Helical poly(N-propargylamide)s with functional catechol groups: Synthesis and adsorption of metal ions in aqueous solution

Article abstract of DOI:10.1016/j.reactfunctpolym.2010.09.006

A novel type of helical N-propargylamide (PA) copolymer was synthesized by catalytic polymerization of monomer 1 (M1), which provided pendent functional catechol groups, and monomer 2 (M2), which endowed the expected copolymer backbones with helical struc

Full text of DOI:10.1016/j.reactfunctpolym.2010.09.006

Reactive & Functional Polymers 70 (2010) 938–943
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Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Helical poly(N-propargylamide)s with functional catechol groups:
Synthesis and adsorption of metal ions in aqueous solution
a,b
Lei Li a,b, Yan Li b, Xiaofeng Luo a,b, Jianping Deng a,b, , Wantai Yang
⇑
a State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
b College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel type of helical N-propargylamide (PA) copolymer was synthesized by catalytic polymerization of
monomer 1 (M1), which provided pendent functional catechol groups, and monomer 2 (M2), which
endowed the expected copolymer backbones with helical structures. With [(nbd)Rh+Bꢀ(C6H5)4
(nbd = 2,5-norbornadiene)] as catalyst, PA copolymers could be obtained with moderate molecular
weights (3800–14,000) in high yields (P96%). The functional catechol groups enhanced the hydrophilic-
ity of the hydrophobic mono-substituted polyacetylenes and aided the helical PA copolymers in showing
a considerable adsorbance toward metal ions [Fe(III), Cr(III), Ni(II), Zn(II), Cu(II) and Cu(I)] in aqueous
solution. The prepared copolymer in which M1/M2 was 0.1/0.9 [mol/mol, poly(10.1-co-20.9)] showed
the highest adsorption capacity among the examined helical copolymers. In particular, for Fe(III), the
Received 1 August 2010
Received in revised form 11 September
2010
Accepted 14 September 2010
Available online 18 September 2010
Keywords:
Helical poly(N-propargylamide)s
Catechol groups
Metal-complexing
Adsorption
maximum adsorption capacity was 186 mg gꢀ1
.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
[11]. Stimulated by this strategy of nature, synthetic helical poly-
mers have gained intense attention and a mass of crucial achieve-
ments have been made in this field in the last two decades. Typical
synthetic helical polymers include polymethacrylates [12], polyis-
ocyanates [13], polysilanes [14] and polyacetylenes [15]. Among
artificial helical polymers, substituted polyacetylenes with helical
main chains and functional pendent groups have gathered much
attention [16–18]. Several research groups have made large contri-
butions to this significant research field, including the Okamoto
and co-workers [19], Aoki and co-workers [20], Masuda and co-
workers [21], Yashima and co-workers [22] and Tang and co-work-
ers [23] groups. Our group has prepared a novel N-propargylurea-
based copolymer containing both carbazole and urea moieties in
side chains [24]. Furthermore, we succeeded in preparing a unique
category of optically active nanoparticles based on helical substi-
tuted polyacetylenes [25–28].
In this study, helical N-propargylamide copolymers containing
catechol pendent groups were synthesized to develop highly func-
tional materials. Substituted polyacetylene backbones afforded the
resulting copolymers in a helical structure, while catechol pendent
groups provided polymers with metal ion-adsorption abilities. It
was found that poly(10.1-co-20.9) had high adsorption capacity to-
ward Fe(III), Cu(II) and Ni(II), and an especially high capacity in
the case of Fe(III). Furthermore, both Langmuir and Freundlich
adsorption isotherms were found to be applicable to all of the
adsorptions. More importantly, we can expect a series of promising
functional polymers and advanced materials based on these newly
obtained functional helical copolymers.
The unique properties of marine life provide a wealth of inspi-
ration for the design and preparation of novel materials. For exam-
ple, 3,4-dihydroxyphenylalanine (DOPA), a material extracted from
marine mussels [1], has been extensively investigated for its excel-
lent moisture-resistant adhesive characteristics. This interest orig-
inally rose from the observation that marine mussels are able to
attach themselves to almost any material surface [2], mainly due
to the catechol groups in DOPA. These catechol groups can form
strong hydrogen bonds with other materials [3], and, therefore,
catechol is widely used to design functional polymers [4–9]. Nev-
ertheless, the introduction of catechol as pendent groups to substi-
tuted polyacetylenes has not yet been reported in the literature. A
series of functional polymeric materials can be expected by com-
bining functional catechol groups with substituted polyacetylene
backbones.
Naturally occurring biomacromolecules that form the structural
basis of life usually adopt helical conformations, such as proteins
and DNA [10]. The sequence, hydrophilicity and chain helicity of
nucleic acid bases and amino acids play significant roles in
determining the high-order structures of biomacromolecules
⇑
Corresponding author at: State Key Laboratory of Chemical Resource Enginee-
ring, Beijing University of Chemical Technology, Beijing 100029, China. Tel./fax: +86
10 6443 5128.
E-mail addresses: dengjp@mail.buct.edu.cn (J. Deng), yangwt@mail.buct.edu.cn
(W. Yang).
1381-5148/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2010.09.006

L. Li et al. / Reactive & Functional Polymers 70 (2010) 938–943
939
ACH2ACH2AC@OA), 2.95 (s, 1H, CH„CA), 3.87 (s, 2H, ACH2AC„),
6.48–6.65 (m, 3H, PhAH), 8.25 (s, 1H, ANHACH2A), 8.60–8.68 (d,
2H, PhAOH). 13C NMR (DMSO, 600 MHz, 20 °C): d 30.84, 37.69
(2ꢂ), 73.05, 81.61, 115.94, 116.12, 119.35, 132.61, 143.71,
145.36, 172.11. Anal. calcd for C12H13NO3: C, 65.74; H, 5.98; N,
6.39. Found: C, 65.87; H, 5.77; N, 6.32.
2. Experimental section
2.1. Measurements
1H and 13C NMR spectra were recorded on a Bruker AV600
spectrometer (600 MHz, Fallanden, Switzerland). FT-IR spectra
were recorded with a Nicolet NEXUS 670 spectrophotometer
(Madison, WI). The molecular weights and molecular weight
distributions of (co)polymers were determined by GPC (Waters
515-2410, Milford, MA) that was calibrated using polystyrene as
the standard and THF as the eluent. Elemental analyses were car-
ried out with a Thermo EA 1112 elemental analysis apparatus.
UV–Vis spectra were recorded on a JASCO J-810 spectropolarimeter
(Tokyo, Japan). The initial and equilibrium concentrations of metal
ions were measured using a Spectr AA 55-B atomic absorption
spectrophotometer.
Synthesis of monomer 2 and the related results were reported
in the preceding paper [30].
2.4. Polymerization and copolymerization
All polymerizations (Scheme 1) were carried out with
(nbd)Rh+Bꢀ(C6H5)4 as a catalyst in dry THF at 30 °C for 1 h under
the following conditions: [monomer] = 1.0 M, [catalyst] = 10 mM.
To exclude the effects of oxygen, the monomer solution and cata-
lyst solution were prepared and then bubbled three times with
nitrogen for 10 min. The monomer solution was transferred to
the catalyst solution under an atmosphere of nitrogen and then
the monomer underwent polymerization. After polymerization
was complete, the reaction mixture was poured into a large
amount of hexane to precipitate the formed polymer. The polymer
was then separated via filtration, washed with hexane, and dried
under reduced pressure.
2.2. Materials
All solvents were of analytical grade and were used as received
without further purification, with the exception of tetrahydrofuran
(THF), which was distilled under reduced pressure in advance.
3-(3,4-dihydroxyphenyl)propionic acid (Aldrich), propargylamine
(Aldrich), N-methylmorpholine (Acros), isobutyl chloroformate
(Aldrich) and propargylamine (Aldrich) were used without further
purification. The rhodium catalyst (nbd)Rh+Bꢀ(C6H5)4 (nbd = 2,5-
norbornadiene) was synthesized as previously reported [29]. The
solutions of metal ions were prepared from FeCl3ꢁ6H2O,
Cr(NO3)3ꢁ9H2O, NiSO4ꢁ6H2O, CuCl2ꢁ2H2O, CuBr and ZnCl2 and were
dissolved in doubly distilled water.
2.5. Adsorption experiments
Aqueous stock solutions (1000 mg Lꢀ1) of Cr(III), Fe(III), Ni(II),
Cu(II), Cu(I) and Zn(II) ions were prepared using FeCl3ꢁ6H2O,
Cr(NO3)3ꢁ9H2O, NiSO4ꢁ6H2O, CuCl2ꢁ2H2O, CuBr and ZnCl2, respec-
tively. The stock solutions were then diluted to give standard solu-
tions of appropriate concentrations (3, 6, 9, 12, 15, 30, 60, 90, 120,
150 and 200 mg Lꢀ1 for Fe(III), Ni(II) and Cr(III); 4, 8, 12, 16, 20, 40,
80, 120, 160 and 200 mg Lꢀ1 for Cu(II), Cu(I) and Zn(II)). In each
case, 10 mL of the metal ion solution was pipetted into a flask con-
taining 0.0050 g of polymer. The solutions were adjusted to pH 4–5
with aqueous HCl and NH3 solutions. Ultrasonic vibration was used
for 60 min at a constant temperature of 25 °C to disperse the poly-
mer. Then, the mixtures were kept for a different static adsorption
time. The time intervals used were: 1.5 h, 3.0 h, 6.0 h, 9.0 h and
12.0 h. After the adsorption, the polymer was filtered and the
residual solution was kept.
2.3. Synthesis of monomers
The main procedure used to synthesize monomer 1 is described
below. 3-(3,4-dihydroxyphenyl)propionic acid (2.73 g, 15.0 mmol),
isobutyl chloroformate (1.97 mL, 15.0 mmol) and 4-methylmor-
pholine (1.65 mL, 15.0 mmol) were added to THF (100 mL),
sequentially. The solution was stirred at 30 °C for 10 min and then
propargylamine (1.03 mL, 15.0 mmol) was added to the solution.
After 4 h, a white precipitate formed. This precipitate was isolated
via filtration and the filtrate was collected. Ethyl acetate (ca.
50 mL) was added to the filtrate to extract the desired product
and the combined solution was washed three times with 2 M
HCl, followed by a wash with saturated aqueous NaHCO3 to neu-
tralize the solution. Next, the solution was dried over anhydrous
MgSO4, filtered, and concentrated to give the target monomer. Fi-
nally, the crude monomer was purified via flash column chroma-
tography on silica gel (hexane/EtOAc = 1/2, v/v).
The amount of adsorption (adsorption capacity) was calculated
based on the differences in each metal ion concentration before
and after the adsorption period, the volume of aqueous solution
(10 mL), and the weight of the polymers (0.0050 g) according to
formula (1) [31]:
Qe ¼ ðC0 ꢀ CeÞ ꢂ V=W ꢂ 1000
ð1Þ
The data collected on monomer 1 are as follows: 58% yield, pale
yellow oily liquid. FT-IR (KBr): 3370 (AOH), 3290 (ANAH), 2120
(C„CA), 1642 (AC@O), 1444, 784 (phenyl), 2962, 1519, 1355,
where C0 and Ce are the initial metal ion concentration and the final
or equilibrium metal ion concentration (mg Lꢀ1), respectively; V is
the volume of the metal ion solution (mL) and W is the weight of
polymer used (g). Qe (mg gꢀ1) thus represents the amount of each
metal ion adsorbed per unit weight of polymer at the final or equi-
librium concentration.
1282, 1196, 1113, 813, 649, 588 cmꢀ1 1H NMR (DMSO, 600 MHz,
.
20 °C):
d
2.34 (q, 2H, ACH2ACH2AC@OA), 2.66 (q, 2H,
3. Results and discussion
3.1. Synthesis and characterization of N-propargylamide polymers
Copolymerizations of M1 and M2 were carried out in the pres-
ence of catalytic (nbd)Rh+Bꢀ(C6H5)4, and the results are given in
Table 1. The average molecular weight (Mn) of the (co)polymers de-
creased with increasing amounts of M1, and the (co)polymers had
a moderate molecular weight distribution (Mw/Mn) (1.49–1.96).
The corresponding (co)polymers could be obtained in high yields
Scheme 1. Synthetic route to helical N-propargylamide (PA) polymers.

940
L. Li et al. / Reactive & Functional Polymers 70 (2010) 938–943
Table 1
Copolymerization results of M1 and M2 with varied monomer ratiosa.
c
c
Entry
Monomer feed (1/2)
(mol/mol)
Yieldb (%)
Mn
Mw/Mn
Monomer unit of 1
in copolymerd (mol.%)
Poly(1)
100/0
40/60
30/70
20/80
10/90
0/100
99
97
96
98
98
3800
6900
8700
11,100
12,700
14,000
1.49
1.69
1.81
1.75
1.79
1.96
100
42
29
21
9
Poly(10.4-co-20.6
Poly(10.3-co-20.7
Poly(10.2-co-20.8
Poly(10.1-co-20.9
Poly(2)
)
)
)
)
100
0
a
b
c
With (nbd)Rh+Bꢀ(C6H5)4 as catalyst; [M]0 = 0.1 M; [M]0/[Rh] = 100; in THF, polymerization temperature = 30 °C.
Precipitated in hexane.
Measured by GPC (polystyrene as the standards; THF as an eluent).
d
Determined by 1H NMR spectra in CDCl3.
(P96%). Considering the loss of the product during purification, it
is believed that the yields of the copolymers were quantitative and
the composition of the products were consistent with the corre-
sponding monomer compositions, which were further confirmed
by the calculated results of the composition test and the 1H NMR
spectra measurements [30,32], shown in Fig. 1. Additionally, refer-
ring to earlier investigations [30], the copolymerizations between
M1 and M2 can be considered random copolymerization.
Taking poly(10.1-co-20.9) as representative, the 1H NMR spec-
trum of poly(10.1-co-20.9) was depicted in Fig. 1. From this spec-
trum, the catechol protons (PhAH, signal (a) of M1 and the
protons of methyl in M2 (CH3A, signal (b) were at 6.55 and
0.91 ppm, respectively. The chemical shifts at 6.09 and 3.75 were
mainly associated with the protons of ACHA and ACH2A in main
chains of polyacetylene [33]. On the basis of the above 1H NMR
spectrum analysis, the actual molar ratio of the two monomers
in the resulted copolymer could be calculated from the peak inten-
sities of signals a (M1) and b (M2), which were 1.00 and 30.08,
respectively. That is, the molar ratio of M1 to M2 was 9/91 in the
resultant copolymer, which nearly coincided with the feed compo-
sition (10/90).
Based on previous studies [30,32–35], a strong absorption peak
observed at approximately 390 nm in the UV–Vis spectra of substi-
tuted polyacetylenes polymers indicates the presence of a helical
conformation in the polymer main chains. On the other hand,
when the polymers adopt random structures, absorption bands
at approximately 320 nm, rather than 390 nm, can be seen. The
secondary structures of the present polyacetylenes polymers were
examined by UV–Vis spectroscopy using THF as the solvent. The
UV–Vis spectra of poly(1-co-2)s with varied ratios of M1 and M2
are depicted in Fig. 2. For the homopolymer of M1, two obvious
Fig. 2. UV–Vis spectra of poly(1-co-2)s measured in THF (c = 0.10 mM) at room
temperature.
absorption bands can be observed at 280 and 320 nm, while no
absorption peaks appeared at 390 nm. The formation of the peak
at 320 nm can be ascribed to the conjugated effect of the polymer
main chains. Thus, it is revealed that this homopolymer did not
form helical structures, but rather adopted random structures,
due to the simultaneous occurrence of both intra- and inter-molec-
ular hydrogen bonding between the many hydroxyl groups in side
chains. In addition, the appearance of the intense absorption at
280 nm may be due to the absorption of phenyl in catechol group.
For the homopolymer of M2 [30], a strong absorption peak was ob-
served at 390 nm, which demonstrates the existence of helical
structure in the polymer main chains. For the copolymers, the
absorption at 390 nm dramatically decreased with increasing
amounts of M1. The investigations above demonstrated that the
expected copolymers could be smoothly prepared; however, the
addition of M1 has an adverse effect on the formation of a helical
structure in the copolymer backbones. Among the copolymers,
poly(10.1-co-20.9) was chosen as a model adsorbent for adsorbing
metal ions in aqueous solution, as it possessed the highest helical
content and simultaneously contained the anticipated catechol
moieties in pendent groups.
3.2. Adsorption capacity
Fig. 3 shows the adsorption capacities of poly(10.1-co-20.9) to-
ward Cr(III), Fe(III), Ni(II), Cu(II), Cu(I) and Zn(II) ions under the
same conditions. As seen in this figure, poly(10.1-co-20.9) exhibited
different adsorption capacities toward different metal ions, and the
highest adsorption capacity appeared in the case of Fe(III), while
the lowest adsorption capacity was seen in Zn(II). In addition, the
Fig. 1. 1H NMR spectra of the poly(10.1-co-20.9) in CDCl3 (Ha: 2H, catechol protons
in M1; Hb: 6H, methyl protons in M2.).

L. Li et al. / Reactive & Functional Polymers 70 (2010) 938–943
941
therms [40]. The most widely used Langmuir equation, which is
valid for monolayer adsorption onto a surface with a finite number
of identical sites, can be expressed by:
Ce=Qe ¼ 1=bqm þ Ce=qm
ð2Þ
where the meanings of Ce and Qe in formulae (2) are the same as
those used in formula (1); qm is the maximum adsorption per
monolayer (mg gꢀ1), b is the Langmuir constant, related to the affin-
ity of binding sites (mL mgꢀ1) and representing a measure of the en-
ergy of adsorption. A linearized plot of Ce/Qe against Ce allows qm
and b to be determined quantitatively.
The widely used empirical Freundlich equation, based on the
adsorption on a heterogeneous surface, is expressed by:
Fig. 3. Adsorption capacity of poly(10.1-co-20.9) towards Cr(III), Fe(III), Ni(II), Cu(II),
Cu(I) and Zn(II): initial concentrations, 150 mg Lꢀ1 for Fe(III), Ni(II) and Cr(III),
160 mg Lꢀ1 for Cu(II), Cu(I) and Zn(II), 10 mL; room temperature; pH 4–5; 12 h.
log Qe ¼ log Kf þ 1=n log Ce
ð3Þ
where Kf and n are Freundlich constants indicating sorption capac-
ity and intensity, respectively. Kf and n can be determined from a
linear plot of log Qe against log Ce.
copolymer exhibited the same adsorption capacity toward Cu(II)
and Cu(I) as well as for Ni(II) and Cr(III). These investigations sug-
gest that metal ions were adsorbed according to the metal-chelat-
ing mechanism between metal ions and the hydroxyl (AOH)
functional groups in catechol [36–38]. Among the metal ions,
Ni(II), Cu(II) and Fe(III) were used as adsorbates to further investi-
gate the adsorption capability of poly(10.1-co-20.9). There were dra-
matic changes in the color of the metal ion aqueous solutions
before and after adsorption (taking Cu(II), Fe(III) and Ni(II) as
example, the photographs of the three metal ion aqueous solutions
before and after the adsorption by poly(10.1-co-20.9) are shown in
Fig. S1 in Supporting information). After adsorption and filtration,
which excluded the polymer absorbent, the three metal ion solu-
tions became almost transparent. The following experiments pro-
vided quantitative data for the metal ion adsorption.
For Langmuir and Freundlich models, R2 indicates the correla-
tion coefficient for adsorption isotherm [41]. The relevant values
of R2 for the adsorptions of Fe(III), Cu(II) and Ni(II) by poly(10.1
-
co-20.9) were calculated, as illustrated in Fig. S2. For Fe(III) and
Ni(II), R2 was approximately 0.99 in both the Langmuir and Fre-
undlich examples, while for Cu(II) R2 was 0.96 and 0.99, respec-
tively. It is thus demonstrated that both of the two types of
adsorption isotherms were applicable to all the adsorptions above.
3.4. Kinetics of adsorption
Taking Fe(III) as an example, the adsorption behaviors of
poly(1-co-2)s with different monomer compositions were further
explored to obtain deeper insights into the adsorption. In Table
2, the homopolymers of M1 and M2 hardly adsorbed Fe(III), despite
the data showing that all of the copolymers presented considerable
adsorption capacities. Moreover, the adsorption capacities of the
copolymers gradually increased with decreasing amounts of M1
in the copolymer, and poly(10.1-co-20.9) showed the best adsorp-
tion effect. This result can be explained by the fact that an excess
of hydrophilic hydroxyl groups (from the catechol groups) not only
made the copolymerization more difficult in hydrophobic solvents
but also negatively affected the formation of helical structures in
the copolymer backbones. Thus, with an appropriate amount of
catechol groups and high helical conformations, it is able to
achieve higher adsorption capacities.
3.3. Adsorption isotherm plots
Fig. 4 depicts the adsorption isotherms of Ni(II), Cu(II) and
Fe(III) on poly(10.1-co-20.9). The adsorptions were conducted at var-
ied initial concentrations, i.e., 30, 60, 90, 120 and 150 mg Lꢀ1, for
Fe(III) and Ni(II), and 20, 40, 80, 120 and 160 mg Lꢀ1 for Cu(II).
Within the concentration range, it was found that with an increase
in the concentration of Fe(III), the adsorption capacity of poly(10.1
co-20.9) toward Fe(III) ascended gradually, while only little change
was seen for Cu(II) and Ni(II). The adsorption of poly(10.1-co-20.9
-
)
toward Fe(III), Cu(II) and Ni(II) showed obvious Langmuir adsorp-
tion characteristics, among which monodisperse layer adsorption
occupied absolute predominance [39]. Thus, it is demonstrated
that the present adsorptions followed Langmuir model.
Fig. 5 shows the adsorption kinetic curves of helical polymers
with and without functional pendent groups, namely, poly(10.1
-
co-20.9) and poly(2), toward Fe(III). For poly(10.1-co-20.9), it appears
that, although the adsorption rates were fast at the beginning,
reaching the adsorption equilibrium required a relatively long
time. As seen in this figure, within the initial 1.5 h time period,
the adsorption of Fe(III) dramatically increased as a function of
adsorption time. However, after 3 h, the adsorption nearly leveled
off. After approximately 12 h, poly(10.1-co-20.9) exhibited a maxi-
mum adsorption capacity (Qe = 186 mg gꢀ1), an observation that
can be explained by the catechol groups present in the helical
copolymer. The catechol groups not only enhanced the hydrophi-
The Langmuir and Freundlich models (Fig. S2 in Supporting
information) are often used to describe equilibrium sorption iso-
Table 2
Adsorption capacity (mg gꢀ1) of Fe(III) by poly(1x-co-2y)s.
x/y in poly(1x-co-2y)s
(mol/mol)
100/0
8
40/60
90
30/70
158
20/80
163
10/90
186
0/100
6
Adsorption capacitya
(mg gꢀ1
)
Fig. 4. Adsorption isotherm curves of Ni(II), Cu(II) and Fe(III): initial concentrations,
30, 60, 90, 120 and 150 mg Lꢀ1 for Fe(III) and Ni(II); 20, 40, 80, 120 and 160 mg Lꢀ1
for Cu(II); 10 mL; room temperature; pH 4–5; 12 h.
a
Calculated by Eq. (1); initial concentrations 150 mg Lꢀ1 for Fe(III) 10 mL, room
temperature, pH of 4–5, 12 h; poly(1x-co-2y)s 0.0050 g.

942
L. Li et al. / Reactive & Functional Polymers 70 (2010) 938–943
Fe(III), Cu(II) and Ni(II). The characteristic peaks of a chelating ef-
fect emerged at approximately 1062 cmꢀ1
.
4. Conclusions
In this study, we synthesized a novel series of helical N-propa-
rgylamide copolymers containing functional catechol moieties in
side chains. The hydrophilic catechol groups from monomer 1
(M1) improved the hydrophilicity of the normally hydrophobic
mono-substituted polyacetylenes. The addition of monomer 2
(M2) provided these copolymers with a helical structure. Such
functional helical copolymers exhibited different adsorption
capacities toward various metal ions. Poly(10.1-co-20.9) exhibited
high adsorption capacity towards Fe(III), Cu(II) and Ni(II), and the
maximum adsorption of Fe(III) was 186 mg gꢀ1. In addition, all
the adsorptions followed Langmuir and Freundlich adsorption iso-
therms. We are convinced that the described functional helical
copolymers represent novel, highly functional materials with high
adsorption capacity toward metals and other adsorbates. More-
over, the helical copolymers may find other practical applications,
for instance in chemical sensing and in metal carrier catalytic
fields.
Fig. 5. Adsorption kinetic curves of poly(10.1-co-20.9) and poly(2) toward Fe(III):
initial concentrations, 150 mg Lꢀ1, 10 mL; room temperature; pH 4–5.
licity of the polymer but also lead to hydrogen bonding and/or
coordination bonds, which are two factors needed to form organo-
metallic complexes. For poly(2), however, the polymer scarcely
worked on the Fe(III) in aqueous solution. The probable reason
for this result was the lack of catechol groups in poly(2), which
led to its poor solubility in aqueous solution and, therefore, its
weak adsorbability. In addition, little change occurred to the
adsorption capacity of poly(10.1-co-20.9) with a further prolonga-
tion of adsorption time, demonstrating that no desorption oc-
curred. This can be ascribed to the strong metal complexing
ability of these catechol groups, and furthermore implies that the
formation of organometallic complexes between ꢀOH groups and
Fe(III) was irreversible [3,42], thus maintaining a constant amount
of adsorbed Fe(III).
Acknowledgements
This work is supported by the ‘‘Program for New Century Excel-
lent Talents in University’’ (NCET-06-0096), the ‘‘National Science
Foundation of China’’ (20974007), the ‘‘Program for Changjiang
Scholars and Innovative Research Teams in University’’ (PCSIRT,
IRT 0706), and the ‘‘Major Project for Polymer Chemistry and Phys-
ics Subject Construction from the Beijing Municipal Education
Commission (BMEC)’’.
3.5. Mechanism of adsorption of metal ions
The different chelating abilities of poly(10.1-co-20.9) towards
Fe(III), Cu(II) and Ni(II) metal ions could be explained based on
the structure of the metals’ outermost d-orbital electrons. The elec-
tron configuration of Fe(III) is [Ar]3d5, while for Cu(II) and Ni(II) it
is [Ar]3d9 and [Ar]3d8, respectively. The possible reason for the
adsorption of metal ions may include two aspects: firstly, due to
the existence of the 3d unoccupied orbital, a lone pair of electrons
from the oxygen atom of catechol directly coordinates to the metal
center, therefore forming a coordination bond within the com-
pound [42]. Secondly, AOH groups in the catechol structures can
form stronger hydrogen bonds with metal ions than those hydro-
gen bonds formed between water and metal ions [3]. Therefore,
firm organometallic complexes between poly(10.1-co-20.9) and me-
tal ions can be formed (Scheme 2).
The chelating effects of the polymer to metal ions can further-
more be verified by FT-IR-spectra (Fig. S3 in Supporting informa-
tion). From curve a, the sharp peak at 3300 cmꢀ1 corresponds to
unchelated hydroxyl groups, while broad absorption peaks in
curves b–d, shifted from 3300 to 3450 cmꢀ1, refer to the hydroxyl
groups chelated with metal ions. Moreover, the peaks of phenolic
hydroxyl groups at 1220 cmꢀ1 changed after the adsorption of
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.reactfunctpolym.2010.09.006.
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Scheme 2. Proposed mechanism for the chelation of helical copolymer toward
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