Preparation of Polymer-Supported Polyazamacrocycles. The Role of

Ind. Eng. Chem. Res. 2000, 39, 3589-3595

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Preparation of Polymer-Supported Polyazamacrocycles. The Role of the Polymeric Matrix in the Preparation of Polymer-Supported Polyazamacrocycles B. Altava,† M. I. Burguete,† J. C. Frı´as,† E. Garcı´a-Espan ˜ a,*,‡ S. V. Luis,*,† and J. F. Miravet† Department of Inorganic and Organic Chemistry, University Jaume I, E-12080 Castello´ n, Spain, and University of Valencia, C/Dr. Moliner 50, 46100 Burjassot (Valencia), Spain

Different approaches have been studied for the preparation of resins containing different polyazamacrocycles. Preparation of monolithic resins by polymerization of vinylic derivatives of the corresponding macrocycles is shown to be a more versatile strategy for this purpose. The use of energy-dispersive analysis by X-ray has revealed to be a very useful tool for the rapid evaluation of the interaction of those materials with both anions and cations and has allowed the corresponding selectivity trends to be obtained in a fast and simple way. Important effects of the polymeric matrix are observed in some cases. Introduction Polymer-supported polyazamacrocyclic compounds as well as some of their open-chain analogues have some important potential applications.1-6 The immobilized polyamine fragments can interact, sometimes in a selective way, with a variety of species such as transition-metal cations, inorganic and organic anions, and even neutral molecules, depending on the pH of the medium.2 Accordingly, different polymers containing such functionalities have been prepared in recent years. Potential applications of those materials in general separation processes, hydrometallurgy,4 environmental sciences and technologies,5 catalysis,6 and biomimetic chemistry have been developed.3c,d In this field Bradshaw and Izatt have afforded an interesting practical application of supported selective macrocylic agents.7 The most general approach for the preparation of those functionalized polymers is the grafting of the desired moiety onto a Merrifield’s resin or a related polymer, because of the easy N-functionalization of the nitrogen atoms of polyaza receptors. In this process, the polymeric backbone is very often considered as a fully inert matrix that plays no role in the synthetic procedure or in their further applications. This is not the case, however. Thus, for instance, we have recently shown how the chemistry involved in the grafting of open-chain polyamines on polystyrene resins experiences important changes when compared with the solution processes.3b In this example, several of the nitrogen atoms are often involved in the grafting, originating final resins with a higher degree of cross-linking. So, the grafting process is accompanied by important modifications in the chemical and morphological nature of the receptor and the polymeric backbone. Structural changes in the receptor can affect its properties, but also changes in the polymeric matrix itself can play an important role in determining the activity of the supported species.3b,8 Accordingly, more work is needed in order to better * To whom correspondence should be addressed. E-mail addresses: [email protected] and [email protected]. † University Jaume I. ‡ University of Valencia.

understand the processes occurring in the polymeric matrix and the role played by the backbone. Here we report on the preparation of supported macrocyclic ligands by two different methods: anchoring to a reactive Merrifield resin and copolymerization of the properly functionalized macrocyclic compound. For the latter approach, monolithic preparation was chosen over suspension polymerization because of the simpler experimental procedure required to prepare the monolithic materials, which, in general, affords more reproducible results and higher yields of polymer. Experimental Section Cyclam was purchased from Aldrich, and receptors B323, D323, and B33 were synthesized as described previously.9c Synthesis of P-cyclam. A mixture of cyclam (0.135 g, 0.675 mmol) and triethylamine (470 µL, 3.37 mmol) in dry dimethylformamide (DMF; 25 mL) was stirred at room temperature under an argon atmosphere. A chloromethylated resin was then added (1.08 mmol Cl/ g, 1% DVB, 0.25 g, 0.27 mmol), and the suspension was stirred at 75 °C for 24 h. The resulting polymer was filtered, washed with DMF (3×) and CH2Cl2 (6×), and vacuum-dried to give the supported polymer P-cyclam. IR (cm-1; KBr): C-Cl peak absent at 1265. Anal. Calcd for [(C8H8)0.88(C10H10)0.01(C9H8)0.11]2(C10H22N4)0.11: N, 2.6. Found: N, 2.4. The nitrogen content corresponds to 0.42 mmol/g (DF ) 0.11, 100% conversion). [DF stands for degree of functionalization, which is the percentage of aromatic groups that have been functionalized. Thus, for instance, a chloromethylated Merrifield resin with a loading of 1 mmol/g of chlorine corresponds to the composition (C8H8)0.88(C10H10)0.02(C9H9)0.11 with DF ) 0.11.] The preparation of all of the other supported polyazamacrocycles was carried out in an analogous way. P-B323 was prepared from B323. IR (cm-1; KBr): peak absent at 1265. Anal. Calcd for [(C8H8)0.88N, 1.3. (C10H10)0.01(C9H8)0.11]4(C16H24N4‚6H2O)0.11: Found: N, 1.4. The nitrogen content corresponds to 0.24 mmol/g (DF ) 0.11).

10.1021/ie000098q CCC: $19.00 © 2000 American Chemical Society Published on Web 10/02/2000

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P-D323 was prepared from D323. IR (cm-1; KBr): peak absent at 1265. Anal. Calcd for [(C8H8)0.88(C10H10)0.01(C9H8)0.11]4(C20H32N4)0.11: N, 1.3. Found: N, 1.4. The nitrogen content corresponds to 0.25 mmol/g (DF ) 0.11). P-B33 was prepared from B33. IR (cm-1; KBr): peak absent at 1265. Anal. Calcd for (C8H8)0.88(C10H10)0.01(C9H8)0.11(C14H21N3‚4H2O)0.11: N, 1.9. Found: N, 2.1. The nitrogen content corresponds to 0.49 mmol/g (DF ) 0.11). P-cyclam(Boc)3 (where Boc is tert-butoxycarbonyl) was prepared from previously triprotected cyclam. A mixture of triprotected cyclam (0.755 g, 1.48 mmol) and 1,8- diazabicyclo[5.4.0]undec-7ene (DBU; 267 µL, 1.79 mmol) in dry DMF (25 mL) was stirred at room temperature under an argon atmosphere. Then a chloromethylated resin was added (1.19 mmol of Cl/g, 1% DVB, 0.5 g, 0.595 mmol), and the resulting suspension was stirred at 80 °C for 72 h. The polymer formed was filtered, washed with DMF (3×) and CH2Cl2 (6×), and vacuum-dried to yield P-cyclam(Boc)3. IR (cm-1; KBr): peak present at 1694 and peak absent at 1265. Anal. Calcd for (C8H8)0.87(C10H10)0.01(C9H9O)0.06(C25H47N4O6)0.06: N, 2.6. Found: N, 2.5. The nitrogen content corresponds to 0.44 mmol/g (DF ) 0.11). P-cyclam-B. The polymer P-cyclam(Boc)3 was suspended in a solution that contained hydrogen bromide/ hydrogen acetate (HBr/HAc) and was stirred for 48 h. The resulting polymer was then filtered, washed with H2O (3×), H2O/MeOH (3×), MeOH (3×), MeOH/ CH2Cl2 (3×), CH2Cl2/TEA (3×), and CH2Cl2 (5×), and vacuum-dried to P-cyclam-B. IR (cm-1; KBr): absent peaks at 1694 and 1265. Anal. Calcd for (C8H8)0.87(C10H10)0.01(C9H9O)0.06(C10H23N4)0.06: N, 3.0. Found: N, 2.7. The nitrogen content corresponds to 0.44 mmol/g (DF ) 0.11). Preparation of the Monolithic Polymers Synthesis of the Monomer 1-(4-Vinylbenzyl)1,4,8,11-tetraazacyclotetradecane (Vcyclam). Cyclam (0.5 g, 2.5 mmol) and K2CO3 (76 mg, 0.55 mmol) were stirred in a CH3CN/EtOH solution under an argon atmosphere. Then a solution of 4-vinylbenzyl chloride (0.5 mmol) in CH3CN was added and the mixture refluxed for 24 h. The organic phase was eliminated at reduced pressure. The crude obtained was purified by chromatography on silica gel using MeOH/NH3 as the eluent to obtain the pure product as a waxy solid. Yield: 95%. 1H NMR (δ, CDCl3): 1.46 (m, 2H), 1.65 (m, 2H), 2.25-2.70 (m, 16H), 3.36 (s, 2H), 5.01 (d, J ) 10.8 Hz, 1H), 5.51 (d, J ) 17.7 Hz, 1H), 6.50 (q, 1H), 7.067.16 (m, 4H). 13C NMR (δ, CDCl3): 26.0, 28.0, 47.1, 47.8, 48.9, 49.2, 49.5, 50.7, 53.6, 54.2, 57.5, 113.4, 125.9, 129.3, 136.2, 136.4, 138.2. Synthesis of the Monomer 2-(4-Vinylbenzyl)2,6,9,13-tetraaza[14]paracyclophane (VB323). Compound B323 (0.2 g, 0.33 mmol) and Zn(OTf)2 (where OTf is triflate; 0.12 g, 0.33 mmol) were dissolved in dry CH3CN (25 mL). Anhydrous K2CO3 (0.46 g, 3.33 mmol) and 4-vinylbenzene chloride (52.6 µL, 0.33 mmol) were added at 0 °C, and the mixture was stirred overnight. The solvent was vacuum evaporated, and the resulting crude was treated with concentrated aqueous ammonia (5 mL) and extracted with CH2Cl2. The organic phase was removed at reduced pressure, giving a crude that was purified by chromatography on silica gel using MeOH/NH3 as the eluent to obtain the pure product as

an oil or waxy solid. Yield: 15%. 1H NMR (δ, CDCl3): 1.37 (m, 2H), 1.45 (m, 2H), 2.15-2.25 (m, 4H), 2.322.41 (m, 4H), 2.42-2.55 (m, 4H), 3.22 (s, 2H), 3.46 (s, 2H), 3.62, (s, 2H), 5.05 (d, J ) 10.9 Hz, 1H), 5.58 (d, J ) 17.6 Hz, 1H), 6.55 (q, 1H), 7.09 (s, 4H), 7.23 (s, 4H). 13C NMR (δ, CDCl ): 26.3, 28.0, 44.6, 46.4, 46.9, 49.1, 3 49.5, 53.0, 59.0, 59.1, 113.3, 126.1, 128.5, 128.9, 129.1, 136.2, 136.5, 139.0, 139.6. Preparation of the Cross-Linking Agent EDADMA. Methacrylic acid chloride (943 µL, 9.56 mmol) was slowly added to a solution containing ethylenediamine (322 µL, 4.78 mmol), tetraethylammonium (TEA; 2.67 mL, 19.1 mmol), and 4-(dimethylamino)pyridine (DMAP) in tetrahydrofuran (THF) at -35 °C. The solution was stirred for 2 h and then heated at room temperature and the solvent eliminated by vacuum evaporation. The resulting crude was purified by chromatography on silica gel using MeOH as the eluent to give the final product as a waxy solid. Yield: 75%. 1H NMR (δ, CDCl3): 1.91 (s, 6H), 3.47 (s, 4H), 5.31 (s, 1H), 5.72 (s, 1H). 13C NMR (δ, CDCl3): 18.6, 40.4, 120.1, 139.2, 169.4. All of the polymerizations to obtain monolithic materials were carried out in glass vials. The monoliths were then crushed to small pieces in order to study their properties as receptors. Synthesis of the Monolithic Polymer Vcyclam(Zn)/DVB (P2.Zn). Zinc triflate and the monomer derived of cyclam were mixed to give the monomer Vcyclam(Zn) (160 mg, 0.23 mmol), which was introduced together with divinylbenzene (DVB; 344 mg, 2.12 mmol) and 757 mg of toluene/dodecanol (1:5) in a test tube. The mixture was degasified by passing an argon flow for 15 min. Then, 5 mg of 2,2′-azobis(isobutyronitrile) (AIBN) was added, and the resulting mixture was introduced in a bath at 75 °C. After 24 h the resulting polymer was washed with MeOH and vacuum-dried. The loading was 0.78 mmol/g. Yield: 90%. FT-Raman (cm-1): 1631, 642. Anal. Calcd for (C10H10)0.93(C19H32N4O6S2F6Zn)0.07: N, 3.9; S, 3.8. Found: N, 3.8; S, 4.0. Synthesis of the Monolithic Polymer of Vcyclam/ DVB (P2). This polymer was obtained by demetalation of P2.Zn with 6 M HCl. The resulting polymer was washed with H2O (3×), H2O/MeOH (3×), MeOH (3×), MeOH/TEA (3×), TEA (3×), TEA/CH2Cl2 (3×), and CH2Cl2 (3×) and vacuum-dried. FTIR (cm-1): 1631, 642. Anal. Calcd for (C10H10)0.93(C19H32N4)0.07: N, 3.9. Found: N, 4.3. Synthesis of the Monolithic Polymer VB323(Zn)/ DVB (P1.Zn). Prepared following the same procedure as that for VB323(Zn) and DVB. Yield: 50%. FT-Raman (cm-1): 1630, 642. The obtained charge was 0.58 mmol/ g. Anal. Calcd for (C10H10)0.9(C27H36N4O6S2F6Zn)0.1: N, 3.2; S, 3.3. Found: N, 3.3; S, 2.6. Synthesis of the Monolithic Polymer of VB323/ DVB (P1). This polymer was obtained by demetalation of P1.Zn with 6 M HCl. The resulting polymer was washed with H2O (3×), H2O/MeOH (3×), MeOH (3×), MeOH/TEA (3×), TEA (3×), TEA/CH2Cl2 (3×), and CH2Cl2 (3×) and vacuum-dried. FTIR (cm-1): 1630, 642. Synthesis of the Monolithic Polymer of Vcyclam/ EGDMA (P3). P3 was prepared from Vcyclam and EGDMA. Yield: 98%. FTIR (cm-1): 1729. The loading was 0.35 mmol/g. Anal. Calcd for (C10H16O4)0.9(C19H32N4)0.1: N, 2.7. Found: N, 2.5. Synthesis of the Monolithic Polymer of Vcyclam/ EDADMA (P4). P4 was prepared from Vcyclam and

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3591 Table 1. Swelling Properties of Polymers Obtained by Grafting Merrifield’s resin P-cyclam P-cyclam(Boc)3 P-cyclam-B P-B33 P-B323 P-D323

loadinga

d2c/d1b (%)

d3d/d1b (%)

1.08 0.42 0.44 0.48 0.49 0.24 0.25

54.9 50.4 38.5 53.2 40.8 42.1 39.5

2.8 8.1 -2.0 1.0 16.9 10.1 7.9

a Expressed in mmol/g. b Mean diameter for the dry polymer particles. c Mean diameter for the CH2Cl2-swollen polymer particles. d Mean diameter for the H2O-swollen polymer particles.

EDADMA. Yield: 80%. FTIR (cm-1): 1648, 1532. Anal. Calcd for (C10H16O2N2)0.9(C19H32N4)0.1: N, 14.8. Found: N, 14.5. Synthesis of the Monolithic Polymer of mstyr/ EGDMA (P5). P5 was prepared from styrene and EGDMA. Yield: 95%. FTIR (cm-1): 1728. Synthesis of the Monolithic Polymer of mstyr/ EDADMA (mest/EDADMA). mstyr/EDADMA was prepared from styrene and EDADMA. Yield: 65%. FTIR (cm-1): 1651, 1528. Anal. Calcd for (C10H16O2N2)0.9(C8H8)0.1: N, 13.5. Found: N, 13.1. Results and Discussion Grafted Polyazamacrocycles. Two different kinds of well-studied macrocycles were selected for our works cyclam and polyaza[n]cyclophanessand were anchored in PS/DVB chloromethylated resins.9 In both cases, the methodologies for the selective N-functionalization of the macrocycles have been deeply studied in solution.10,11 The grafting reaction was carried out at 75 °C, using a 3-fold excess of the macrocycle, DMF as the solvent, and Et3N as the base. Quantitative conversion of the chloromethyl groups was observed through the monitoring of the disappearance of the C-Cl band at ca. 1260 cm-1 in the IR or, even better, Raman spectra.12 Nevertheless, the nitrogen content of the resulting polymers revealed that anchoring had taken place through more than one nitrogen atom (see Table 1). For macrocycles B323 and D323 (Scheme 1), the analyses suggested that all of the nitrogen atoms had been functionalized (resins P-B323 and P-D323), while for B33 and cyclam, functionalization had occurred on two of the nitrogen atoms (resins P-cyclam-1 and P-B33). For the case of B323 and D323, this correlates with the observation made in solution that, in their N-benzylation, the incorporation of one benzyl group favors the entrance of the next.13 For cyclam, anchoring through only one nitrogen atom could be accomplished using a derivative containing three nitrogen atoms protected with Boc groups.10d In this case, the DBU needed to be used as the base, and the bulky nature of the macrocycle precluded a complete replacement of the chloromethyl groups. Removal of the protecting groups with HBr/HAc gave polymer P-cyclam-B. Attempts to obtain the grafting of the polyaza[n]paracyclophanes by only one of the nitrogen atoms, using the reported methodology for their monofunctionalization,11 were unsuccessful. According to those data, incorporation of polyazamacrocyclic structures through grafting is generally accompanied by a significant increase in cross-linking (except for P-cyclam-B). This, clearly, has to affect to the diffusion properties inside the polymer beads. Table

1 gathers some data on the swelling behavior of those polymers with two different solvents: CH2Cl2 and H2O. As can be seen, the increase in cross-linking is accompanied by a decrease in the swelling factor in methylene chloride. Such a decrease is not observed, however, for P-cyclam-B that presents no additional cross-linking. The presence of nitrogen atoms makes the polymeric matrix more compatible with water, but the swelling factor continues to be, in all cases, very low, which makes the application of such materials in aqueous solutions difficult. Monolithic Polymers Containing Polyazamacrocycles. An alternative approach that could solve some of the above-mentioned problems related to the diffusion of the cations into the polymer is the preparation of monolithic polymers through the polymerization of vinylic monomers containing polyazamacrocyclic functionalities. At the difference with the resins discussed above, monolithic polymers present a macroporous structure with a permanent porosity. The functional groups are located at the inner surface of the monoliths, being easily accessible to different kinds of solvents. According to this, those materials, which can be prepared in an array of different forms, have found important uses, in particular for separation processes.14 The more important factors affecting the pore size distribution are the cross-linking monomer, the porogen, the concentration of the radical initiator, and the temperature.14,15 Vinylic monomers derived from cyclam and B323 were prepared as shown in Scheme 2. For the preparation of the cyclam derivative, selective monofunctionalization could be carried out in excellent yields by using a large excess of the macrocycle over the alkylating agent (4-chloromethylstyrene).10 The corresponding vinylic derivative of B323 was prepared in the presence of Zn(OTf)2, making use of the general methodology previously described by us.11 As the crosslinking agents, three different species were selected (see Chart 1). The first one was DVB, and the other two were acrylic derivatives in order to improve the compatibility of the resulting polymers with aqueous solutions. Thus, the divinylic derivatives EGDMA and EDADMA were prepared in good yields from the acid chloride of the acrylic acid and ethylene glycol or ethylenediamine, respectively. Polymers P1-P5 were prepared from a monomeric mixture containing 90% of the cross-linking agent and 10% of the polyazamacrocyclic derivative using AIBN as the initiator. The porogenic agents were dimethyl sulfoxide (DMSO), DMSO/dodecanol, or dodecanol/ toluene depending on the solubility of the monomers (see Table 2). Two additional polymers P6 and P7 were prepared from EGDMA or EDADMA and styrene in order to analyze the role of the donor atoms present in these cross-linking agents on the properties of resins P3-P5. All of the polymers showed the expected morphology for macroreticular resins, as can be illustrated by the scanning electron micrograph (SEM) image of the polymer P1. For polymers P1 and P2, polymerization was carried out in the presence and absence of Zn(II), but no big differences were found in the properties of the resulting polymers.16 Interaction of Monolithic Polymers with Cations and Anions. Polymers P1-P5 are able to interact with different host species. In this respect the use of

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Scheme 1

Scheme 2

Table 2. Monolithic Polymers Obtained from B322 and Cyclam monomer monomera

porogen

VB323 DVB Vcyclam DVB Vcyclam EGDMA

DMSO DMSO dodecanol and toluene Vcyclam EDADMA DMSO Vcyclam EDADMA DMSO and dodecanol styrene EGDMA dodecanol and toluene styrene EDADMA DMSO a

Chart 1

EDAX (energy-dispersive analysis by X-ray) proved to be a very useful tool to semiquantitatively evaluate the selectivity trends of those insoluble materials in a simple and fast way. The technique requires the analysis of, at least, three or four regions of the polymer and the average of the different elemental contents measured for each individual spot. An example is given in Figure 1 for the polymer P1 obtained from the vinylic derivative of B323 in the presence of Zn(II). The bottom trace in Figure 1 shows the spectrum obtained after polymerization and allows us to observe the presence of Zn, F, and S, which corresponds with the nature of

loading time yield (mmol/g) (h) (%) polymer 0.58 0.78 0.35

24 24 24

50 90 98

P1 P2 P3

1.42 1.40

24 24

80 65

P4 P5

24

98

P6

24

65

P7

4.7

Cross-linking agent.

the metal salt used (Zn(OTf)2). After treatment with 6 M HCl and further neutralization, the polymer (central trace) shows the absence of those three elements. When this polymer is treated with an aqueous solution (at pH ) 6) containing equimolar amounts of Cu(II), Zn(II), Hg(II), Ni(II), and Mn(II), the spectrum obtained (upper trace) reveals that Cu(II) is very selectively complexed. This is the only metallic element detected along with very minor amounts of Ni. The presence of the bands for S and Cl reveals the preferential uptake of SO42- and Cl- as the counterions. This high selectivity for Cu(II) follows the trends observed for B323 in solution.9b According to the former results, similar studies were carried out for polymers P1-P7. For this purpose, a few milligrams of each resin were stirred for 24 h in a solution containing a 0.04 M concentration of Zn(II), Cu(II), Ni(II), and Na(I) as their perchlorate salts at pH ) 6. After this time, the resin was washed with H2O and MeOH and vacuum-dried. The polymers were then analyzed by EDAX, and the selectivity trends obtained are gathered in Figure 2a. The selectivity trends are

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Figure 1. EDAX micrograph of polymer P1.

Figure 2. (a) Plot of the affinity of the polymers for the different metal ions. Molar ratios are calculated as the ratio between a given cation and the less adsorbed one. (b) Plot of the interaction between P1 and P2 with different transition-metal ions in the absence of Cu2+.

represented for each resin as the molar ratio between the cation considered and the cation detected in smallest concentration. Obviously, for those cases where only one cation is detected, the only molar ratio observed is 1. It can be observed that Cu(II) seems to be again the cation more strongly complexed by most polymers. Only P4, which is chemically analogous to P5 but has been prepared using a different porogenic mixture, shows a detectable complexation for Ni(II). This evidences the high importance that the exact morphology of the “inert” polymeric matrix has in the observed selectivity of a

given material. It is important to note that control polymer P7 containing EDADMA amide moieties also depicts a selective complexation of Cu(II). On the contrary, control P6 having only oxygen donor atoms shows a clear selectivity for Na(I) over the transitionmetal cations. However, even in this case, the uptake of some Cu(II) is detected. A similar study was carried out for polymers P1 and P2, derived from B323 and cyclam, respectively, using an equimolar solution of different cations (Na(I), K(I), Cs(I), Ni(II), Fe(II), and Hg(II)) in which Cu(II) was absent. The results (Figure 2b) allowed us to observe a very different behavior for the two macrocycles. Thus, the B323-derived resin (P1) showed a large selectivity for Fe(II), with the uptake of Hg(II) not being detected. On the contrary, in the polymer containing cyclam fragments (P2), only Ni(II) and Hg(II) were detected, with a moderate selectivity for the later. This roughly corresponds with the trends observed for the association constants of cyclam: log K ) 20.3 for Ni(II) and 26.4 for Hg(II), being much lower for the other cations considered.17,18 The same methodology can be used to analyze the selectivity trends for the interaction of P1-P7 with different anions. When an equimolecular solution of the sodium salts of the anions F-, Cl-, Br-, I-, SO42-, and PO43- at pH ) 2.5 was used, the results obtained are those shown in Figure 3a. It can be seen that none of the resins interact, at an appreciable extent, with For PO43-. This fact may be ascribed to the extensive protonation that phosphate and fluoride bear at this pH value, reducing their effective negative charge.17 It is interesting to note that for polymers containing DVB as the cross-linking agent, both for the B323 derivative (P1) and the cyclam derivative (P2), the larger halogenide anions are selectively complexed (I- > Br- > Cl-). A rather reverse trend is shown by the polymeric cyclam derivatives containing acrylic cross-linking agents, in particular P4 and P5. The control polymer P7 derived from EDADMA shows itself a clear selectivity for Cl-, while control polymer P6, containing no nitrogen atoms, shows an appreciable selectivity for SO42-. Nevertheless, when the interaction of those resins with anions is considered, it is important to take into account that selectivities can be greatly affected by the presence of transition-metal cations in the solution and

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kinds of polymer-supported macrocycles. The present results show that the relative ratios of the association constants observed in solution for analogous macrocycles are one important factor in order to predict the selectivity trends. Nevertheless, small morphological changes associated with the details of the polymerization protocol can have an important effect on the final selectivities observed, and this reveals how the polymeric backbone is never an “inert” matrix but plays an important role in the final activity of the supported functionalities. Acknowledgment We are indebted to DGICYT Project PB96-0792 and Generalitat Valenciana Project GV-D-CN-09-140 for financial support. Supporting Information Available: SEM micrograph of polymer P1 (Figure S1). This information is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited

Figure 3. (a) Plot of the affinity of the polymers for the different anions. Molar ratios are calculated as the ratio between a given anion and the less adsorbed one. (b) Plot of the interaction between P1 and P2 with different anions in the presence of different metal ions.

by the protonation degree of the polyamines incorporated. As a matter of fact, one of the strategies used for the development of efficient and selective anion receptors is the use of metal complexes with nonsaturated first coordination spheres that can add exogenous anionic ligands.2b,9b This is illustrated in the results shown in Figure 3b for the interaction of Cl-, Br-, I-, Cr2O72-, and PO43- (always in equimolecular amounts) with P1 and P2 in the presence of several bivalent transition-metal ions. For the polymer containing cyclam fragments, there are no large differences in selectivity trends with respect to those using the sodium salts, even if the selectivity for I- over Cl- is increased. However, a more dramatic effect is observed for the polymer derived from B323, which corresponds to a macrocycle being able to form low symmetry metal complexes with only three out of the four nitrogen atoms involved in the binding to the cation.9b For resin P1, a clear selectivity for PO43- is observed, an anion that does not interact in the absence of the transition-metal cations which can be probably ascribed to formation of mixed complexes. Conclusions In conclusion, the present work shows that monolithic polymers containing polyazamacrocyclic receptors represent a very interesting class of resins for the development of novel materials for separation processes. Their use allows one to avoid some of the problems associated with resins obtained through grafting procedures, in particular the increase in cross-linking that is accompanied by a decrease in the accessibility of the reactive sites and a low compatibility with aqueous solutions. The use of EDAXS represents a useful technique for the rapid and simple screening and evaluation of these

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Received for review January 18, 2000 Revised manuscript received June 14, 2000 Accepted June 15, 2000 IE000098Q