected for the detailed investigation of the solubility properties. Experimentally, the water solubility turned out to be greatly affected by the presence of Ag+, as shown by turbidity temperature measurements of Ag+−polymer complex solutions (Figure 7). The cloud point of the polymer

Figure 7. Cloud temperature of a water solution (10 mg mL−1) of PG1bis-EO3 polymer (DP = 24) as a function of the amount of AgNO3 (molar equivalent per monomer repeat unit).

solutions (10 mg mL−1) is found to drastically increase by addition of Ag+ aliquots (as AgNO3), going from 18.4 °C (neat polymer) to nearly 100 °C after addition of 2 mol equiv of Ag+ per monomer repeat unit (i.e., 100% complexation). Only addition of Ag+ is found to affect the cloud point value. As a comparison, no significant shift of the cloud point is observed by addition of 2 mol equiv of Cs+ (18.3 °C), not even by addition of 100 mol equiv of Na+ (17.8 °C). This impressive extension of the solubility domain far beyond the LCST of the neat polymers reveals the repulsive effects between Ag+− denpol complexes that prevent them from aggregation. The appearance of these interactions indicates that a substantial fraction of the counterions was solvated and moved away from the outer shell, leaving positively charged denpol complexes. The repulsions, however, compete with the collapse of the EO shell and with the increasing hydrophobic character of the polymers at higher temperatures that results in a progressive increase of LCST at low charge fractions. To complete the characterization with the properties in organic solvents, we have prepared a complex of PG1bis-EO3 (DP = 37) with 2 equiv of Ag+ per monomer repeat unit, and we compared the solubility in organic solvents of this charged denpol with that of the starting polymer (uncomplexed). It results that both materials are equally soluble in THF, toluene, and dichloromethane and also equally insoluble in cyclohexane and ether. This suggests that both materials just show the solubility of the dry EO shell, whose surface was therefore not modified by the counterions associated with the silver complexes. A more extensive study would, however, be needed to interpret these results, which is beyond the scope of the

Figure 8. Shape evolution of the spherocylinder model for polymers as a function of DP, deduced by SANS. The polymer investigated here (DP = 24) is characterized by an aspect ratio of 1.5 (reproduced with permission from ref 26).

The sequestration of Ag+ by the denpols likely transforms these denpols in polycations, whose mutual repulsive interactions should be the probable explanation for the vanishing of the LCST (see above). In this case, the system should behave as a polyelectrolyte or a charged colloid and give rise to typical scattering functions (see for instance refs 45−47 with cited references therein). The SANS study reported here checks this hypothesis for one previously reported polymer, PG1bis-EO3 of DP = 24, whose model structure corresponds to a spherocylinder with an aspect ratio of 1.5 (Figure 8). Thus, various concentrations of PG1bis-EO3 (DP = 24) and AgNO3 in deuterated water (D2O) or tetrahydrofuran (TDF) were studied in order to fully describe the influence of Ag+ on the structure of the polymer in solution. The first examination of the form factors (VP(q)) in Figures 9a,b enables to follow the polymer morphology change upon addition of Ag+. The same form factors were found in the 7081


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Figure 9. Guinier representation of the cylinder cross section (a) and Porod representation (b) of the form factors determined by SANS for solutions of 2 wt % PG1bis-EO3 (DP = 24) in D2O or TDF. The concentration of the background salt NaNO3 is 50 g L−1; the amount of AgNO3 is 1 equiv per monomer repeat unit.

observed, indicating the enhancement of the aggregation process, in consistency with the experimental lowering of the LCST. The contrast between the effects of NaNO3 and AgNO3 is spectacular: scattered intensities obtained in the presence of a stoichiometric amount of Ag+ are considerably reduced at small q values and show a typical correlation peak. This correlation peak is caused by the electrostatic repulsions between particles that lead to a decrease in the osmotic compressibility. Its position in the reciprocal space q* is associated with the average distance between neighboring charged particles or polyelectrolytes in the dilute regime.45,47 A further confirmation is provided by the addition of NaNO3 (50 g L−1) to the Ag+− polymer complex: the scattering curves then just coincide with that in TDF, indicating that the charges on the polymer still avoid the formation of aggregates. In this case, no more structuration occurs since the electrostatic repulsions are screened up to a reduced range (by adding NaNO3, the Debye screening length is decreased from 28 to 3.8 Å, with the assumption that NO3− ions are free). Even in salt-free solutions, the electrostatic repulsions between charged polymers are partially screened out by NO3− ions dispersed in the solvent and reduce the charge fraction viewed by vicinal polymers. The correlation peak width and intensity therefore reproduce this effective charge fraction feff, which in classical polyelectrolyte solutions46,48 is limited by the Manning−Oosawa condensation process (above a certain threshold, each additional charge fraction is fully screened out by counterions). For monovalent counterions, the condensation process thus occurs when the distance between charges along the chemical sequence goes below the Bjerrum length lB.48 In classical linear polymers, this distance is readily deduced from f and the repeat unit length, while the dielectric constant involved in the expression of lB corresponds to the solvent. With the denpol architecture, its calculation is not as easy, since the dispersion of Ag+ in the polymer shell increases substantially their separation distance and modifies also the dielectric constant with respect to the solvent. Despite these peculiarities, the overall dependence upon charge fraction turns out to be just the same as for linear polyelectrolytes in dilute solution or charged colloids [refs 45−47 and references cited therein], with a progressive growing of the correlation peak, followed by the freezing of the entire curve above a certain threshold (slightly beyond f = 0.5, as taken from Figure 11).

intermediate and asymptotic regions, whatever the nature of the solvent (D2O or TDF), and the presence (or not) of AgNO3 and of background salt (NaNO3, 50 g L−1). This means that the overall shape of the polymer does not change significantly upon addition of Ag+ and that the spherocylinder model structure deduced from the previous SANS study still remains relevant in all cases, including the polymer salt environment. At a first sight, the absence of significative change of the shape of PG1bis-EO3 (DP = 24) upon complexation may be surprising taking into account the high Ag+ sequestration involved (2 Ag+ per monomer repeat unit). An unchanged denpol volume is, however, expected from the structure modeling of the complex that showed that the insertion of a silver complex between the dendron branches does not significantly change the natural distance between them (see above). Moreover the congestion of dendritic structure freezes the denpol aspect ratio, so that the repulsive interactions between complexes have no influence on the denpol shape, but only on the maximum amount of incorporable silver. Dispersion of the Ag+−Denpol Complexes in Water. At small scattering vectors (q), however (Figure 10), strong deviations are found in the scattering curves. As previously reported,26 the scattering curves of solutions in D2O lie above that in TDF, with a gap increasing with concentration, because of the existence of aggregates in aqueous solution even below the LCST. In presence of background salt, a further increase is

Figure 10. Scattered intensities normalized to the contrast and the particle volume fraction at small scattering vectors determined by SANS for PG1bis-EO3 (DP = 24) solutions in D2O or TDF (1 or 2 wt %), in the presence or not of AgNO3 (1 equiv per monomer repeat unit) and of background salt NaNO3 (50 g L−1). 7082


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with dilution, in accordance with the expected C1/3 power law at high concentration (see Figure 12a). Surprisingly, a progressive deviation from the power law is noticed at high dilution, in the way that polymers seem closer than expected (Figure 12b). Aggregates forming only at low concentration and effects of the polymer polydispersity are readily ruled out as well as a change of the interaction regime since the distance between polymers is greater than the Debye screening length in the whole concentration range. A tentative explanation arises from the broadening of the correlation peak noticed at high dilution: a peak shift could then be mimicked by a broader distribution of polymer distances, in relation with the weakening of electrostatic repulsions with distance. Nature of the Ag+−Denpol Complexes. Overall, SANS measurements allowed the structure characterization of the denpol−Ag+ complex in aqueous solution. In particular, the polyelectrolyte behavior of the complexes was clearly confirmed and validated as being responsible for the disappearance of the LCST. Indeed, Ag+ extraction from water transforms the polymers to polycations and generates electrostatic repulsions preventing them from aggregation. SANS also established that the spherocylinder shape inherited from the neutral polymer is preserved by the complexation. This is actually the most original feature of these polyelectrolytes and comes from the sterical congestion of the parent denpols. The congestion imparts objects with a specific globular shape, making them comparable to colloidal particles, though they consist of welldefined single macromolecules. Insight into the complexation process was provided by the deshielded NMR chemical shifts, suggesting that complexes are constituted by interactions of oxathioether units with silver cation (Figure 5). It implies that a single dendritic moiety could potentially form a silver complex, which was further confirmed by molecular modeling (Figure 6). Constraints related to the defined denpol shape and to charge separation, however, limit the incorporation of silver to 2 complexes per polymer repeat unit, as evidenced by titration calorimetry. As a corollary, the location of complexation sites in the repeat units implies the penetration of cations in the depth of the shell and suggests that the modifications of the denpol surface induced by the complexation are negligible. This could then explain that, apart from the appearance of the polyelectrolyte properties, charged and neutral denpols show similar properties, among which are similar solubilities in organic solvents.

Figure 11. Scattered intensities normalized to the contrast and the particle volume fraction at small scattering vectors determined by SANS for polymer PG1bis-EO3 (DP = 24) solution at 2 wt % in D2O with a variable amount of AgNO3 ( f = 0−2 equiv). For f = 0, the scattering curve is just the form factor VP(q) of the polymer.

The specificity of denpols only comes in the foreground in the form factor scattering range (q > 0.06 Å−1). The superimposition of all curves confirms once more that the silver complexes preserve the spherocylinder shape of the neutral polymer. From a geometrical point of view, the denpols look more like globular colloids than thread-like linear polymers, and this specificity modifies the structure of the polyelectrolyte solutions. While the shape of denpols is defined by their architecture, linear polymers adopt different conformations whose degree of coiling or stretching varies with the charge fraction. These conformations then determine an early overlap concentrations C*, when distances between linear polymers become comparable with polymer sizes and lead to contacts and entanglements. The small spherocylinder aspect ratio of denpols (≈1.5 for PG1bis-EO3, DP = 24) should on the contrary delay C* to high concentrations (beyond 15% for PG1bis-EO3, DP = 24) and leave a broad concentration range associated with the so-called dilute regime. In this range, the position of the correlation peak q* should just depend on the average distance between particles, according a characteristic C1/3 power law. The relevance of this approach was tested with eight solutions of the same denpol−silver complex chosen beyond the condensation threshold (PG1bis-EO3, DP = 24, f = 1 equiv of Ag+, in the 0.5−8% range), of concentration below C*. Scattering curves confirm the progressive small-angle shift of q*

Figure 12. (a) Scattered intensities normalized to the contrast and the particle volume fraction at small scattering vectors determined by SANS for solutions in D2O, with 1 equiv of AgNO3 of polymer PG1bis-EO3 (DP = 24) at variable concentration of polymer. (b) Scattering vectors q* versus concentration of polymer PG1bis-EO3 in D2O with 1 equiv of AgNO3. Theoritical C1/3 variation in dilute regime (dashed line). 7083


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*E-mail [email protected].

To summarize, the relative selectivity regarding the extraction of Ag+ from water appears as a consequence of the ability of these ions to form stable complexes with oxathiaether branches. In relation with the specific denpol architecture, the extraction then yields unconventional polyelectrolyte materials.

Present Addresses #

J.R.: Technische Universität Berlin, Hardenbergstraße 40, 10623 Berlin, Germany. ∥ C.B.: Institut Charles Gerhardt Montpellier, CNRS UMR 5253, Université Montpellier, Place Euge′ne Bataillon, 34095 Montpellier Cedex 5, France.

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CONCLUSIONS Metal binding properties have been investigated on a series of novel well-defined bis- and tris-dendronized polymers bearing peripheral ethyleneoxy (oxathiaether) chains of two different lengths (PG1bis-EOx and PG1tris-EOx, with x = 2 or 3) and of variable DPs. The polymers showed a high and selective extractability for silver with regard to more than 30 different cations tested, including alkali and alkaline earth metal ions, the first row transition metal (Mn2+ to Zn2+), and other soft metal (e.g., Cd2+, Pb2+, and Hg2+) cations. The complexation studies using isothermal titration microcalorimetry confirmed the high affinity of the polymers for Ag+. In contrast to extraction results, they also showed a high complexation level for Hg2+. With the bis-dendronized materials and Ag+, the complexation is found to take place within the dendritic structure with a stoichiometry of one cation per dendron, involving Ag···S and Ag···O interactions from the oxathiaether linkages of the dendritic branches, as evidenced by molecular modeling and NMR spectroscopy analysis. A higher complex stability is observed with bisdendronized polymers as compared to the monomer analogues, suggesting a better preorganization of the dendritic chelating arms when attached to the polymer backbone. For the highest congested tris-dendronized polymers the stoichiometry dropped off to less than one cation per dendron, due to steric constraints or more probably due to electrostatic repulsions between charged Ag+ particles. The effect of Ag+ complexation leads to a Coulombic stabilization of the denpols that drastically affects their thermoresponsive properties by causing a sharp elevation of their LCST. SANS studies proved without doubt that the high sequestration of Ag+ by a dendronized polymer with oxathiaether branches transforms this polymer in a novel type of polyelectrolyte with all typical features of polyelectrolytes but with a specific dendronized polymer worm shape. This shape is not significantly modified by the formation of the polyelectrolyte, but as expected from the rapid vanishing of the LCST, this process involves the dissociation of the aggregates preexisting in silver-free aqueous solutions. Finally, the oxathiaether-based dendrons described in this report constitute remarkable chelating units for Ag+ and Hg2+, with potential applications as Ag+ sequestration for waste treatment, chemosensors, or preparation of antimicrobial and antibacterial surfaces.49

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors are grateful to A. Helary, and Drs. F. Boué and D. Lairez for technical support and valuable discussions for SANS measurements at the Laboratoire Léon Brillouin (LLB), CEASaclay, France.

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ASSOCIATED CONTENT

S Supporting Information *

Complementary experimental data on complexation: extraction percentages (%E) of various metal picrates and microcalorimetric titrations traces. This material is available free of charge via the Internet at http://pubs.acs.org.

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