Towards achieving selectivity in metal ion binding by fixing ligand–chelator complex geometry in polymers

Reactive & Functional Polymers 44 (2000) 79–89 www.elsevier.com / locate / react

Towards achieving selectivity in metal ion binding by fixing ligand–chelator complex geometry in polymers a, b c a Alok Singh *, Dhananjay Puranik , Yan Guo , Eddie L. Chang a

Laboratory for Molecular Interfacial Interactions, Code 6930, Naval Research Laboratory, Washington, DC 20375, USA b Geo Centers Inc., 10903 Indian Head Hwy, Ft. Washington, MD 20744, USA c Department of Biochemistry and Molecular Biology, Georgetown University, Washington, DC 20007, USA Received 10 March 1999; received in revised form 3 September 1999; accepted 10 September 1999

Abstract The synthesis of polymers with pre-organized metal ion binding sites for optimal binding of selected metal ions is reported. An acyclic N-vinylbenzyl substituted chelator, triethylenetetramine, is complexed with copper(II) ions and crosslinked with matrix monomer TRIM [2-ethyl-2-(hydroxymethyl) propane-1, 3-diol trimethacrylate] to form the polymer. UV–visible spectra indicate that the coordination geometry of the monomeric metal–ligand complex is highly conserved in the polymer. Unlike macrocyclic ligands, the acyclic ligating molecules demonstrate flexibility in forming chelation rings that are specific to metal-ion size and geometry. The complexation step is used as a means to optimize the chelator conformation for copper ions. The resulting polymers, after removal of metal ions, show selectivity towards the templated metal ions. The small size of the metal ions facilitated their access to the templated sites embedded in the porous polymers. This scheme has the potential to become a generalized procedure for making metal ion selective polymers.  2000 Elsevier Science B.V. All rights reserved. Keywords: Polymerizable metal ion chelator; Metal ion selective polymer; Selective metal ion sponge; Metal chelating polymer; Macroporous polymers

1. Introduction There has been great interest in making functionalized materials by using the principles of molecular recognition and molecular imprinting for selective binding of targeted molecules or molecular assemblies [1–4]. In particular,

*Corresponding author. Tel.: 11-202-404-6060; fax: 11-202767-9594. E-mail address: [email protected] (A. Singh)

imprinting techniques have been used for the incorporation of metal ions into stable polymers [1]. In these materials, a monomeric complex, formed between a metal ion and monomer chelator(s), is crosslinked with matrix-forming monomer(s) to synthesize the desired imprinted polymers [8–11]. The metal, or metal ions, incorporated into such a polymeric matrix can affect the mechanical, physical, and chemical properties of the host polymers Metal-ion selectivity of synthetic organic molecules has also been the subject of extensive

1381-5148 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S1381-5148( 99 )00082-6

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investigations [5–7]. These studies used synthetic cyclic molecules, cryptans and crown ethers, to demonstrate that the selectivity of these chelators towards inorganic ions and small molecules is similar to that observed in biological systems. A common approach for achieving metal-ion selectivity involves the use of cyclic chelators that require the synthesis of several intermediates and a prior knowledge of how cyclization can affect binding and metalion selectivity. Metal-ion chelating sites have also been introduced into polymer matrices to assist recognition of large organic molecules [12–15]. But the target molecules in these cases, being larger than metal ions, had problems accessing the polymer interior. This is particularly true when the imprinted sites are embedded within the polymer. Imprinted sites grafted onto the outer surfaces of polymeric microspheres do not have this accessibility problem, but surface density is always much less than a bulk distribution of sites, and surface sites can achieve selectivity only through non shape-directed chemical interactions [14]. Our goal is to remove selected metal ions from complex solutions using polymers with pre-organized recognition sites that have been templated to bind specific metal ions. We also need to control polymeric properties to ensure the preparation of non-swelling, porous, and rigid but not brittle polymers. The templated

acyclic chelators represent a form of pre-organization that bypasses the need for both multistepped ring-closing chemical reactions and exploratory steps to find detailed architecture of a coordination environment. Because the target ions are much smaller than the organic molecules used in conventional approaches, we anticipate easier accessibility and reversibility in binding of metal ions to these pre-organized binding sites. To ensure access to the binding sites, the metal-ion templates are incorporated into a macroporous, polymeric matrix. Aspects of this approach can be found elsewhere; for example, imprinted polymers have been reported to show good selectivity for substrates made from isomeric organic molecules [12,13]. The issue of substrate accessibility to those imprinted sites has not been solved, however. This approach also differs from previous work that used polymeric polyamines to template the target ions, as for example from Nishide’s group [11]. Scheme 1 shows the synthetic routes followed in this work. Fig. 1 illustrates the strategy for reversible metal ion binding in polymers. Essentially, the multidentate, acyclic amine, triethylenetetraamine (TETA), binds to a target metal ion. The binding of the target ion stabilizes the ligand into a pseudo closed-conformation that will be, by definition, a stable chelator conformation specific for that particular metal

Scheme 1. Polymer synthesis.

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Fig. 1. Reversibility in metal ion binding in polymers.

ion. We then perform a standard locking-in of this conformation by polymerization.

2. Materials and methods All chemicals including triethylenetetramine hydrate (TETA), 1, 4-vinylbenzyl chloride (Vb), copper(II) sulfate pentahydrate were obtained from Aldrich. TRIM [2-ethyl-2-(hydroxymethyl) propane-trimethacrylate] was obtained from TCI. All glassware was thoroughly cleaned using non-ionic detergent and then rinsed with copious amount of deionized (DI) water before performing metal ion analysis. Elemental analysis was performed by Oneida Research Services (Whitesboro, NY, USA). Melting points were determined on a Fisher– Jones melting point apparatus. UV–Vis spectra were recorded on Beckmann DU650 spectrophotometer. Infrared spectra were recorded on a Nicolet Impact 400D FT-IR instrument, NMR spectra were recorded on a Bruker AC250 instrument at 250 MHZ. Polymer textures were examined on an ElectroScan Model 2020 environmental scanning electron microscope (ESEM). Specific polymer surface area and pore volumes were determined from nitrogen absorption measurements using the BET method.

2.1. Synthesis of N-(4 -vinyl)benzyl triethylenetetraamine ( Vb–TETA), 2 Vinyl benzyl chloride (5.4 g, 35 mmol) was reacted with TETA (3.0 g, 20 mmol) suspended

in 50 ml ether by stirring the reaction mixture at 308C overnight. The reaction turned orange yellow after 3 h of refluxing. The reaction was stopped by addition of 1 ml of water. Ether was removed by rotary evaporation and the residue was dissolved in 40 ml water. Organic material containing vinyl benzyl chloride and disubstituted TETA were removed by chloroform extraction (3 3 20 ml). Water solution contained unreacted TETA and 2. Water was removed by freeze-drying and the title compound was recovered by careful chloroform–methanol (9:1) extraction to yield 1.4 g (14% yield) Vb–TETA as viscous oil. Final purification was achieved by using a short column (1 3 15 cm) of silica gel (230 mesh). Vb–TETA showed an R f value of 0.3 on silica-gel plates (chloroform, metha1 nol, and 1:1). NMR H (CD 3 OD) d ppm 2.9 (m, 12H), 3.6 and 3.4 (2H), 5.4 (d, 1H), 5.9 (d, 1H), 6.91 (m, 1H), 7.56 (m, 4H); 13 CMR: 39.0,41.0,46.7, 48.6, 51.7, 53.4, 56.6, 58.4 (–CH 2 N); 112.8, 125.5,127.7, 128.4, 136.0 ppm. Analysis: calcd. for C 15 H 26 N 4 : C, 68.65; H, 9.98; found: C, 63.28; H, 8.33. Proton and carbon-13 NMR showed that product is mixture of isomeric monosubstituted TETA.

2.2. Synthesis of N,N-di-(4 -vinyl)benzyl triethylenetetraamine ( DVb–TETA), 3 To 2.04 g (14 mmol) triethylenetetraamine suspended in 20 ml diethyl ether, 4.27 g (28 mmol) vinylbenzyl chloride (VBz) in 10 ml ethyl ether was carefully added at room temperature. The solution was stirred for 72 h at

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room temperature. Solvent was then removed from the reaction mixture by rotary evaporation under reduced pressure. Care was taken to keep reaction mixture at room temperature and protected from light. Unreacted TETA was removed from solid residue by washing the chloroform solution with water. Absence of TETA in aqueous layer was confirmed by addition of few drops of 10% copper sulfate solution, which produces intense blue color in the presence of amine ligand. Material was obtained by removing chloroform, and purified by column chromatography on silica gel (70– 230 mesh silica, CHCl 3 –MeOH, 9:1) leaving 1 pure DVb–TETA (0.48 g, 9% yield). NMR H (CHCl 3 ) dppm 3.18 and 3.84 (m, 16H), 5.56 (d, 2H), 6.06 (d, 2H), 6.97 (m, 2H), 7.6 (m, 8H). Carbon-13 NMR is not included because of additional carbon chemical shifts due to the presence of isomeric impurities. Analysis: calcd. for C 24 H 34 N 4 : C, 76.12; H, 9.05; found: C, 66.54; H, 7.26.

2.4. Preparation of N,N9 -divinylbenzyl triethylenetetraamine ( DVb–TETA–Cu) copper complex, 5

2.3. Preparation of N-vinylbenzyl triethylenetetraamine–copper ( Vb–TETA–Cu) complex, 4

2.5. Synthesis of polymers from Vb–TETA–Cu incorporated in TRIM matrix, 6

Copper sulfate (1.25 g, 5 mmol) was dissolved in minimum amount of water (2–3 ml) and diluted with 25 ml ethanol. This solution was added to 1.37 g (5.23 mmol) TETA–vinylbenzyl dissolved in 40 ml methanol. The blue precipitate was filtered and dried. The filtrate was concentrated and resuspended in methanol. The resulting precipitate was filtered and collected. The reconcentration / refiltration process was repeated to collect Vb–TETA–Cu complex. The combined fractions of blue complex were dried under vacuum to give 1.43 g 4 in 65% yield. The water-soluble complex has an R f value of 0.15 (silica–MeOH or MeOH–chloroform, 1:1) and charred on the TLC plate. This complex has shown a broad peak at 605–610 nm as solid sample mulled with nujol and a large defined peak at 610 nm in methanol. Analysis: calcd. for C 15 H 26 N 4 O 4 SCu: C, 42.70; H, 6.21; N, 13.28; found: C, 42.33; H, 7.43; N, 12.55.

To a solution of DVb–TETA (0.36 g, 0.95 mmol) in 20 ml chloroform–methanol (7:3), 0.55 g (2.1 mmol) CuSO 4 ?5H 2 O in 15 ml water was added. The solution was stirred thoroughly. The organic layer turned navy blue due to the formation of the copper complex. Water layer containing excess copper was discarded and the organic layer was washed with water until no free copper ions were detected in aqueous fraction. Removal of solvent afforded 0.48 g of navy blue 5 in quantitative yield. UV–vis spectrum showed absorption maxima at 688– 690 for the solid sample spread as film dispersed in nujol and at 651 nm in methanol solution. Analysis: calcd. for C 24 H 34 N 4 Cu: C, 65.16; H, 7.69; N, 12.67; found: C, 65.06; H, 7.01; N, 7.47.

Synthesis of metal-ion containing polymer was achieved by copolymerizing 4 with matrix monomer TRIM. Polymers were synthesized following the reported procedure with some modifications [16,17]. Complex 4 was dissolved in THF with the aid of laboratory bath sonicator. This step also helped in degassing the solvent. A weighed amount of TRIM was added to the solution and nitrogen was bubbled through the mixture thoroughly for at least 15 min. The solution was mounted on an oil bath, and heated by maintaining oil bath temperature at 708C while stirring the solution with a magnetic stirrer. At this time radical initiator azobis isobutyronitrile (AIBN) was added to start polymerization. After about 16 h, the temperature of the reaction mixture was raised to 858C. Polymer formation can be observed at this time. Finally, after maintaining at 858C for 1 h the reaction mixture was heated to 908C till polymer formation is observed (1–3 h). A free flowing, blue powder was formed at the comple-

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tion of polymerization. Polymer 6 was collected by filtration, washed thoroughly using water and methanol to remove any soluble impurities. Polymers were stored at room temperature after drying under high vacuum.

2.5.1. Synthesis of polymer containing 5 mol% Vb–TETA–Cu complex, 6 a A 30-ml THF solution of 172 mg (0.4 mmol) Vb–TETA–Cu was copolymerized after mixing with a solution of 3.3 g (9.76 mmol) TRIM in 30 ml methanol. To the homogenous solution 100 mg AIBN was added and the reaction mixture was stirred at 708C for 16 h. The temperature was then raised to 858C, which caused the solution to turn into solid powdery suspension within 1 h. The solid polymer was filtered off, washed thoroughly with water and methanol, and dried under vacuum to give 4 g polymer 6a. UV–vis spectrum of the polymer as thin film, dispersed in nujol revealed a broad absorption peak at 586 nm. 2.5.2. Synthesis of polymer containing 10 mol% Vb–TETA–Cu complex, 6 b Following the general procedure, 210 mg (0.5 mmol), 4 dispersed in 50 ml THF was crosslinked with 1.69 g TRIM (5 mmol) in 25 ml THF with the aid of 20 mg AIBN. After 18 h a thick, viscous slurry was obtained, which was diluted with additional 25 ml THF, followed by the addition of 10 mg AIBN. The mixture was sequentially heated at 708C for 4 h and 808C for 1 h to obtain the polymer. After the usual work-up 1.73 g 6b was obtained as solid blue powder. UV–vis spectrum of the polymer as thin film dispersed in nujol revealed a broad absorption peaked at 586 nm. 2.6. Synthesis of polymers from DVb–TETA– Cu incorporated in TRIM matrix, 7 Complex 5 was crosslinked with TRIM essentially following the procedure described for 6. For low loading of metal-ion complex monomer THF was used as solvent while for 10% loading, MeOH was used. The solution was

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thoroughly bubbled with nitrogen to displace dissolved oxygen. After the addition of radical initiator AIBN, the stirred solution was heated for 16 h at 708C, 3 h at 808C, and 1 h at 908C. At this time formation of solid polymer was observed. The work-up procedure was essentially the same as for 6.

2.6.1. Synthesis of polymer containing 5 mol% DVb–TETA–Cu complex, 7 a A 100-mg (0.18 mmol) amount of light blue, DVb–TETA–Cu complex was dissolved in 25 ml THF and to this solution 1.9 g (5.6 mmol) TRIM in 25 ml methanol was added to the clear blue solution. A 100-mg amount of AIBN was added to initiate the polymerization after thoroughly displacing oxygen by flushing nitrogen. The polymer was filtered off, washed with methanol and dried thoroughly in vacuum to give 1.8 g material. UV–vis spectrum of the polymer as thin film, dispersed in nujol, revealed a broad absorption peaked at 641 nm. 2.6.2. Synthesis of polymer containing 10 mol% DVb–TETA–Cu complex, 7 b Following the general procedure, divinylbenzyl–TETA–Cu Complex (133 mg, 0.24 mmol) was crosslinked with 845 mg, 2.5 mmol TRIM solution in 25 ml methanol. AIBN (100 mg) was added as polymerization initiator. After usual work-up, 900 mg dry polymer was recovered. UV–vis spectrum of the polymer as thin film, dispersed in nujol, revealed a broad absorption peaked at 641 nm. 2.7. Measurement of metal ion bound to templated polymers The following protocol was used in the quantitative analysis of metal ions bound to various polymers. In a typical experiment, copper ions bound to the polymer were released by treating 100 mg polymeric material with 10 ml 1 M HCl. A 0.5 ml aliquot of the supernatant was withdrawn and diluted to 10 ml by adding DI water. For analysis, to 0.5 ml diluted sample were added 5.5 ml water, 1 ml hydroxylamine

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hydrochloride (10%), 1 ml bathocuproine (0.1%), and 2 ml ammonium acetate (10%) [18]. The absorption values of the resulting colored solution were measured at 480 nm. By using a standard curve prepared by using preweighed analytical samples, the absorbance was directly correlated with the amount of copper ion present in the test sample. For measuring the reversible binding capacity, a weighed amount (50–100 mg) of metal bound polymer was soaked in 1 M HCl, washed with DI water (53), resoaked with 15 ml 2000 ppm copper sulfate solution, and washed for five times with DI water. Rebinding of the copper was measured following the protocol described in preceding paragraph. For mixed metal studies a solution containing 2168 ppm Cu and 2152 ppm Zn from copper and zinc sulfate salts was used. For example, 0.107 g Vb–TETA–Cu–TRIM polymer was first soaked in 10 ml 1 M HCl to remove the bound copper polymer, washed with 150 ml water after soaking for a few minutes each time to ensure removal of the acid. The polymer was then treated with the Cu / Zn solution for 36 h to assure completeness of competition in binding. The solution was analyzed for copper only.

2.8. Measurement of binding constants for polymers 6 and 7 Titrations were performed using an Orion 920A pH meter according to previously published procedures [19,20]. The electrode was standardized against pH 7.0 and 4.01 buffer standards prior to each titration. The following protocol has been used to measure formation constants in polymers 6b and 7b. A thoroughly dried, weighed amount of Vb–TETA–TRIM– Cu was treated with 10 ml, 1 M HCl to remove copper ions. The resulting polymer was washed with DI water (53), 10 ml, 1 M HCl (13), and again with DI water (53). Finally the polymer was soaked in 2 ml conc. HCl twice for 1 min each time. Excess HCl was removed by washing with water and the polymer was thoroughly

dried under vacuum. Standardized 0.10 M NaOH was titrated into 20-ml samples holding, e.g., 51.0 mg of polymer. The data were fitted using HYPERQUAD, a suite of programs designed to analyze chemical equilibria, to obtain both the deprotonation and stability constants.

3. Results and discussions The trifunctional methacrylate monomer, TRIM, was used as a cross-linking matrix monomer to produce macroporous polymers. Poly(TRIM) therefore provides good accessibility to small organic molecules and metal ions into the polymer interior [15]. In the current approach, instead of cyclic chelators, N-substituted, acyclic, multidentate amines are utilized for creating metal-ion specific binding sites. This approach bypasses our imperfect state-of-knowledge concerning how best to preconfigure the ligand about a specific ion, and helps to avoid the time-consuming, multistep synthesis typical for making macrocycles. Previous synthesis of polymeric materials capable of metal-ion binding was achieved by following molecular-imprinting approaches [1– 4]. The current approach utilizes molecular templating, which, in general, is similar to the template-mediated synthesis used in preparative organic syntheses. As shown in Fig. 1, metal ion sites in the polymer are re-enforced to preserve their conformation in the absence of metal ion. The monomer metal–ligand complexes, 4 and 5, are crosslinked with matrix monomers forming a porous cast around the complex to retain its geometry. Scheme 1 depicts the synthetic route followed for making such polymers. The classical template-directed synthesis of 4 by reacting TETA–CuSO 4 complex with vinylbenzyl chloride did not work. On the other hand, formation of 4 and 5 was easily achieved following a two-step synthesis, which involved reaction of 1 with Vb followed by treating with metal salt to form the metal–ligand complex. Alternatively, compound 2 was prepared by adding equimolar

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amounts of sodium amide and TETA hydrate before the addition of vinylbenzyl chloride. In this case, we believe that the role of sodium amide was to scavenge water from TETA and to allow vinylbenzyl chloride to react with TETA. Water solubility of 2 allowed the separation of 3 from the reaction mixture during synthesis. NMR revealed that in the case of either 2 or 3, vinylbenzyl substitution was not limited to any single nitrogen atom in TETA and other positional isomers were formed. For 2, only two positional isomers are possible. Proton NMR revealed the existence of only monosubstituted product. Carbon-13 spectrum showed chemical shifts indicating the presence of two monosubstituted isomers. In the case of 3, synthesis may lead to three disubstituted isomers. Additional chemical shifts in carbon spectrum confirmed the formation of additional disubstitution products. For both monomers the elemental analysis showed lower values for carbon and hydrogen indicating that the monomers are not single isomeric molecules. Both 2 and 3 were complexed with copper sulfate pentahydrate to produce 4 and 5. These complexes were characterized by absorption spectroscopy either in an organic solvent or by dispersing the complex in nujol. Absorption maxima at 598 nm for TETA–Cu complex indicated the expected square planar conformation of the complex was formed. Monomer Vb– and DVb–TETA–Cu complexes (4 and 5) showed absorption maxima at 610 nm and 651 nm, respectively, indicating that monosubstitution caused no major alteration of the geometry but that the DVb–TETA–metal complex has been distorted from the planar conformation. Metal ion binding results with templated polymers 6 and 7 were compared with the polymers consisting similar composition without any bound metal ion. We synthesized poly(TRIM) cross linked with styrene as well as untemplated divinylbenzyl TETA. Polymerization proceeded smoothly in both cases. Monomers containing metal complexes polymerized vigorously producing crackling sounds when the

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oil-bath temperature was raised to 908C. For safer polymerization and for obtaining reproducible polymer batches, we sequentially raise the bath temperature from 60 to 908C in 108C increments. Heating directly at 70 and 908C produced only a slight amount of polymer even after maintaining that temperature overnight. But heating the oil bath sequentially allowed the formation of polymers in a safer way. Solvents played an important role in polymerization reaction. Crosslinking of 4 with TRIM proceeded smoothly in THF, while the DVb–TETA–Cu–TRIM polymerization required methanol as the solvent. This was particularly true for cases where 10 mol percent or higher loading of the monomer metal complex was involved. All polymers prepared were free flowing fine powders and were found to be stable up to 2008C, above this temperature decomposition was observed. Information concerning the binding geometry of the templated sites may be gained by examining the electronic spectra of the polymers. Visible spectra of 6 and 7 at 5 and 10% doping were recorded as dispersions in nujol. In each case, broad absorption peaks with clear maxima were observed. Polymers from 6a and b showed absorption maxima at 586 nm, which was close to absorption maxima of its monomer precursor 2 indicating that the expected square planar geometry of the copper complex was retained. Absorption maxima at 641 nm for 7a,b indicated some distortion in the complex geometry. Identical visible spectra of polymers (582 and 643 nm for 6 and 7, respectively) obtained before and after subjecting the material to metal-ion removal / redoping cycle showed that the binding sites in the polymers did not change on cycling. Examination of the polymer under the scanning electron microscope showed a clear difference in texture between metallated and unmetallated Vb–TETA polymers (Fig. 2). Fig. 2A shows that the metallated Vb–TETA polymer, 6b, formed sheet-like materials, while the copper-bound polymer, 7b, appeared as uniform

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Fig. 2. Scanning electron micrographs of metal ion templated poly(TRIM); (A) 10% Vb–TETA in poly (TRIM), 6b, without Cu, (B) 10% Vb–TETA in poly (TRIM), 6b, with Cu, (C) 10% DVb–TETA in poly (TRIM), 7b, without Cu, (D) 10% DVb–TETA in poly (TRIM), 7b, with Cu.

spherical beads (Fig. 2B). In contrast to DVb– TETA polymer, Vb–TETA polymer changed morphology on metallation,. Surface area of the polymer 7b was found to be 233 m 2 / g. The surface area for TRIM-based polymers is reported in the range of 200–285 m 2 / g) [15]. Results of copper binding studies are compiled in Table 1. All values are averages of at least two measurements per experiment. Binding capacity values for poly(TRIM) cross linked with styrene and poly(TRIM) cross linked with DVb–TETA monomer (without any metal complex), which is |2 mg / g polymer. In either case, values for copper binding were within the

range of experimental errors. The initial cycle of metal binding after removal of metal-ion from the polymer shows close resemblance to the calculated metal ion binding capacity of polymer. Thus, an initial binding capacity of 22 mg Cu ion per gram of polymer was measured for 6b, which was in good agreement with the calculated values for this polymer. A maximum uptake of 25 mg copper ions per gram polymer is possible in polymer consisting of 10 mol% 4. Table 1 shows average values for bound copper. These values are smaller then reported in the preceding line because of the averaging method followed. These values were taken from a large

A. Singh et al. / Reactive & Functional Polymers 44 (2000) 79 – 89 Table 1 Metal-Ion uptake by various polymers Polymer

Bound Cu (mg / g) in polymer

Poly(styrene–TRIM) Poly(DVb–TETA–TRIM (10%)

1–3 1.9–1.3

Vb–TETA–Cu–TRIM 5% incorporation (6a) 10% incorporation (6b)

1361 1765

DVb–TETA–Cu–TRIM 5% incorporation (7a) 10% incorporation (7b) ]

762 1765

number of experiments performed by multiple person. An increase in binding capacity with additional doping of binding sites suggests that the binding efficiency of polymers can be improved by increasing incorporation of TETA in the polymer. Therefore, increased amounts of TETA in polymers may affect the physical characteristics of the polymer including binding specificity due to physical distortion of binding sites caused by constraints produced by overincorporation of binding sites. We have not examined the issue of how to optimize the maximum percentage of doping without losing selectivity. Polymers 6a and b showed higher binding capacity than polymer 7a or b, which are derived from disubstituted TETA copper complex. Both the mono- and divinylbenzyl– TETA polymers retained most of their metalbinding capability after several bind–release– bind cycles. Table 2 shows an initial decrease in binding capacity upon second doping cycle. One possible reason for the decrease may be due to permanent trapping of metal ion in the sites present in the deeper core of the polymer matrix. A lower, copper ion uptake value of 4.6 mg / g was observed when polymer was first

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soaked in zinc solution before soaking in the copper solution. It was unexpected. No change in binding capacity was observed after subsequent redoping cycles. To test the metal binding selectivity, CuSO 4 – TETA polymer samples were initially treated with 1 M HCl to remove the copper ions. In one test the demetallated polymer was soaked in a zinc–copper (1:1) solution (3000 ppm concentration), washed with water, and the amount of bound Cu was measured. In another test, the demetallated polymer was soaked in a concentrated Zn solution, then in concentrated Cu solution before measuring the amount of copper adsorbed. In a test to assess competitive binding of metal ions, 6b showed a binding capacity of 13 mg copper per gram polymer. This value suggested that the presence of copper template had preference for copper over zinc. Experiments were repeated by reusing the same polymer to evaluate their rebinding capacity. Lower binding values were obtained in the second cycle which were found stabilized in the subsequent cycles. Comparable binding values were obtained for each experiment type, which indicated the ruggedness of the imprinted sites. In order to make the process cost-effective, we prepared a technical grade polymer from a mixture of both the Vb–TETA monomer and access free TETA. After co-polymerizing Vb– TETA–Cu complex with TRIM, non-crosslinked TETA–Cu complex was removed from the polymer by extensive water washing, aided with ultra sound agitation. The resulting polymer also showed metal-ion binding selectivity, albeit with lower values for binding capacity, as expected. The protonation constants of the vinylbenzyl–

Table 2 Metal ion uptake by 10% Vb–Teta incorporated polymer (6b) Sample

Copper only Zn solution then copper Zinc1copper

Cu binding (mg / g) 1st cycle

2nd cycle

3rd cycle

13.1 13.1 14.0

8.3 4.6 8.4

7.6 7.9 7.9

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Table 3 Log K protonation constant Ligand

[HLA] / [L][H] [H2LA] / H[L][H] [H3LA] / [H2L][H] [H4LA] / [H3L][H]

Log K TETA

Vb–TETA–TRIM

DVb–TETA–TRIM

9.8 9.4 6.8 3.5

11.5 11.3 3.4 2.8

11.0 10.8 7.1 3.2

and divinylbenzyl–TETA polymers, 6 and 7, were determined by titrating them against 0.1 M NaOH. The results were compared with those obtained for TETA. The values are compiled in Table 3. We found that the first two protonation constants for DVb– and Vb–TETA polymers were much higher than for TETA, whereas the third and fourth constants of the DVb–TETA polymer were lower than the corresponding values of TETA. An increase in basicity, such as observed, is expected for the alkylation of an amine [21]. Interestingly, the values for the third and fourth constants for the DVb–TETA polymer have become similar to that of the cyclic tetraamine, cyclam [19], implying that some aspects of the locked conformation for the TETA in the DVb–TETA polymer might be similar to that of cyclam.

ions. Easy removal of metal ions from the polymer sites demonstrates that the poly(TRIM) matrix is porous enough to allow access to small ions and the approach may be extended for making templated polymeric materials with other metal ions. One may envisage that these polymeric materials could function as metalspecific sponges for removing targeted heavymetal ions for environmental cleanup.

Acknowledgements We thank Mr. Daniel Zabetakis for recording electron micrograph on the polymer samples. Financial Support from ONR is gratefully acknowledged.

References 4. Conclusion Selective metal ion binding sites have been successfully incorporated into macroporous polymers following the techniques of templatedirected synthesis. Metal–ligand templates embedded in polymer matrices provide selectivity towards templated metal ions. The advantages of the approach lie in avoiding the use of covalent interactions to create selective metal ion binding sites in polymers. The additional advantage is in the use of simple polymer synthesis with faster kinetics, and the mild conditions under which the complexes are formed. We have also demonstrated that, without the initial template step, polymers of the acyclic chelator, per se, do not bind to metal

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