Reactive & Functional Polymers 70 (2010) 545–554

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Reactive & Functional Polymers

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Engineering selectivity into polymer-supported reagents for transition metalion complex formation

Amanda N. Pustam, Spiro D. Alexandratos *

Department of Chemistry, Hunter College of the City University of New York, 695 Park Avenue, New York, NY 10065, United States

a r t i c l e i n f o

Article history:Available online 9 May 2010

Keywords:Polymer-supportedSelectivityMetalLigand

1381-5148/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.reactfunctpolym.2010.05.002

* Corresponding author.E-mail address: alexsd@hunter.cuny.edu (S.D. Alex

a b s t r a c t

Polymer-supported reagents are widely employed in the complexation of metal ions for applications inseparations science, organic reactions (as catalysts) and in analytical chemistry (for processes requiringmetal ion enrichment, detection and quantification). Various strategies for the continued developmentand optimization of polymer-supported reagents have evolved. This review cites ways in which poly-mer-supported reagents can be designed to achieve improved metal ion selectivities from the viewpointof the mechanisms involved in the complexation of transition metal ions from aqueous solutions.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction Chelating polymers offer enhanced selectivity for the separation

The selective removal of toxic metal ions from the aqueousenvironment remains a challenging problem. This issue has beenaddressed by the development of innovative methods for seques-tering metal ions. Existing technologies include solvent extraction[1,2], precipitation [3–5], electrochemical methods [6], membraneseparation [7,8], and ion exchange using natural [9–11] and syn-thetic materials [12].

Research in the area of metal ion separations is ongoing, in part,to meet environmental regulations. Further, there is a global impe-tus towards a reduction of metal ion waste collected from any sitefor treatment and/or storage at landfills. This can be achievedthrough recycling and reusing the metal ions. A concern that arisesis to identify ways of separating metal ions, especially those thatexhibit similar chemical behavior such as trivalent lanthanidesfrom transplutonium actinides [13].

The various technologies noted above are useful for specificapplications, though they are not without shortcomings [14]. Forinstance, while liquid–liquid extractions are efficient for separa-tions involving high concentrations of metal ions, large quantitiesof secondary waste are generated which require further treatment.Some extractant loss into the aqueous phase is likely due to theirfinite solubility. Ion-exchange resins are attractive alternativesfor separations from dilute solutions since the extractant is boundto a solid phase, simplifying the separation process. This makes itsuitable for separations of mixtures with complex matrices andalso environmentally safe. However, they can exhibit poor metalion selectivity and kinetics. The advent of chelating polymers inthe 1950s has allowed the synthesis of ion-selective resins [15].

ll rights reserved.

andratos).

and concentration of metal ions [16], while the varying stabilityconstants of metal ion complexes under different solution condi-tions can allow for greater degrees of separations. Chelating resincomplexes oftentimes have higher stability constants comparedto their corresponding monomer complexes [17]. This has beenattributed to an entropy effect; a large positive change in entropydue to the proximity of functional groups and the high local con-centration of ligands within the resin coupled to the release ofwaters of hydration upon complexation. Chelating resins includea wide range of ligands, including iminodiacetates [18,19], hydrox-ylamines [20], amides [21], Schiff bases [22], phosphates and phos-phonates [23].

Polymer-supported reagents are polymers that have beenchemically modified with various ligands. These modified materi-als are important as ion exchangers because they can be preparedto a desired selectivity with appropriate ionophoric reagents. Theyalso have high mechanical and chemical stability and recovery ofmetal ions can occur without destruction of the support. Poly-mer-supported reagents are effective chelants because of high up-take capacities of specific metal ions. There has been muchresearch on improving equilibration rates for these materials. Thisreview focuses on organic polymers; modified inorganic polymers[24], organic–inorganic hybrids [12,25], and impregnated resins[26] will be the subject of future reviews.

The need for designing selective polymers by determining ‘‘thebasic principles by which a polymeric reagent can recognize a tar-geted substrate” [27] has been expressed. The objective of this re-view is to contribute to the understanding of the behavior ofpolymer-supported reagents with primarily transition metal ionsin aqueous solutions. Detailed considerations about the interac-tions of lanthanide ions and polymer-supported reagents havebeen reviewed [28]. Resins are organized by classifying them based

CO

P OHOH

OCH2P OH

OH

O

A B Fig. 1. Polystyrene-bound a-ketophosphonic acid (A) and phosphonic acid (B)resins.

OCH2CHO CH2CHO]n CH2CHO[CH2

O(CH2)2

O(CH2)2

CH2

O(CH2)2

R

CH2

O(CH2)2

O(CH2)2

OCH2CHO CH2CHO]m CH2CHO[CH2

O

Fig. 2. Structures of NCR-S (R = SCH2CH3), NCR-SS (R = SCH2CH2SCH2CH3) and NCR-SN (R = SCH2CH2N(CH2CH3)2).

546 A.N. Pustam, S.D. Alexandratos / Reactive & Functional Polymers 70 (2010) 545–554

on the mechanisms by which metal ions interact with the ligandswhen bound to the support. The mechanisms identified aregrouped according to the properties of the metal ions and are theionic radius, polarizability, and coordinate geometry.

2. Synthesis and characterization

Polymer-supported reagents are most commonly prepared bymodification of a crosslinked copolymer, usually polystyrene,poly(vinylbenzyl) chloride and poly(glycidyl methacrylate), withdivinylbenzne or ethylene glycol dimethacrylate as the crosslinker.Polymers can be prepared as beads by suspension polymerization.By this method, the beads can have different levels of porosity. Thebead form of the polymer makes it amenable to continuous pro-cesses and enhances its versatility.

Different functional groups can be immobilized on the polymerdue to the applicability of organic reactions on a solid phase. Thesuccess of the functionalization relies largely on the choice of asuitable solvent to swell the beads, especially if they are micropo-rous (gel) beads. Swelling is important so that the reagent can havegreater accessibility into the bead, resulting in a high degree offunctionalization and thus a high metal ion saturation capacity.To ensure complete reaction, excess reagents are used. Macropo-rous resins are porous beads with channels for access of reagentsinto the interior of the bead. Swelling is not as significant for thesebeads compared to gel beads. Changes in the FTIR and NMR spectraof the polymer can indicate whether a particular reaction sequencewas successful. The degree of functionalization can be quantifiedby elemental analysis.

Alternatively, polymer-supported reagents can be preparedfrom a functionalized monomer bearing a complexing group thatis copolymerized with a crosslinking agent. This method is not al-ways amenable to suspension polymerization.

Evidence of metal ion interactions with functional groups isgathered from extended X-ray absorption fine structure (EXAFS)[29] spectra, X-ray photoelectron spectroscopy (XPS), electron spinresonance spectra (ESR), solid state NMR and IR spectroscopy.Images of the bead’s texture, size, and shape can be obtained fromscanning electron microscopy (SEM). Pore size and surface area canbe determined with porosimeters.

3. Ion-complexing polymer-supported reagents

Selectivity is a property of ion-complexing polymer-supportedreagents that refers to their ability to complex a specific metalion in the presence of other ions. This is especially necessary wherethe targeted metal ion is toxic and in much lower concentrationsthan other metal ions present, as is common in groundwater con-tamination. Because of the varying environmental conditions inwhich metal ions are found, different chelating ion exchangershave been studied, their applicability dependent on the situation.

Much of the development of ion-complexing resins is based onevidence from experimental work from solvent extraction [30],analysis of X-ray crystallography data [32] and molecular mechan-ics calculations [31–33]. These studies focus on observing theinteraction(s) of small molecule complexants with metal ions.

The following survey of the literature illustrates strategies thatare used to enhance metal ion selectivity in polymers.

3.1. Immobilizing ligand(s) with two or more coordinating sites

Multiple ligand interactions can result in metal ion binding withgreater affinity and selectivity. This effect is seen by comparingcomplexation by resins with carbonyl and phosphoryl coordinatingsites (Fig. 1) [34]. The uptake of Cu(II) and Pb(II) from 0.10 N HNO3

solutions was 89.5% and 97.5%, respectively, for the a-ketophos-

phonic acid resin (A) compared to 27.8% and 55.6%, respectively,for the phosphonic acid resin (B). Cooperation of the carbonyland phosphoryl moieties in binding the metal ions results in highersorption levels.

In another study, the sorption capacity of pseudocrown etherresins with pendent functional groups containing nitrogen and sul-fur, were determined for Cu(II), Co(II), Pb(II), and Hg(II) in the pHrange 1–4 [35]. The resins were identified as NCR-S, NCR-SS andNCR-SN based on the identity of the donor atoms in the R group(Fig. 2). All resins showed the highest affinity for Hg(II), comparedto the other metal ions. Of the three resins, NCR-SN had the great-est capacity and NCR-S had the lowest capacity. Note that the pen-dent functional group of NCR-S has one coordinating sulfur atomper ligand while NCR-SN has two (nitrogen and sulfur) coordinat-ing atoms per ligand. X-ray photoelectron spectroscopy confirmedthat the sulfur and nitrogen of the NCR-SN resin were involved inbinding the Pb(II), Cu(II) and Hg(II) ions.

Macrocyclic ligands such as crown ethers, calixarenes, resorcina-renes, cyclodextrins, and porphyrins, contain several donor sites formetal ion interactions and can be immobilized on polymers. Macro-cycles bind metal ions through the macrocycle effect [36]. Theimmobilization of 25,26,27,28-tetraethoxycarbonylmethoxyca-lix[4]arene on Amberlite XAD-4 via an azo linkage is pre-organizedfor Pb2+ and works best at pH 6.5 [37]. This resin interacts withPb2+ through a coordinative bond to each carbonyl oxygen on the li-gand (Fig. 3). The specific attraction for lead ions is also attributed tothe nature of the donor groups (see Section 3.4 below) which favorspolarizable cations such as lead.

Low-molecular weight complexing agents maintain their com-plexing ability after immobilization on polymer supports. A macro-porous resin, PDTA-4 (Fig. 4), was synthesized by attaching 1,3-diamine-2-hydroxypropane-N,N,N0,N0-tetraacetic acid to XAD-4via an ester link [38]. The resin tested by column studies was foundto be very selective for uranium(IV), thorium(IV) and zirconium(IV)(over Zn(II), Ni(II), Mn(II), Mg(II), Fe(III), Co(II), Cd(II), and Bi(III)) atpH below pH 2.5.

The complexing agent may be chosen as a result of its selectiv-ity for metal ions in biological systems [39]. The goal is to designligands or biomimics which would perform similarly when immo-bilized on polymers. Examples of ligands immobilized that are

CH2 CH CH2 CH

CH

NN

O

OEtO

O

O OEtO

O OEt

O

OEtO

NH3+Cl-

CH2HCH2Cn

n

CH2

Fig. 3. 25,26,27,28-Tetraethoxycarbonylmethoxycalix[4]arene supported XAD-4.

O CH

CH2N

CH2N

CH2COOH

CH2COOH

CH2COOH

CH2COOH

OHCH2C

HCH2C

n

n

Fig. 4. 3-Diamine-2-hydroxypropane-N,N,N0 ,N0-tetraacetic acid immobilized ontoXAD-4 (PDTA-4).

A.N. Pustam, S.D. Alexandratos / Reactive & Functional Polymers 70 (2010) 545–554 547

biomimics include hydroxamate [40] and catechol-containing[41,42] moieties.

HC CH2

COHN

CH2HN

CO

CH2SH

HC

CO

CH2

HNCH2

HNCO

CH CH2n m

Fig. 5. Poly [N-((acryloylamino)methyl)mercaptoacetamide].

3.2. The microenvironmental effect

The environment around a ligand may be altered by the pres-ence of other functional groups and affect the ligand’s ability tobind metal ions. The selectivity of Cu(II) over Co(II) was studiedby synthesizing polystyrene/poly(N-vinylimidazole-co-ethyl acry-late) interpenetrating polymer networks (IPNs) with varying ratiosof the N-vinylimidazole and ethyl acrylate [43]. Different ratios ofimidazole: ester cause changes in the polymer’s polarity andhydrophilicity. All IPNs containing imidazole had a much higheraffinity for Cu(II) than for Co(II). As the percentage of vinylimidaz-ole (%VIm) decreased from 100% to 53%, the Cu(II) affinity in-creased. This was indicated by an increase in the bindingconstant from 3130 N�1 to 8203 N�1. A further decrease in %VImresulted in a decrease in the binding constant for Cu(II) becausethe much-reduced polarity caused metal complex formation tobe disfavored.

Some immobilized ligands may react with metal ions after coor-dination. They are classified as Reactive Ion Exchangers (RIEX).Dual mechanism bifunctional polymers are included in this cate-gory [44]. These polymers contain two different functional groups,in which one ligand enhances the metal ion’s accessibility into thematrix while the other ligand recognizes and interacts with the

target metal ion. This interaction is accompanied by a chemicalreaction such as precipitation, reduction or complexation.

3.3. Ion-imprinted polymers (IIP)

Ion-imprinted polymers [45] operate from the principles ofmolecular recognition [46,47]. A template of a particular metalion within a polymer matrix is formed, and the polymer forms a‘‘memory” of that metal ion upon its removal. When the polymeris later contacted with a solution containing different metal ions,the polymer can extract the templated ion preferentially. Severalmethods by which these polymers can be prepared have beenidentified [45]. Further details are provided in a subsequent sectionunder the heading ionic radius.

3.4. Application of hard–soft acid–base theory to immobilized donorgroups

The affinity and selectivity a ligand has for metal ions is relatedto the stability of the metal ion complex formed upon binding. Ahydrophilic resin (Fig. 5) with thiol functional groups on a poly-acrylamide backbone was prepared [48] and its affinity for severalcations was quantified over a pH range of 0–8. The resin had noaffinity for alkali and alkaline earth ions while its affinity for firstrow divalent transition metal ions showed the same order as theIrving–Williams series: Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) >Zn(II).

3.5. Effect of spacer groups

Spacer groups are moieties that connect the ligand to the poly-mer support. Introducing ethylene oxide and ethylene sulfide spac-ers between chloromethylated polystyrene and thedi(isobutyl)phosphine sulfide ligand increased the resin’s capacityfor gold ions [49]. The synthesis of polymers with different spacerarms (Fig. 6) showed that ethylene oxide and sulfide spacers in-crease the hydrophilicity of the polymers and also participate inchelating the metal ions. Polymers 3 and 4 (Fig. 6) were the mosteffective for gold and palladium: the capacity for gold was 2.82and 6.50 mmol/g for polymers 3 and 4, respectively, while it was0.40 mmol/g for both polymers 1 and 2.

Spacer groups enhance the flexibility of the polymer by havingthe ligand at a distance from the polymer backbone and improvingaccessibility of the metal ion to the ligand [50]. In this way, the li-gand may show complexation behavior that approaches that of afree ligand in solution.

4. Factors affecting the affinity and selectivity of polymer-supported reagents

Many ion exchangers with different functional groups havebeen reported; some are more selective depending on the condi-

CH2 O (CH2CH2)4 P iBuiBu

S

n

CH2 (OCH2CH2)3 P iBu

iBu

Sn

Polymer 2

Polymer 3

CH2 SCH2CH2 (OCH2CH2)2 P iBu

iBu

Sn

Polymer 4

CH2 P iBu

iBu

Sn

Polymer 1

Fig. 6. Polymers with ethylene oxide and ethylene sulfide spacer groups.

CH2

OCH2COOH

CH2

NHCH2

CH

4

H2C n

Fig. 7. Calix[4]arene tetracarboxylic acid immobilized on polyallylamine (PAA-Calix).

548 A.N. Pustam, S.D. Alexandratos / Reactive & Functional Polymers 70 (2010) 545–554

tions in which they will be applied. Sorption studies report metalion affinities in terms of percent metal ion complexed, distributioncoefficients, and selectivity coefficients. However, it is oftentimeschallenging to understand the mechanism by which the resinscan complex metal ions. Solution conditions can influence theapparent affinities and selectivities. For example, metal ion specia-tion plays a significant role in complexation to the ligand and it isdependent on solution pH and the presence of counterions such asnitrate, chloride, and sulfate. Organic compounds in the environ-ment may act as complexing ligands and affect speciation [51].

The physical and chemical properties of the polymer matrix arealso variables. For example, the extent of crosslinking of polyacryl-amide affected the affinities of glycine [52] and ethylenediamine li-gands [53]. Polymer hydrophobicity can affect its compatibilitywith aqueous solutions and uptake capacities. The Cu(II) capacityfor a polystyrene-based resin with 2-aminomethylpyridyl ligandswas 8 g/L while the capacity of a methacrylate-based resin withthe same ligand was twice as great [54].

When external factors are constant, metal ion affinities andselectivities are governed by the electronic compatibility of the li-gand and metal ion. To design ligands with enhanced selectivity,the donor properties of the ligand must be matched with theacceptor properties of the metal ion [55]. This concept is expressedin several reviews where ion-selective ligands are identified [56].

It has been noted that ‘‘that no attempt has been made to devel-op a relationship between selectivity and different properties of achelating group present in a selective ion-exchange resin” [57].Additional relevant data are needed to establish a relationship be-tween selectivity and properties of a chelating group. However, theIrving–Williams series is one example of a trend elicited by diva-lent first row transition metal ions in their interaction with poly-mer-supported reagents, proteins and small molecules. Theaffinity order, Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II) is re-lated to the stability of the complex formed [58].

The following sections categorize the properties of metal ionsthat most strongly affect their sorption by polymer-supported re-agents based on literature data. The emphasis is on transition me-tal ions. The examples of polymer-supported reagents illustrate therelationships between ionic properties and observed selectivities.

5. Correlation of metal ion properties with the selectivities ofpolymer-supported reagents

5.1. Ionic radius

Evidence for ionic size as a factor in complexation was found instudies of small molecule complexes, particularly with macrocyclicpolydentates [59–63]. Macrocycles encapsulate metal ions. Ionicradius determines which metal ion gets complexed and the stabil-ity of the complex. For example, the lanthanide(III) complexes of1,4,7,10-tetrakis(carbamoylmethyl)–1,4,7,10-tetraazacyclodecaneshowed an increase in stability constants across the series withoptimum stability found with Sm(III) followed by a decrease toLu(III) [64].

The cyclic tetramer p-t-octylcalix[4]arene tetracarboxylate hasa high selectivity for Pb(II) over Fe(III), Al(III), Cu(II) and Zn(II) ions[65]. Immobilization of this extractant on polyallylamine, PAA, pro-duced a lead-selective resin [66]. The resin, PAA-Calix (Fig. 7),exhibited a selectivity order Pb� Cu� Zn = Ni = Co, where themaximum sorption of lead occurs at a pH of 4.5. Sorption studiesusing unfunctionalized PAA confirmed that the sorption behaviorof PAA-Calix was due to the calix[4]arene component and not thefree amino groups of PAA. A similar selectivity for lead ion byPAA-Calix and by the soluble tetramer was found. It was postulatedthat the size of the lead ion most closely matches the size of thecoordinating sites of the calixarene derivative. While this is a sig-nificant factor, the preference for lead was also shown by othermacrocyclic ligands bearing oxygen donor sites such as crownethers [67].

A sandwich complex of Zn(II) with the 4-vinylbenzyl derivativeof 1,4,7-triazacyclononane (TACN) was copolymerized in the pres-ence of divinylbenzene [68]. A 2:1 ratio of TACN:Zn in the polymermatrix was obtained. The polymer (Fig. 8) was then treated with6 N HCl to remove the Zn(II) ions. The demetallated resin wasselective for Zn2+ in the presence of Ni2+, Co2+ and Mn2+

(Mn2+ < Ni2+ < Co2+n Zn2+) at pH 4.5. The 2:1 ratio of TACN:Zn

was retained. The Zn(II) thus acted as a template to produce cavi-ties within the polymer that were of the same size as the ion andthose cavities led to the recognition of Zn(II) in the subsequentloading cycle. However, when Cu(II) ions were present, the resinhad a higher affinity for Cu(II) compared to Zn(II) at the samepH. While Cu(II) and Zn(II) ions have similar ionic radii, the resultsindicate that ionic radius is important but other factors also affectthe selectivity (vide infra). The competition between Cu(II) andFe(III) was studied at pH 2. A high selectivity was noted for Cu(II)relative to Fe(III) and this was attributed to the difference in ionicradii between the two ions.

A complex similar to the one described above was preparedusing mercury ions instead of zinc [69]. The demetallated polymer-ized sandwich complex of [{mono-N-(4-vinylbenzyl)-1,4,7-triaza-

N

NH

NH

N

N

HN

Zn2+

H

Fig. 8. Polymerized Zn2+ sandwich complex.

CO

O ECH3

n

CO

OEH

LCH3

nFig. 9. Structure of p(GMA-O) (E = O) and p(GMA-S) (E = S) before and afterimmobilization by ligand L.

N

OO

O O

N

H

H

7, 16-diaza-1, 4, 10, 13-tetroxacyclooctadecane, [18]aneN2O4

NO

N

O O

HH

7, 13-diaza-1, 4, 10-trioxacyclopentadecane, [15]aneN2O3

NHO O

OO

13-aza-1,4,7,10-tetroxacyclopentadecane, [15]aneNO4

Fig. 10A. Aza crown ethers.

A.N. Pustam, S.D. Alexandratos / Reactive & Functional Polymers 70 (2010) 545–554 549

cyclononane (TACN)}2 Hg](OTf)2 was selective for Hg(II) at pH 2.High selectivity coefficients KHg2þ=Mnþ

� �were found for mercury

in the presence of metals with smaller ionic radii (Fe3+, Cu2+,Cd2+) and larger ionic radii (Pb2+, Ag+). In the absence of Hg(II),the demetallated resin complexes low levels of other ions (0.87,41.2 24.1 and 130.6 lmol/g Pb2+, Fe3+, Ag+ and Cu2+, respectively).These observations suggest the prevailing factor determiningselectivity is ionic size. However, the ratio of TACN:Hg in the poly-mer was 1:1; when a metal ion with a large ionic radius (such asHg2+) is used as the template, the large distance between the TACNligands may allow them to coordinate metal ions in a manner inde-pendent of the sandwich arrangement. The polymer may not berecognizing the metal ions based on the cavity size created fromimprinting. The random arrangement of TACN ligands have a lowaffinity for Cu2+, Co2+, Ni2+, Zn2+ and Mn2+ and a high affinity forHg2+. Thus the polymerized sandwich arrangement of TACN li-gands achieves good selectivity for metal ions with smaller ionicradii by the template method through spatial recognition.

An ion-exchange resin with a high capacity and selectivity forthe uranyl ion was prepared by imprinting. The synthesis involvedthe copolymerization of styrene and uranyl vinylbenzoate cross-linked with divinylbenzene [70]. When the uranyl ion was re-moved, the binding sites for recognition of the uranyl ion wereretained by the polymer. When the polymer was contacted witha solution containing a mixture of ions, there was a preferencefor UO2þ

2 compared to Ni2+, Cd2+, Cu2+, Fe3+ at pH 3.5. A comparisonof sorption properties of the imprinted polymer to a non-imprintedpolymer (polymer prepared without the uranyl ion) for the uranylion demonstrated that imprinting had improved its affinity andselectivity. The rebinding capacity (lmol/g) was 0.407 for the ura-nyl-imprinted polymer and 0.021 for the non-imprinted polymerat pH 3.5. Furthermore, a Ni(II)-imprinted polymer was preparedin a manner similar to the uranyl-imprinted polymer. There waslower sorption of uranyl by the nickel-imprinted polymer at pH3.5 and 5.3 (0.058 and 0.321 lmol/g, respectively). This was dueto a difference in the size of the cavity created by the nickel iondue to the orientation of the functional groups from the size ofthe uranyl ion.

A polar support, poly (glycidyl methacrylate-co-ethylene glycoldimethacrylate), (p(GMA-O)), and its thiirane analog (p(GMA-S))were prepared (Fig. 9) and modified by the incorporation of azacrown ether-type ligands; [15]aneNO4, [15]aneN2O3 and [18]ane-N2O4, (Fig. 10A) [71]. The metal ion affinity of p(GMA-O)-[15]ane-N2O3 generally decreased with decreasing metal ion radii:Ag+ > Pb2+ > Cd2+ > Cu2+ > Co2+ > Zn2+ � Ni2+ (exceptions were Ag+

and Cu2+). The pendent hydroxy groups of the p(GMA-O)-[15]ane-

N2O3 resin destabilized the complexes of small metal ions com-pared to those of larger metal ions which explained thepreference of p(GMA-O)-[15]aneN2O3 for larger ions. Metal nitratesolutions were used for Ag+ and Pb2+ ions experiments. The orderdescribed above, (Cd2+ > Cu2+ > Co2+ > Zn2+ � Ni2+), was not ob-served for p(GMA-S)-[15]aneN2O3, which showed a reversal inthe affinity of Cd2+ and Cu2+ from buffered metal chloride solutionsat pH 3–7, under non-competitive conditions. This difference in thetrend was attributed to a replacement of hydroxy groups with thiolgroups, which have a stronger binding affinity.

Smaller ions such as Cd2+, Cu2+ and Zn2+ could not fit in the largecavity of p(GMA-O)-[18]aneN2O4 and had a low uptake capacity.For larger metal ions such as Ag+ and Pb2+, metal uptake byp(GMA-O)-[18]aneN2O4 and p(GMA-O)-[15]aneN2O3 resins weresimilar.

A Hg(II)-imprinted polymer is prepared by polymerizing meth-acrylic acid and trimethylolpropane trimethacrylate in the pres-ence of Hg(II)-1-(2-thiazolylazo)-2-naphthol (TAN-Hg, Fig. 10B)[72]. Extraction efficiencies of Hg(II) are compared for the im-printed polymer, P(TAN-Hg), a blank P(B) that was prepared with-out the template, and a polymer prepared with the ligand only

Fig. 10B. Hg(II)-1-(2-thiazolylazo)-2-naphthol.

550 A.N. Pustam, S.D. Alexandratos / Reactive & Functional Polymers 70 (2010) 545–554

P(TAN). At pH 7, almost 100% extraction of Hg(II) is obtained withP(TAN-Hg), while P(B) and P(TAN) showed 70% and 40%, respec-tively. High selectivity for Hg(II) is confirmed by competitive stud-ies of Hg(II) sorption in the presence of CH3Hg(I), Cd(II), Co(II),Cu(II), Ni(II), Pb(II) and Zn(II). The selectivity coefficient for Hg(II)is large in all cases, with the highest when compared to organicmercury. The distribution coefficient (D) for Hg(II) with P(TAN-Hg) is the highest (49) while the corresponding D values for theother ions range from 0.67 for Pb(II) to 0.07 for Zn(II).

5.2. Coordination geometry of the metal ion

The coordination number and geometry of transition metal ionshave been found to affect the apparent selectivities of polymer-supported reagents. This holds for oligomeric chelators [73] as wellas for larger molecules such as proteins [74]. Evidence was ob-tained from single crystal X-ray diffraction (XRD) and EXAFS spec-tra of complexes in the solid state and from computer modelingcalculations of complex formation [75]. Infrared spectra also iden-tify the ligands that participate in complexation from shifts in theband frequencies.

The coordination number is defined as the number of ligand do-nor atoms connected to the central metal ion. Coordination geom-etry refers to the spatial arrangement of the ligands around thecentral ion. It is possible for a metal ion with a particular coordina-tion number to have more than one geometrical arrangement. Theionic selectivity of polymer-supported reagents can be enhancedby creating pre-organized binding sites or fixing the position ofthe binding site so that a given metal ion can adopt a preferredcoordination number and geometry. One way this is achieved isthrough the preparation of ion-imprinted polymers [76,77]. Inthe preparation of these polymers, a cavity is created in whichthe specific geometric configuration for a particular metal ion is re-tained. When the imprinted polymer is contacted with a solutionof metal ions, the ion which best fits the geometric requirementsof the template ion will be complexed preferentially. Covalentand non-covalent interactions make it possible to fix binding sitesand improve selectivities and stabilities of metal ion binding. Theimprinted polymer is more selective for the template ion comparedto the non-imprinted polymer for the same metal ion. The geome-try around an ion and its size are related. As discussed earlier, ion-imprinted polymers also show selectivity based on ionic radius.

A surface-templated resin was prepared by emulsion polymer-ization using monomers, dioleylphosphoric acid and divinylben-zene in the presence of copper acetate; L-glutamic aciddioleylester ribitol was added as a surfactant [78]. The resin waswashed with 1 M HCl to remove the bound copper and its affinityfor Cu(II) investigated. A Zn2+ imprinted polymer and a non-im-printed polymer were also made and their affinity for Cu2+ exam-ined. All resins showed an increase in %Cu2+ ion sorbed uponincreasing solution pH. The sorption increase was most significantfor the Cu2+ imprinted polymer: it removed 100% of the copperions compared to the non-imprinted and Zn2+-imprinted polymers(both sorbed about 50% Cu2+ at pH 6). Imprinting with Cu2+ ar-ranged the dioleylphosphoric acid groups so that a favorable coor-

dination (most likely square planar) around the Cu2+ ion wasformed upon contact. This spatial arrangement was not met bythe Zn2+ imprinted (which favors a tetrahedral arrangement) northe reference resin, the latter having a random arrangement ofgroups.

Another Cu(II)-selective imprinted polymer containing methac-ryloylamidohistidine crosslinked with ethylene glycol dimethacry-late was successfully developed [79]. In the presence of Zn(II),Ni(II) and Co(II) at pH 7 and 25 �C, Cu(II) was preferentially sorbed:42.1 mg/g Cu(II) was sorbed compared to 18.3, 11.7 and 8.2 mg/gof Zn(II), Ni(II) and Co(II), respectively. The uptake of metal ionson the non-imprinted polymer was comparable: 16.8 and14.4 mg/g for Cu(II) and Zn(II), 10.8 and 9.3 mg/g for Ni(II) andCo(II). Distribution coefficients, Kd, indicated that the imprintingwas successful – Kd for Cu(II) was 5329 while the next highest Kd

was 577 for Zn(II), while the Kd for non-imprinted polymer was506 for Cu(II) and 405 for Zn(II). Analysis of ratios of selectivitycoefficients for Cu(II) in the presence of Zn(II), Ni(II), or Co(II)showed that Cu2+ uptake was 7.4, 9.5 and 12.3 times more selec-tive from a mixture of Cu2+/Zn2+, Cu2+/Ni2+ and Cu2+/Co2+, respec-tively, for imprinted than non-imprinted polymers.

Metal ion specific binding sites can be designed using acyclicmultidentate ligands. Cu(II) selectivity was obtained by synthesisand polymerization of a Cu(II)–triethylenetetramine complex[80]. The ligand was a mono- or di-subsituted vinylbenzyl chloridetriethylenetetramine complex (mVb-TETA or dVb-TETA) whichbinds Cu(II) through its nitrogen donor sites (mVb-TETA–Cu ordVb-TETA–Cu). Copolymerization of the copper complex with 2-ethyl-2-(hydroxymethyl)propane-1,3-diol trimethacrylate (TRIM)formed a macroporous polymer. The binding of Cu(II) to the ligandgives a conformation that will be specific for an ion with the geo-metric requirements of Cu(II) and, when polymerized, this confor-mation will be ‘‘locked-in”.

Ultraviolet–visible (UV–Vis) spectra indicated that the geome-try of the Cu(II)–ligand complex was retained when polymerized;for the monosubstituted TETA–Cu(II) complex, a square planargeometry (polymer and monomer had absorption peaks at similarwavelengths); a distortion of the planar geometry was observed forthe divinylbenzyl-TETA–Cu(II) complex. Polymers with 5% and 10%mVb-TETA–Cu and dVb-TETA–Cu were prepared. The Cu(II) uptakewas 17 mg/g for 10% mVb-TETA–Cu and dVb-TETA–Cu, 13 mg/g for5% mVb-TETA–Cu and 7 mg/g for 10% dVb-TETA–Cu The corre-sponding values of Cu(II) complexed for polystyrene-TRIM andpoly(dVb-TETA–TRIM) prepared without the copper template,were 1–3 and 1.9–1.3 mg/g. The selectivity for Cu(II) was deter-mined with the 10% mVb-TETA–Cu resin by contacting it with asolution of Zn(II) ions followed by a solution of Cu(II) ions or a solu-tion of Zn(II) and Cu(II) ions in a 1:1 ratio. The amounts of Cu(II)bound were the same in both cases; the maximum binding capac-ity was 13 mg Cu/g polymer.

Polymer-supported catechol ligands were selective for Fe3+ atpH 1–3 in the presence of divalent (Cu2+, Zn2+, Mn2+, Ni2+, Mg2+)and trivalent (Al3+, Cr3+) metal ions [81]. The selectivity was 11times greater (see Table 1) when the sulfonated catechol ligand(Fig. 11) was changed to a sulfonated 3,3-linear tris(catechol)amide (PS-3,3-LICAMS), a ligand predisposed for octahedral geom-etry (Fig. 12). It was evident that the catechol ligands were in-volved in chelate formation since the sulfonated andunsulfonated polymers (PS-CATS and PS-CAT) had similar selectiv-ities for Fe3+ from aqueous solution. The FT-IR spectrum of thecomplex, [(PS-CATS)2Fe]� showed free sulfonic acid bands at1220 and 1175 cm�1 while for the [(PS-3,3-LICAMS)Fe]3� complex,free sulfonic acid bands were observed and the C–O stretch shiftedfrom 1270 cm�1 to 1257 cm�1.

Immobilization of the sulfonated bis(catechol) linear amide (PS-2–6-LICAMS) ligand, (Fig. 13), also showed a high selectivity for tri-

OH

SO3Na

NaO3S

OH

n

CH2

Fig. 11. Polystyrene-supported sulfonated catechol, (PS-CATS).

CH2N(CH2)3

CO

N(CH2)3 NH

CO

NaO3S

OH

OHNaO3S

CO

OH

OHNaO3S

OH

OH

n

Fig. 12. Polystyrene-supported sulfonated 3,3-linear tris(catechol)amide, (PS-3,3-LICAMS).

NH HN

N

CH2H2C

N

Fig. 14. N,N0-bis(2-pyridylmethyl)-2,20-diaminobiphenyl (tetradentate ligand).

NH2 HN

N

CH2

Fig. 15. N-(2-pyridylmethyl)-2,20-diaminobiphenyl (tridentate ligand).

CH2 NH HN CH2

n

Table 1Km values for the sulfonated catechol polymers.

Relative equilibrium selectivity coefficients,Km with Mg2+ as standard, i.e. (Mn+/Mg2+)

PS-CATS

PS-3,3-LICAMS

PS-2–6-LICAMS

Fe3+ 171 1817 65Cr3+ 66 443 28Al3+ 43 121 6Mn2+ 5.0 6 1Cu2+ 0.8 4 1Zn2+ 0.8 2 1Ni2+ 0.3 3 1Mg2+ 1 1 1

A.N. Pustam, S.D. Alexandratos / Reactive & Functional Polymers 70 (2010) 545–554 551

valent ions. When the divalent ion affinity of PS-2–6-LICAMS, apotentially square planar ligand, was compared to PS-3,3-LICAMSand PS-CATS, PS-2–6-LICAMS was found to be the most selectivefor divalent ions, although no selectivity was observed with anydivalent ion (Table 1). Also, changing the cavity size (from two tosix CH2 moieties) on the linear amide chain between the two cat-

CH2N

CO

(CH2) NH

CO2-6

OH HO

OH HO SO3NaNaO3S

n

Fig. 13. Polystyrene-supported sufonated bis(catechol) linear amide, (PS-2–6-LICAMS).

echol groups in PS-2–6-LICAMS had no effect on the order ofselectivity.

Derivatives of a tetradentate ligand, N,N0-bis(2-pyridylmethyl)-2,20-diaminobiphenyl, and a tridentate ligand, N-(2-pyridyl-methyl)-2,20-diaminobiphenyl were bonded to crosslinked chlo-romethylated polystyrene [82]. Small molecule chelatecomplexes of the tetradentate ligand were prepared with Cu(II)and Pd(II), confirming the rationale that this ligand is predisposedto form complexes with a square or tetrahedral geometry due toarrangement of its donor nitrogen atoms. Solvent extraction stud-ies in chloroform solutions of the tetradentate and tridentate li-gands (Figs. 14 and 15) confirmed that Cu(II) is selectivelychelated from a mixture of Cu(II) and Fe(III). Iron(III) commonlyforms 6-coordinate octahedral complexes.

Phenolic tridentate and tetradentate analogs of the above li-gands were immobilized via the phenolic group (Figs. 16 and 17).The O-alkylated polymers had good binding affinities for Cu(II).For the tetradentate group, when the polymer was attached orthoto the methylamino groups, the Cu(II) uptake capacity averaged�1 mmol/g. When methyl groups were introduced ortho to thepyridine donor atoms (in both rings), affinity for Cu(II) was re-duced. This could result from a change of ligand stereochemistry

CH2O

N

Fig. 16. O-alkylated polymer of N-(2-pyridylmethyl)-2,20-diaminobiphenyl.

CH2O

N

H2C NH NH CH2

N

n

Fig. 17. O-alkylated polymer of N,N0-bis(2-pyridylmethyl)-2,20-diaminobiphenyl.

OO

S SNH

2S2

O

S S

10-aza-1, 4- dioxa-7, 13- dithiacyclopentadecane, [15]aneNO

552 A.N. Pustam, S.D. Alexandratos / Reactive & Functional Polymers 70 (2010) 545–554

or steric effects restricting access to binding sites. There was noaffinity for Fe(III) by any of the polymers.

5.3. Metal ion polarizability

Polarizability, a, refers to the ability of an atom or ion to be dis-torted by an electric field [83]. Polarizability is related to softness:atoms which hold onto their electrons less firmly are more polar-izable and are described as ‘‘soft”. ‘‘Hard” indicates low polarizabil-ity and the electron cloud of the atom is less easily deformed.According to the Lewis concept of acids and bases, the metal ionin a complex or coordination compound is the electron pair accep-tor or Lewis acid while the ligand is the electron pair donor or a Le-wis base. Soft acids prefer to coordinate with soft bases while hardacids prefer hard bases. Hard–soft acid–base (HSAB) theory is usedto predict the stability of compounds [84].

HSAB theory has been correlated with frontier orbital energiesof the reactants. When soft acids and soft bases interact, the energydifference between the highest occupied molecular orbital and thelowest unoccupied molecular orbital (HOMO–LUMO) is small, sothat electron transfer occurs and covalent bonding results [85]. Alarge HOMO–LUMO band gap is found in hard–hard interactionsand the bonding is electrostatic.

The first classification of Lewis acids and bases into class ‘‘a” andclass ‘‘b” was based on their preferential binding. Later, class ‘‘a”donors and acceptors were defined as ‘‘hard” and class ‘‘b” as‘‘soft”. Molecules and ions of intermediate character were termed‘‘borderline”.

Polarizability is related to the electronegativity of an atom:highly electronegative atoms are hard; softness decreases fromcarbon to fluorine across the second row of the periodic table.Polarizability is also a function of atomic number/size: largeratoms are softer than smaller atoms of similar electronegativity.The charge on an atom affects its polarizability: metal cations be-come harder as the oxidation number increases. Table 2 givesexamples of metal ions and atoms/molecules categorized by HSABtheory. There have been attempts to quantify hardness/softnesswith some physical parameter(s). A parameter known as absolutehardness, g, was identified [86]. Absolute hardness was calculatedfrom ionization potential and electron affinity values. However,determining absolute hardness for polyatomic ions was not accu-rate, making it difficult to rank acids and bases according to the de-gree of hardness. In the following examples, a correlation is

Table 2Classification of Lewis acids.

Class a or hard Class b or soft Borderline

H+, Li+, Na+, K+, Cu+, Ag+, Au+, Tl+,Hg+, Cs+

Fe3+, Co2+,Ni2+, Cu2+,

Be2+, Mg2+, Ca2+, Sr2+, Sn2+, Al3+, Sc3+,Ga3+, In3+, As3+, Ir3+, Si4+, Ti4+, Zr4+,Th4+, Pu4+,

Pd2+, Cd2+, Pt2+,Hg2+, CH3Hg+

Zn2+, Pb2+,

VO2+ Tl3+, Tl(CH3)3, BH3 B(CH3)3,SO2, NO+

UO2þ2 , (CH3)2Sn2+, RS+, RSe+, RTe+

BeMe2, BF3, BCl3, B(OR)3, Al(CH3)3, I+, Br+, HO+, RO+, I2,Br2, ICN, etc.

RPOþ2 ;ROPOþ2 ;RSOþ2 ;ROSOþ2 ; SO2;R3Cþ ,RCO+, CO2, NC+

Trinitrobenzene,etc.Chloranil, quinones,etc.Tetracyanoethylene,etc.O, Cl, Br, I, R3C(?)M0 (metal atoms)Bulk metals

R = alkyl or aryl group.

observed between the polarizabilities of ions and ligands basedon HSAB theory.

The immobilization of mixed crown ethers containing N, S andO atoms (Fig. 18) on copolymers of glycidyl methacrylate and eth-ylene glycol dimethacrylate (p(GMA-O), (Fig. 9) produced resinsthat extracted Ag+ in the presence of divalent ions (Pb2+, Cu2+,Cd2+, Zn2+) with negligible affinities. For example, (p(GMA-O)-[15]aneNO2S2) had a Ag+ capacity of 1.04 mmol/g [87] and(p(GMA-O)-[16]S4N) had a capacity of 1.39 mmol/g [88] in thepH range of 1–6. Resins containing macrocycles with a highernumber of sulfur atoms showed higher Ag+ complexation:(p(GMA-O)-[15]aneN2O2S), with one sulfur atom, had a maximumAg+ capacity of 0.58 mmol/g at pH 4.7. The ability of crown ethersto complex Ag+ is related to the polarizability of the donor atoms[89]. Note that (p(GMA-O)-[12]aneNOS2), with a smaller ring size,had a capacity of 1.05 mmol Ag+/g resin [87].

Ag+ is a soft Lewis acid and will have an affinity for soft baseswith sulfur atoms. Hence the observed trend is expected basedon the relative polarizabilities. Further evidence for the affinity ofAg+ for sulfur ligands was the observation that the hydrolysed resin(p(GMA-S)), (Fig. 9), with thiol groups had a capacity of 1.5 mmolAg+/g resin, while p(GMA-O), when treated similarly, sorbed only0.01 mmol Ag+/g resin.

Chelating resins containing bis(sulfonamide) groups (Fig. 19)were immobilized on Amberlite XAD-2, a commercial styrene–divinylbenzene macroporous resin [90]. Metal sorption was stud-ied from pH 1–7 and was optimum at pH 5.5 for Cu(II) and Zn(II)and pH 6.0 for Cd(II) and Pb(II). Copper and zinc uptakes weregreater than cadmium and lead. Generally, the metal capacity in-creased, though not significantly, with III > II > I. IR spectra suggest

NH

[12]aneNOS2

OO

NH HNS

7-aza-1-oxa-4, 10-dithiacyclododecane,

7, 13-diaza-1,4-dioxa-10-thiacyclopentadecane, [15]aneN2O2S

S S

S SHN

7-aza-1, 4, 10, 13-tetrathiacyclohexadecane, [16]aneS4N

Fig. 18. Crown ether ligands with sulfur, nitrogen and oxygen donor atoms.

SO2NHn

R NHSO2 n

I : R= (CH2)2II : R= (CH2)3

III : R= (CH2)2NH(CH2)2

Fig. 19. Bis(sulfonamide) resins.

A.N. Pustam, S.D. Alexandratos / Reactive & Functional Polymers 70 (2010) 545–554 553

that metal ions coordinate to the resin via the amino-nitrogens.There were shifts in the IR spectra for N–H and C–N bands whilebands due to C–H and SO2 were not greatly affected. The IR datawere comparable for metal complexes with diamine or polyaminegroups. Of the metals studied, Cd(II) and Pb(II) are more polariz-able than Cu(II) and Zn(II). Nitrogen-containing ligands are consid-ered borderline, preferring metals that are of intermediatehardness/softness. Amines are known for their affinity for Cu(II)[91].

Resin III has the greatest ability to separate Cu(II) from mixturesof Cu(II) and Pb(II) compared to other binary mixtures with Cu(II).The separation factors, K, are: 38.74 (resin III), 22.36 (resin II) and12.81 (resin I) for the Cu(II)–Pb(II) binary mixture. This is as a con-sequence of the high affinity for Cu(II) and lower affinity for Pb(II)by the donor nitrogen atoms.

Poly(2-hydroxyethylmercaptomethylstyrene–diethanolamine)or (PSME–EDA) is a novel chelating resin that contains S, N, and Oatoms (Fig. 20) [92]. This resin has a high affinity for mercury(II)ions from a solution of mercury nitrate buffered at a pH of 5.4. Up-take of Hg(II) ions involved both N and S atoms on the resin as seenby X-ray photoelectron spectroscopy (XPS) (after complexationwith Hg(II), the binding energy for N1s and S2p increased whilethere was almost no change for O1s). A new peak for N1s suggestedthat Hg(II) was sorbed as Hg(NO3)2. This is consistent with Hg(II), ahighly polarizable metal, will have strong affinities for ligands withthe polarizable sulfur and nitrogen.

Polymers of vinyl 2-hydroxyethyl sulfide (PVHES) and vinyl 2-hydroxyethyl sulfide–acrylamide (PVHES–AA) were prepared withthe cross linker N, N-methylenebisacrylamide (Fig. 21). Their effi-ciency as sorbents for metals such as Au(III), Hg(II), Pd(II), Pt(IV),Cu(II), Fe(III) were examined from dilute and concentrated HCland HNO3 solutions [93]. Copper and iron were not sorbed inappreciable amounts. Among the highest distribution coefficientswere, for PVHES, Au(III) 19800; Pd(II) 11606; Hg(II) 1913; and forPVHES–AA, Au(III) 12000; Pt(IV) 200; Pd(II) 168; Hg(II) 167. All

CH2SCH2CH2NCH2CH2OH

CH2CH2OHn

Fig. 20. Poly(2-hydroxyethylmercaptostyrene–diethanolamine) PSME–EDA.

(CH2CH)x (CH2CH)y (CH2CH)z

S

CH2

CH2

OH

C ONH2

CHN

CH2HN

C

O

O

(CH2CH)n

Fig. 21. Copolymer of vinyl (2-hydroxyethyl) sulfide and acrylamide crosslinkedwith methylenebisacrylamide.

the metals that were sorbed are soft acids and show preferencefor resins bearing soft bases such as sulfur groups in the polymers.

6. Concluding remarks

Much is known about polymer-supported reagents and theyhave been intensely studied. The key variables by which transitionmetal ions are selectively complexed based on their propertieshave been outlined: ionic radius, geometry, and polarizability.Solution conditions (pH, counter ions, solvent polarity) often influ-ence the apparent sorption. While relying on one property to pre-dict affinities can be an oversimplified approach, it does oftentimesprovide a good first approximation to the observed selectivities.The results do indicate that the polarizability properties of theion and the ligand can dominate the final results. An understandingof the mechanisms of interaction via property-function relationscan be a useful tool in the design of polymer-supported reagents.

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