The Green Route from Carbon Monoxide Fixation to Functional

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The green route from carbon monoxide fixation to functional polyamines: a class of high-performing metal ion scavengers Claudio Toncelli, Frank Alberts, Aaldrik Haijer, Antonius Augustinus Broekhuis, and Francesco Picchioni Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02556 • Publication Date (Web): 11 Sep 2015 Downloaded from http://pubs.acs.org on September 15, 2015

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Industrial & Engineering Chemistry Research

The green route from carbon monoxide fixation to functional polyamines: a class of high-performing metal ion scavengers C.Toncellia,*,1, A. Haijera, F. Albertsa, A.A. Broekhuisa, F.Picchionia

a

Department of Chemical Engineering/Institute for Technology and Management, University of

Groningen, Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands KEYWORDS: functional polymers, Paal-Knorr, heavy metal ion, aliphatic polyketones, structure-performance relationship

ABSTRACT.

The

exploiting

of

Paal-Knorr

reaction

as

post-modification

tool

of

ethene/propene/CO polyketone terpolymers to produce thermoset beads for metal ion uptake is hereby presented. Its prominent green character (i.e. no catalyst neither solvent employed, biocompatible polymer precursor) render this class of functional polymers highly competitive in terms of low environmental impact compared to commercial bio-sorbents. Different crosslinking protocols are implemented for the four polymer resins, all involving formation of imine and/or pyrrolic bridges between the macromolecules. Functional thermosets bearing piperazine (X-PKPip), primary amine (X-PKDap) or glycine-type (X-PKLys) functional groups are

ACS Paragon Plus Environment

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synthesized. Qualitative correlation has been attempted with the metal ion-functional group interaction

evaluation

through

FT-IR

measurements

and

the

correspondent

uptake

performances.During batch studies, the three resins indicated a robust uptake of Cu(II), Ni(II), Ag(I) and Cr(III) whereas in competitive adsorption during selectivity studies they showed a different reactivity pattern in the presence of the same metal ions.

1. Introduction

Heavy metal ion pollution deriving from anthropogenic and endogenic sources constitutes a health hazard for freshwater and marine biota1. Many separation techniques (i.e. liquid–liquid extraction2, bulk-liquid membrane systems3, sorption, precipitation, coagulation and flocculation processes4, ultrafiltration5 as well as electrochemical treatment6 are currently being scrutinized. However, most of them failed to perform when it comes to device up-scaling and product commercialization due to several factors (membrane saturation/deactivation, long residence times and/or use of toxic chemicals)4. Ion-exchange resins prove to be a promising candidate as liquid-solid adsorption by being the perfect compromise between acceptable performances and low-cost of the adsorption/regeneration process7. Indeed, the metal ion can then be desorbed at acidic pH or by increasing the temperature. Nonetheless, metal ion/polymer composites can be employed as precursor for synthesis of metal-organic nanocomposites for fabrication of devices in optoelectronics8, catalysis9 or anti-bacterial surfaces10.

Functional materials with N-chelating groups score an excellent uptake for heavy metal ions and they display high variability in the interaction with metal ions according to the type of functional group. Indeed, macromolecules bearing secondary and tertiary amines11–13, pyrrole and pyridine14 preferably interact with soft metal ions (in terms of Pearson’s hard-soft theory15) while


2

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primary amines16 and imines chelate with hard metal ions. Obviously, when the product needs to perform in an ecosystem, his design must comply with environmentally friendly fabrication process. However, this is not the case for commercial ion-exchange resins, such as functional cross-linked polystyrene. Undeniably, this material presents environmental issues correlated with the monomer toxicity17.

On the other hand, chemically modified polyketones represent a green alternative for obtaining polyamines with tunable amine concentration and cross-linking degree18. Indeed, perfectly alternating aliphatic polyketones (PK) terpolymers produced by carbon monoxide and unsaturated hydrocarbon monomers (e.g. olefins), react, through the Paal-Knorr mechanism, with N-functionalized amines to yield pyrrolic moieties with the functional group pendent as in a comb-like polymer19. The absence of any catalyst or solvent as well as the use of mild operating conditions (T ≈ 90-110°C) represents the key advantage for this post-modification pathway. Moreover, the high tolerance towards different functional groups linked to the amine moiety by a methylene spacer allows the synthesis of a wide range of functional polymers20–22. The simultaneous re-arrangement of 1,4-dicarbonyl groups in pyrrolic rings on the functionalized repetitive unit constitute a unique feature within the class of post-modification reactions. Therefore, the rigidity of the backbone can be modified by simply varying the cross-linking degree and this constitutes a distinctive advantage during polymer processing.

In this work, a paradigm of the potentiality for this system is presented as metal ion uptake behavior of several different functional polymers (derived from polyketones) bearing amines (PKDap, PKPip) and glycine-like (i.e. amino-carboxylic, PKLys) groups along the backbone (Scheme 1). Their structures were well established by 1H-NMR and FT-IR and the carbonyl


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conversion was calculated by elemental analysis. Three cross-linked polymers (X-PKDap, XPKLys, X-PKPip) were synthesized by employing three different cross-linking methods.

The metal ion (i.e. Cu(II), Co(II), Ni(II), Cr(III), Fe(III), Hg(II) and Ag(I)) reactivity in batch studies was then compared to the functional group/metal ion interaction via FT-IR of the investigated complexes. In the second part of the study, selectivity measurements were carried out in the presence, simultaneously, of the four metal ions that showed the best affinity during the previous study.

2.Experimental

2.1. Materials

Alternating polyketones with 30 wt% ethene and 70 wt% propene (PK30, Mw-3970 g/mol) based on the total olefin content were synthesized according to a reported procedure23. 1,2Diaminopropane (1,2-Dap, Acros, >99%), L-Lysine ((C6H14N2O2), Sigma-Aldrich 99%), 1Amino-ethylpiperazine (C6H15N3, Sigma Aldrich 99%), Hexamethylenediamine (HDA, SigmaAldrich 98%), Chromium nitrate (Cr(NO3)3.9H2O, Sigma-Aldrich 99%), Cobalt (II) nitrate (Co(NO3)2.6H2O, Sigma-Aldrich 99%), Iron(III) nitrate (Fe(NO3)3.9H2O, Sigma Aldrich 99%), Nickel (II) chloride (NiCl2, Sigma Aldrich 99%), Copper (II) sulfate (CuSO4.5H2O, Merck 99%), Silver (I) nitrate (AgNO3, Merck 99%), Mercury chloride (HgCl2, Baker 99%), Chloroform (Acros) and Tetrahydrofuran (THF, Acros) were purchased and used as received. De-ionized Milli-Q water has been used.

2.2 Synthesis of cross-linked functional polymers


4

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2.2.1.Chemical modification of polyketones by conventional heating (PKDap, PKPip)

The chemical modifications were carried out following a similar procedure adopted in a previous publication of our group19 in the presence of 1,2-diaminopropane or 1-aminoethylpiperazine as amino-substituted compound. During the reaction, the reactants mixture typically changed from slight yellowish, low viscous fluids into highly viscous dark brown homogeneous paste. At the end of the reaction (after 4 hrs), the resulting products were cooled to room temperature. After grinding into small particles, the obtained polyamines were washed several times with de-ionized Milli-Q water to remove, if any, unreacted amine. After filtering and freeze-drying, light brown polymers were obtained as the final products.

In order to calculate the conversion of reacted 1,4-dicarbonyl units (y) (Scheme 1), elemental analysis was performed using a Euro EA elemental analyzer.

Scheme 1. Paal-Knorr reaction of aliphatic polyketones (PK) with several N-substituted amines: 1-aminoethylpiperazine (I), 1,2-diaminopropane (II) and L-lysine (III).


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The results of this analysis are linked to the conversion by the following formula:

ܰ=ெ

ெ೙ ∙௬∙௡భ

1

ೢభ ∙(ଵି௬)ାெೢమ ∙௬

where Mn represents the atomic mass of nitrogen, n1is the number of nitrogen atoms in the repetitive unit of the reacted polyketone, N is the nitrogen content per gram calculated by elemental analysis, Mw1 is the molecular weight of a non-converted 1,4-dicarbonyl segment (131.6 g/mol) of PK30 and Mw2 is the molecular weight of converted 1,4-dicarbonyl segment (169.7 g/mol for PKDap and 223.6 g/mol for PKPip) (Scheme 1). Mw1and Mw2 are calculated by taking into account the presence of 30 wt % ethylene and 70 wt % propylene in the copolymers.

From that, the conversion (y) can be calculated by:

‫=ݕ‬ெ

ே∙ெೢభ

2

೙ ∙௡భ ାே∙(ெೢభ ିெೢమ )

The 1,4-dicarbonyl yields for each modified polyketone (X-PKLys, X-PKDap and X-PKPip) were calculated on the basis of elemental analysis data (Table 1).

Carbonyl conversion (%)

PKLys

47%

PKDap 65 %


6

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PKPip

47 %

Table 1. Conversion of carbonyl groups for the mentioned polymers (PKLys, PKDap, PKPip)

PKDap

EA: Calcd. N 12.4; C 72.0; H 9.55; O 6.05.Found N 12.25; C 71.95; H 9.50; O 6.25.

FT-IR (KBr): 3300-3340 cm-1 (m, N-H stretching of primary amine groups), 1705 cm-1 (m, stretching C=O), 1625 cm-1 (m, N-H scissoring primary amines), 1373 cm-1 (s, C-N pyrrole stretching), 750 cm-1 (m, N-H wagging)

PKPip


FT-IR (KBr): 3448 cm-1 (m, N-H stretching of secondary amine groups), 1705 cm-1 (m, stretching C=O), 1373 cm-1 (s, C-N stretching pyrrole),663 cm-1 (N-H wagging)

2.2.2.Chemical modification of polyketones by microwave heating (X-PKLys)

The synthesis of X-PKLys was carried out in microwave oven and thus deviates from the above described procedure. The reason behind the use of microwave is mainly linked to the faster kinetics. Moreover, the chemical modification and the cross-linking occur in one-pot by reaction of both amine groups of the employed amino-acid, L-lysine.


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Details of the reaction procedure can be found elsewhere24. The occurring of the Paal-Knorr reaction can be observed by the change in color of the media from slightly yellowish to dark brown or dark orange (Scheme 2). The calculation of the functionalization degree can be determined by elemental analysis as in the previous section.

Scheme 2. Paal-Knorr reaction of PK30 with L-Lysine in microwave. Production of pyrroleamino/carboxylic units (y), iminopyrrole units (z1) and carboxylic-pyrrole moieties (z2).

X-PKLys


FT-IR (KBr): 3434 cm-1 (m, N-H and O-H stretching), 1705 cm-1 (sh, stretching C=O), 1640 cm1

(s, C=N stretching), 1390 cm-1 (s, C-OH bending), 663cm-1(w, N-H wagging)

2.2.3. Cross-linking of PKDap


8

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PKDap was cross-linked via thermal heating for 10 minutes at 150°C. In this case, the network formation has been shown

25

to proceed via reaction of the PKDap free amino groups with the

free carbonyls on other chains to mainly yield a Schiff base.

X-PKDap


FT-IR (KBr): 3358 cm-1 (m, N-H stretching of free amine groups), 1705 cm-1 (m, stretching C=O), 1669 cm-1 (m, C=N stretching), 1458 cm-1 (m, scissoring of CH2 units), 1368 cm-1(m, C-N pyrrole stretching), 1029 cm-1 (m, pyrrole ring breathing), 750 cm-1 (w, N-H wagging)

2.2.4. Cross-linking of PKPip

Immediately, after the Paal-Knorr modification, PKPip was cross-linked by addition of hexamethylenediamine (HDA) solution in methanol (0.04 equivalent with the respect of total amount of 1,4-dicarbonyl group present in the polyketone precursor) drop-wise for 10 minutes at a reactor stirring speed of 500 rpm and at a temperature of 110°C. After a total reaction time of 100 min, the suspension was kept at 110°C without cooler in order to evaporate the residual methanol. The resulting solid was a rubbery, dark brown material that was washed several times with de-ionized Milli-Q water to remove unreacted di-amine. However, in all cases, GC/MS analysis of the water solution after evaporation used to wash the polymer yielded only traces of unreacted di-amine. This suggests a quantitative conversion of the hexamethylenediamine.

X-PKPip


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FT-IR (KBr): 3430 cm-1 (s, N-H stretching of secondary amine groups), 1720 cm-1 (m, stretching C=O), 1670 cm-1 (m, stretching C=N), 1464 cm-1(N-H piperazine ring bending), 1013 cm-1 (m, pyrrole ring breathing)

2.3. Physico-chemical characterization

Atomic Absorption Spectroscopy (AAS) measurements were carried out by using an AAnalyst 200 device interfaced with Perkin-Elmer software 5.0.0.1. All solutions, both before or after adsorption, were filtered and diluted till the respective linearity range for the detecting element. FT-IR spectra were recorded using a Perkin Elmer Spectrum 2000 FT-IR apparatus. 1H-NMR spectra were recorded on a Varian Mercury Plus 400 MHz NMR apparatus using CDCL3 as solvents.

2.3.1. Batch experiments

For all batch experiments, 0.15 g (corresponding to X-PKDap: 0.570 mmol, X-PKLys: 0.315mmol and X-PKPip: 0.630mmol of amino groups) of polymer was poured in a beaker at different temperatures (e.g. 35, 50, 60, 70 and 80°C) in the presence of 20 ml of the selected metal ion solution (i.e. Ag(I), Hg(II), Cu(II), Ni(II), Co(I), Cr(III), Fe(III)) at equimolar ratio with the total nitrogen intake (i.e. taking into account also the pyrrole groups along the modified polyketone backbone). Upon reaching the desired contact time (e.g. 10, 20, 30 and 40 minutes), the solution was filtered off under vacuum and the filtrate was analyzed by AAS (using AAnalyst 200 atomic adsorption spectroscopy interfaced with Perkin-Elmer software 5.0.0.1).


10

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For all the experiments, the pH values were fixed in the range between 4 and 5 (thus avoiding the formation of any precipitate), i.e. at their natural value depending on the chemical composition. The use of a buffer for finer pH adjustments has been considered but discarded in order to ensure a proper comparison with similar studies described in the literature and in the perspective of an industrial application.

2.3.2. Selectivity batch experiment

To evaluate the selectivity of the cross-linked polymers (X-PKDap, X-PKPip and X-PKLys) for Cu(II), Cr(III), Ni(II) and Ag(I), the metal ions in solution were mixed in a beaker (80 mL). The metal ion content (for each single metal) of the solution was equimolar to the total nitrogen equivalent calculated by elemental analysis for 0.15 g of resin. The polymer was stirred for 40 minutes in the presence of the metal solution. The solution was then filtered off and analyzed by inductively coupled plasma (ICP) equipped with a Perkin-Elmer interface.

The metal ion uptake selectivity has been calculated according to the equation:

Mratio = Mads / N

3

Where Mratio is the metal ion uptake selectivity calculated for each couple metal ion/cross-linked resin, Mads is the total amount in mol of adsorbed metal per gram of resin and N is the total amount of nitrogen equivalent per gram of resin.

3. Results and discussion

3.1. Synthesis of chemically modified polyketones (PKDap, PKPip)


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The Paal-Knorr reaction of 1,2-diaminopropane (PKDap) and 1-aminoethylpiperazine (PKPip) with PK30 has been carried out for four hours. The elemental analysis (Table 1) confirmed the fixation of nitrogen on the backbone of the polymer and the 1,4-dicarbonyl conversion resulted to be 64.7% for PKDap (4.3mmol/gresin of amino groups) and 47.0 % for PKPip (4.2 mmol/gresin of secondary and tertiary amino groups). IR and 1H-NMR spectra have been recorded after functionalization (Figure 1).

b

a Figure 1.1H-NMR (a) and FT-IR (b) scans of PKDap.

The FT-IR spectrum of PK30 and PKDap (right in Figure 1) showed the appearance of two peaks at 3300-3340 cm-1 (N-H stretching of free amine groups), at 1625 cm-1 (N-H scissoring primary amines), 1076 cm-1 (pyrrole ring mode), 1033 cm-1 (C-N aliphatic stretching) and 750 cm-1 (N-H wagging) that confirmed the presence of free amine and pyrrole ring26. On the other hand, the drop in intensity of the peak at 1705 cm-1 (stretching C=O) is in agreement with the decrease in carbonyl groups density along the backbone after the reaction.


12

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Furthermore, 1H-NMR spectrum of PKDap (left in Figure 1) and PKPip (left in Figure 2) displays, in comparison with the one of PK30, the appearance of a broad peak at around 5.5-6.0 ppm and at 130 ppm in the 13C-NMR (not shown for brevity), assigned to the pyrrole group in agreement with previous studies19.

a

b

Figure 2.1H-NMR (a) and FT-IR (b) scans of PKPip.

The complicated and random structure of the polymer (i.e. ethylene/propylene distribution along the backbone) resulted in relatively broad peaks, which hinders their detailed interpretation at high fields (low δ).

Moreover, for PKPip a shift of the signal is observed from the region 3.2-2.2 ppm in PK30 spectrum (CH and CH2 in α to carbonyl moieties) towards lower ppm (i.e. 2.9-2.0) as well as a sharp signal at 2.2 ppm (attributed to the CH2 of the piperazine group27). FT-IR spectra of the latter (Figure 2 right) showed, with respect to PK30, a new broad peak at 3448 cm-1 (N-H aromatic and aliphatic stretching) and weaker C=O stretch adsorption at 1705 cm-1.


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3.2. Synthesis of cross-linked modified polyketones (X-PKLys, X-PKPip, X-PKDap)

The reaction of PK30 with L-Lysine was carried out in a microwave at 60°C for 1 hr and it resulted in a conversion of 1,4-dicarbonyl groups to L-lysine derivatives equal to 47 %.

A broad peak at 3434 cm-1 (N-H and O-H stretching) appears in X-PKLys FT-IR spectra (Figure 3a) and confirmed the presence of amine and carboxylic groups.

Figure 3. FT-IR spectra of PK30 and X-PKLys (a), PKDap and X-PKDap (b) and PKPip and XPKPip (c).

Furthermore, the peak at 1705 cm-1 (C=O stretching) decreased in intensity and became a shoulder of the one set at 1647 cm-1 (C=N imine stretching) thus suggesting, as expected, a simultaneous cross-linking reaction. The peaks at 1455 cm-1 (C=C pyrrole stretching) and 1398 cm-1 (C-N pyrrole stretching) indicate the increased of the aromatic/aliphatic ratio of the polymer backbone25.

A comparison between the FT-IR spectra of PKDap before and after cross-linking for 15 minutes at 150°C (reaction of the remaining carbonyl moieties with the pendent primary amine groups)


14

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(Figure 3b) showed the appearance of a peak at 1669 cm-1 (C=N stretching) as well as a decreasing of the shoulder at 1705 cm-1 (C=O stretching) after cross-linking.

The synthesis of X-PKPip has been carried out by cross-linking of the respective polymer with 1,6-hexamethylendiamine (HDA). Firstly, the cross-linking reaction of PKPip is qualitatively confirmed by the appearance of a strong peak at 1670 cm-1 (C=N stretching) simultaneously with weaker ones at 1013cm-1 (C-N imine stretching) (Figure 3c). Along the same line, the decreasing of carbonyls groups intake (decrease of the C=O absorption at 1705 cm-1) and formation of imine and pyrrole rings as cross-linked bridges were registered through FT-IR at the same frequencies (vide supra), e.g. 1013 cm-1 for C=N imine stretching.

3.3. Metal uptake studies in batch experiments

All prepared resins in their cross-linked form (i.e. X-PKLys, X-PKPip and X-PKDap) were used for uptake studies in batch experiments. In contrast to the majority of the works present in the open literature often using high ratio polymer/metal ion, such experiments were carried out according to an equimolar ratio between the metal ion and the N-functional groups on the polymer. This choice can result in relatively lower uptake percentage (with respect to literature data), but it induces competition between the availability of functional groups, thus it enhance the peculiar adsorption pattern of the three resins.

For every combination resin/metal ion 40 minutes of contact time was enough to reach equilibrium in the system, this being confirmed by experiments (not shown for brevity) of metal uptake as function of the contact time. The obtained results (Figure 4) clearly show the different behavior of the different metal ions and resins.


15


1.2 Uptake metal ion ratio

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1 0.8

Page 16 of 30

X-PKDap X-PKLys X-PKPip

0.6 0.4 0.2 0 Cu(II) Ni(II) Co(II) Cr(III) Fe(III) Ag(I) Hg(II)

Figure 4. Metal ion (i.e. Cu(II), Ni(II), Co(II), Cr(III), Fe(III), Ag(I) and Hg(II)) uptake of XPKDap, X-PKLys and X-PKPip after 40 minutes of contact time at room temperature.

In order to rationalize the different uptakes for every polymeric material employed, an FT-IR study has been performed by recording the spectra of the used polymer before and after being in contact with the metal ion solution for the same amount of time (i.e. 40 minutes). FT-IR vibrational shifts and/or changes in adsorption have already been proposed by B.L. Rivas et al.28 as proof of chelation between poly(vinyl)amine and several different metal ions. Such changes in the wavelength as well as in the absorbance can be used as a kind of fingerprints for the interaction of the corresponding group with the metal ion. Therefore, a larger variation in the wavelength (i.e. red or blue shift) or decrease in transmittance of a vibration peak related to a given functional group in the presence of an ion can be interpreted as a higher affinity of the latter for the employed functional group. As an example, FT-IR spectra (Figure 5) for the XPKPip/Cu(II) binary mixture and X-PKPip are shown and discussed.


16

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X-PKPip X-PKPip/Cu(II) 4000 3300 2600 1900 1200 Wavelength (cm-1)

Figure 5. FT-IR scans of X-PKPip before and after 40 minutes mixing at room temperature of equimolar amounts between amine groups and copper(II) ions. Spectra have been normalized with respect to the –CH3 peak at 2873 cm-1.

The spectra of X-PKPip before and after mixing with copper showed increased intensity and a shift towards lower wavelength for the broad band at 3450 cm-1 (N-H stretching) as well as for the peaks at 1713 cm-1 (C=O stretching) and 1654 cm-1 (C=N stretching) 1464 cm-1 (N-H bending piperazine ring) and 1123 cm-1 (C-N aliphatic stretching piperazine ring). This suggests, on a qualitative level, the involvement of such functional groups in the chelation of Cu(II).

The same analysis has been carried out for every possible combination of resin and metal ion, which allows defining a qualitative rank of affinity for every functional group/metal ion couple (Table 2).


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X-PKDap

X-PKLys

Page 18 of 30

X-PKPip R2NH and R3NH, RC=NR’,

Cu(II)

R-NH2, R-CO-R’

R-NH2, R-COOH R-CO-R’

Co(II)

R-NH2, RC=NR

R-NH2, R-COOH, RC=NR’ R2NH and R3NH, RC=NR’

R-CO-R’,

RC=NR’, R-COOH,

C4H5N, R2NH and R3NH, RC=NR’,

R-NH2

R-NH2

R-CO-R’

C4H5N, R-NH2,

R-NH2, R-CO-R’,

R-CO-R’

R-COOH

Ni(II)

C4H5N, R2NH and R3NH, RC=NR’

Fe(III)

R-CO-R’, C4H5N, Cr(III)

C4H5N, R-NH2, R-CO-R’

C4H5N, RC=NR’, R2NH and R3NH

C4H5N, RC=NR’, R-NH2

C4H5N, R2NH and R3NH, RC=NR’

RC=NR’, C4H5N

RC=NR’

R-CO-R’ R-NH2, C4H5N, Ag(I) R-CO-R’ Hg(II)

RC=NR’, C4H5N

Table 2. Specific affinity of the functional groups (e.g. pyrrole, amine, carboxyl, carbonyl, imine) contained in X-PKDap, X-PKLys and X-PKPip towards seven metal ions (e.g. Cu(II), Co(II), Ni(II), Fe(III), Cr(III), Ag(I) and Hg(II)). In case a functional group is not present in the table, it means that it display no (or negligible) vibrational changes upon contact of the polymer with the metal ion solution. R-NH2= Amine, R-CO-R’=Ketone, R-COOH =Carboxyl, RC=NR’=Imine, C4H5N=Pyrrole

The obtained results clearly show relevant differences in the affinity of the metal ions towards specific functional groups. It must be noticed how a quantitative analysis of the data is factually


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hindered by the fact that the three resins show different carbonyl conversions and thus different amounts of functional groups along the backbone. Moreover, the different hydrophilic character of the three resins and their cross-linking method have an influence on the beads swelling, thus on the accessibility of the chelating center for the metal ion. Thus, the relative affinity between metal ion center and chelating group observed via FT-IR measurements does not lead to a complete understanding of the coordination mechanism for metal ion. However, a preliminary, nonetheless significant, targeting of the functional groups involved in the chelation of a specific metal ion when they are anchored to a polymeric substrate represent an interesting approach for the smart design of functional polymers as metal ion scavengers.

Based on Pearson’s theory15 and for the sake of a more rational discussion, three groups of metal ions can be outlined in order to simplify the observed metal ion affinity. The following sections will thus discuss the observed uptake (Figure 4) on the basis of FT-IR analysis (Table 2) according to a classification of the employed metal ions into borderline (Cu(II), Ni(II) and Co(II)), hard (Cr(III), Fe(III)) and soft metal ions (Ag(I), Hg(II)).

3.3.1 Cu(II), Ni(II) and Co(II)

For all three resins it is possible to rank the metal ions according to their uptake into the following:

X-PKDap:

Cu>Ni>Co

X-PKPip:

Ni>Co>Cu

X-PKLys:

Ni>Co>Cu


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The affinity rank for X-PKDap is in agreement with the FT-IR data and the expected behavior according to the Irving-Williams theory, which ranks the complex stability for primary amino groups and divalent cations of the first transition series in the following way: Mn

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