Pd nanoparticles encaged in nanoporous interpenetrating polymer networks: A robust recyclable catalyst for Heck reactions

Article abstract of DOI:10.1016/j.reactfunctpolym.2011.04.007

A novel type of Heck reaction catalyst composed of hydrophilic interpenetrating polymer networks (IPNs) and palladium (Pd) nanoparticles was prepared by simultaneous crosslinking of polyvinyl alcohol and polyacrylamide. The mesh sizes of the IPNs are one order of magnitude smaller than the average sizes of the uniformly dispersed Pd nanoparticles, which functions well to stabilize Pd nanoparticles and prevent aggregation. The Pd particles in the IPNs can catalyze Heck coupling reactions with high activities and be recycled over 20 times, providing more sustainability than any other polymer-stabilized Pd catalyst reported in the literature. The confined reactions inside the IPN nanopores in neat water may provide a green route for C-C coupling reactions.

Full text of DOI:10.1016/j.reactfunctpolym.2011.04.007

Reactive & Functional Polymers 71 (2011) 756–765
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Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Pd nanoparticles encaged in nanoporous interpenetrating polymer networks:
A robust recyclable catalyst for Heck reactions
Kan Zhan a, Huanhuan You a, Wenyu Liu a, Jie Lu a, Ping Lu a, Jian Dong a,b,
⇑
a School of Chemistry and Chemical Engineering, Shaoxing University, Shaoxing, Zhejiang 312000, China
b State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, Jiangsu 210093, 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 Heck reaction catalyst composed of hydrophilic interpenetrating polymer networks (IPNs)
and palladium (Pd) nanoparticles was prepared by simultaneous crosslinking of polyvinyl alcohol and
polyacrylamide. The mesh sizes of the IPNs are one order of magnitude smaller than the average sizes
of the uniformly dispersed Pd nanoparticles, which functions well to stabilize Pd nanoparticles and pre-
vent aggregation. The Pd particles in the IPNs can catalyze Heck coupling reactions with high activities
and be recycled over 20 times, providing more sustainability than any other polymer-stabilized Pd cat-
alyst reported in the literature. The confined reactions inside the IPN nanopores in neat water may pro-
vide a green route for C–C coupling reactions.
Received 20 January 2011
Received in revised form 14 April 2011
Accepted 22 April 2011
Available online 29 April 2011
Keywords:
Green chemistry
Heck reaction
Ó 2011 Elsevier Ltd. All rights reserved.
Palladium
Interpenetrating polymer network
Nanoparticle
1. Introduction
reaction conditions with negligible metal leaching during its use.
Therefore, we have focused on designing ‘‘heterogeneous systems’’
Metal nanoparticles dispersed in polymers are attractive cata-
lysts for a variety of reactions. They are usually in the form of dis-
persions in colloidal systems, membranes or porous resins [1–10].
In particular, colloidal palladium (Pd) nanoparticles protected by
hydrophilic polymers, such as polyvinyl pyrrolidone (PVP) and
polyvinyl alcohol (PVA), can demonstrate excellent catalytic activ-
ity and selectivity in comparison with general Pd catalysts
[1,2,4,7]. Likewise, Pd nanoparticles encapsulated in dendrimers
[5a,11–13], star-shaped block polymers [14], hyper-branched
polymers [15], and crosslinked polymers [16,17] have been pro-
posed as catalysts for C–C bond coupling reactions. Such Pd cata-
lysts for C–C bond coupling reactions are extremely important in
modern organic synthesis; however, additional studies have
shown that Pd nanoparticles used in Heck coupling reactions could
only be reused a limited number of times [5–9,14–29a]. In partic-
ular, when open polymer structures (e.g., colloidal dispersions,
dendrimers or porous resins) are used as support materials
[9,11–17], Pd leaching into the reaction products could become a
severe problem in the pharmaceutical industry. An important goal
in the applications of Heck reactions is the development of a cata-
lyst for ‘‘permanently’’ sustainable and environmentally benign
by confining the Pd particles to interpenetrating polymer networks
(IPNs) with nanometer-scale meshes and encaging the C–C bond
coupling reactions inside the IPNs. Such a catalytic system differs
from the conventional polymer supported metal systems found
in the literature [1–29].
An interpenetrating polymer network (IPN) is composed of two
or more polymer networks, which are interlaced on a molecular
scale [30]. The first advantage of the IPNs is the attainment of a
combination of properties of two component polymers, resulting
in the amphiphilicity required for different solvent conditions. Be-
cause there is no covalent bonding between two component net-
works, each network may retain its own property while the
proportion of each network can be adjusted independently. Inter-
penetration of two networks may also result in better resistance
to harsh reaction conditions (such as high temperature and contin-
uous processing) due to higher mechanical strengths and stability
than the individual homopolymer network or conventional cross-
linked resins. The second advantage of the presence of two poly-
mer networks is that the metal particles may be effectively
prevented from coalescencing during the reactions; however, to
the best of our knowledge, encaging cross-coupling reactions in-
side IPN structures has not been reported. We show here for the
first time that metal-IPN catalysts (referred to as metal@IPN) are
easily separated from the reaction medium, resulting in little con-
tamination to the products, unlike conventional colloidal metal
particles used in homogeneous catalysis. Therefore, the metal@IPN
⇑
Corresponding authors. Address: School of Chemistry and Chemical Engineer-
ing, Shaoxing University, Shaoxing, Zhejiang 312000, China. Tel.: +86 575
88342511; fax: +86 575 88341521 (Jian Dong).
E-mail address: jiandong@usx.edu.cn (J. Dong).
1381-5148/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2011.04.007

K. Zhan et al. / Reactive & Functional Polymers 71 (2011) 756–765
757
2.6 nm
d = 32.9
10.8 nm
n
OH OH OH
O
O
OH OH
O
O
OH
n
OH OH OH OH OH OH
OH OH OH OH
H2N
O
O
H2N
NH2
O
O
H2N
H2N
O
O
O
NH2
O
O
NH2
O
NH2
H2N
O
NH2
O
NH2
O
H2N
O
HN
H2N
CH2
H2N
H2N
O
HN
NH2
H2N
O
O
O
O
NH2
H2N
O
NH2
H2N
O
O
O
Scheme 1. Interpenetrating polymer networks comprised of crosslinked PVA (blue lines) and PAM (red lines) with Pd nanoparticles (solid spheres). The mesh sizes for the
linear distance between consecutive crosslinks and average size of Pd particles shown here correspond to those of Pd@IPN with VA:AM = 0.826:1 in water listed in Table 1.
Structures of the crosslinked PVA and PAM are shown in a magnified view in the bottom. (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
systems can be tailored as reusable, environmentally friendly and
mechanically strong catalysts for rigorous reaction conditions with
desired affinities in different reaction media. The confinement of
the cross-coupling reactions inside the network pores can be
achieved by controlling the mesh sizes and contents of the compo-
nent polymers (see Scheme 1 for a diagram of an IPN with dis-
persed metal particles).
Pd nanoparticles for Heck reactions in water [31b]. We show here
that the Pd nanoparticles in nanoporous IPN systems can serve as
green catalysts with higher activity and better recyclability than
the Pd nanoparticles in charged polymer complexes [31b].
2. Experimental
The importance of our study can be seen from several perspec-
tives. Heterogenization of a homogeneous catalyst (or vice versa,
homogenization of a heterogeneous catalyst) is an important ap-
proach for taking full advantage of both heterogeneous and
homogenous catalysis. It is known that when employing homoge-
neous catalysis by Pd2+ salt, Heck coupling reactions suffer from
poor reusability and require the removal of residual Pd, which
can be a complex process. In pharmaceutical applications, contam-
ination with metallic species in the final products is always closely
monitored. In a recent study, crosslinked polyacrylamide was used
as a Pd metal support to catalyze Heck reactions; however, the cat-
alyst could only be reused six times [31a]. Another recent study
proposed that the use of a charged polymer complex stabilized
2.1. Materials
Analytical grade palladium chloride, sulfuric acid, acetic acid,
glutaraldehyde (25% aqueous solution), PVA, ethanol, acrylamide,
potassium persulfate, N,N0-methylene bisacrylamide, N,N-dimethyl
formamide (DMF), potassium hydroxide, hydrochloric acid, trieth-
ylamine, and hexadecyltrimethylammonium bromide were pur-
chased from Sinopharm (Shanghai). Iodobenzene, 4-iodoanisole,
styrene, methyl acrylate, acrylic acid, p-fluoroiodobenzene, 2-iodo-
thiophene, and 4,40-diiodobiphenyl were obtained from Aladdin
Reagents Company (Shanghai). All of these chemical compounds
were used as received without further purification.

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K. Zhan et al. / Reactive & Functional Polymers 71 (2011) 756–765
2.2. Preparation and characterization of the Pd@IPN catalysts
removed. The equilibrium swelling ratio and the polymer volume
fraction in the swollen state were calculated using the dried and
water-swollen (or DMF-swollen) weights measured.
IPNs can be prepared using two possible methods. Preparation
of IPNs using the simultaneous method requires two different
types of monomers (or polymers) that are mixed homogeneously
and then polymerized (and/or crosslinked) independently by dif-
ferent initiators (and/or crosslinkers). The sequential method of
IPN preparation requires first crosslinking a linear polymer to form
a network structure, then a new monomer, initiator and crosslinker
are swollen into the network and polymerized and crosslinked
among the first polymer to form an interpenetrated network. To
acquire uniform dispersion, the Pd particles were generated by
chemical reduction in solution and protected from aggregation
by the presence of PVA. The PVA-protected metal solutions were
subsequently transformed into IPN structures by the simultaneous
approach that was originally used for synthesizing metal-free IPN
structures in drug control release studies [32–34] but modified
here to optimize the amount of crosslinkers (i.e., to generate mesh
sizes appropriate for encaging the Pd nanoparticles) and the corre-
sponding amount of acid required for catalyzing the crosslinking
reactions to obtain optimal performance in Heck coupling reac-
tions. The PVA–Pd colloidal solutions were mixed thoroughly with
an acrylamide (AM) monomer and persulfate initiator. Two differ-
ent types of crosslinkers were also mixed into the PVA–Pd solution
to crosslink the PVA and polyacrylamide (PAM) independently.
Solid PVA (4.0 g), with a number averaged Mn = 22,100, weight
averaged Mw = 70,000, degree of polydispersity D = 2.79 (as deter-
mined by gel permeation chromatography) and a degree of sapon-
ification >99.8%, were mixed with 30 mL of ethanol and 25 mL of
distilled water in a round-bottom flask. The mixture was heated
to 97 °C under the reflux condition for 2 h to produce a transparent
solution into which 20 mL of PdCl2–HCl with a Pd concentration of
5.64 mmol/L (prepared by mixing 0.0200 g PdCl2 and 20.0 mL of
0.5 M HCl) was added. The mixture was heated under the reflux
condition for 5 h in a silicone oil bath and then cooled to room
temperature. This black homogeneous solution was mixed with
0.2 mL of 10% H2SO4 (catalyst), 0.6 mL of 10% acetic acid (pH con-
troller), and 0.8 mL of 25% glutaraldehyde aqueous solution (‘‘GA’’,
crosslinker) to form solution A. The PVA–Pd solution A when cross-
linked has a nominal crosslinking ratio (X) of 2.2%, which is the ra-
tio of moles of crosslinking agent to moles of polymer repeating
units. Solution B was prepared by combining 7.82 g of acrylamide
and 0.93 g of N,N0-methylene bisacrylamide (‘‘MBAM,’’ crosslinker)
that were then mixed with 0.15 g of potassium persulfate (initia-
tor) and 20 mL of distilled water. The PAM homopolymer obtained
from solution B has a nominal crosslinking ratio (X) of 5.5%. Solu-
tions A and B were mixed thoroughly in different ratios, poured
into petri dishes, and heated at 70 °C for 3 h to form solid IPN gels,
which were washed with distilled water repeatedly overnight and
dried in a 60 °C oven overnight to generate the Pd@IPN catalysts.
Nanoparticles embedded in crosslinked PVA (denoted as Pd@cPVA)
were also prepared from solution A directly without mixing with
solution B for comparative purposes. After heating the PVA–Pd gels
at 70 °C for 3 h, the Pd@cPVA catalysts were washed with distilled
water repeatedly overnight and dried in a 60 °C oven overnight.
The catalysts were analyzed under a JEM-1230 transmission
electron microscope at an accelerating voltage of 80 kV using the
cryo-ultramicrotomy technique. Nanoparticle sizes were deter-
mined by analyzing approximately 180–300 nanoparticles from
several TEM images and calculating the Feret’s diameters of the
particles using ImageJ software (National Institutes of Health,
USA). The standard deviations of the Feret’s diameters were calcu-
lated to measure the widths of the size distributions. To determine
the equilibrium swelling ratio Q, a sample of the Pd-embedded
polymers was soaked in water (or DMF) at 100 °C for 1 day to swell
to equilibrium and weighed in air after the surface solvent was
2.3. Catalysis
In a typical Heck coupling reaction between iodobenzene and
the olefinic compound in DMF solvent, 10 mmol of iodobenzene
and 30 mmol of acrylic acid were mixed with 40 mmol of triethyl-
amine and 10 mL of DMF. The insoluble IPN catalyst (0.50 g) con-
taining 0.00299 mmol of Pd were added to the solution, and the
reactants were heated in an oil bath at 100 °C for 12 h. After the
reaction, the catalysts were filtered, washed with ethanol three
times and dried in the air for the next round of use. The filtrate
was decolored by activated carbon, and the white product was pre-
cipitated out by adding 1 M HCl solution, separated by filtration,
washed repeatedly with cold water, and recrystallized in hot
water.
For a coupling reaction in aqueous solution, 10 mmol of iodo-
benzene, 15 mmol of styrene, 5.5 ml of triethylamine (4 mmol),
and 1.82 g of hexadecyltrimethylammonium bromide (5 mmol)
were mixed with 10 ml of water in a 100 ml round-bottom flask.
After 0.50 g of the IPN catalyst with 0.00299 mmol of Pd were
added into the reactants, the mixture was heated to 100 °C in a sil-
icone oil bath for 10 h. The precipitates containing the product (E)-
stilbene (trans-diphenyl ethylene) and the catalyst were isolated
from the reaction solution by filtration and then washed with
1.0 ml of methanol (or DMF) to dissolve the product and separate
it from the catalyst. The methanol (or DMF) solution was then
mixed with 100 mL of water to precipitate the product that was
then filtered, washed with water repeatedly, and dried to a con-
stant weight of 1.63 g. A similar procedure was used for the cata-
lyzed reaction between iodobenzene and acrylic acid. The yield
from the coupling reaction between iodobenzene and styrene
was 90.5%, while that from the reaction between iodobenzene
and acrylic acid was 95.9%. A melting point (m.p.) of 123.6–
124.2 °C for the product (E)-stilbene (literature 123–125 °C) and
a m.p. of 132.6–133.7 °C for the product (E)-cinnamic acid (litera-
ture 133–134 °C) were obtained.
For a comparative study of unsupported catalysis, 1.0 mL of
PdCl2 solution (5.64 mmol/L) was mixed with 10 mmol of iodoben-
zene, 30 mmol of acrylic acid, 40 mmol of triethylamine, and 10 mL
of water (or DMF). The catalytic reaction was carried out at 100 °C
in a silicone oil bath for 10 h. The product was precipitated out by
adding 1 M HCl solution, separated by filtration, washed repeat-
edly with cold water, and recrystallized in hot water.
2.4. Instrumentation
The reaction products were characterized by 1H NMR and 13C
NMR using an AVANCE III 400 MHz NMR spectrometer (Bruker Bio-
spin). FTIR spectra were recorded on a Nexus 360 FTIR spectrome-
ter (Thermo Nicolet). The Pd metal concentration in reaction
solutions was analyzed using a Prodigy ICP–AES spectrometer
(Teledyne Leeman Labs). The detailed spectroscopic data and their
assignments are provided in the Supplementary data.
3. Results and discussion
3.1. Morphology of Pd nanoparticles in IPN networks
The Pd nanoparticles in the IPNs (referred to as Pd@IPN) are
formed by a two-step process. The particles are first formed in
the presence of a stabilizing polymer prior to crosslinking. Subse-
quent formation of the IPN meshes tightly fastens the Pd particles

K. Zhan et al. / Reactive & Functional Polymers 71 (2011) 756–765
759
as shown in Scheme 1, which depicts an IPN with dispersed metal
particles. PAM absorbs a larger amount of water and has a higher
thermostability than PVA, whereas the latter is more flexible and
has much better film forming properties than the former; thus,
we chose these two polymers for forming IPN networks. The
hydrophilicity, swelling and mechanical properties of the PVA–
PAM IPN supports depend on the PVA/PAM weight ratio and the
contents of the crosslinking agents present in the individual
networks.
During the preparation, the stabilizing PVA was crosslinked by
the acetalization of a very small number of PVA hydroxyl (–OH)
groups with glutaraldehyde. The non-acetalized –OH groups in
PVA remained to serve as the Pd metal binding sites, which were
randomly distributed in the PVA. During the simultaneous poly-
merization of acrylamide (AM), the monomer AM and a small
amount of a divinylic crosslinker, N,N0-methylene bisacrylamide
(MBAM), were diffused into the PVA chains after metal reduction.
Fig. 1 shows FTIR spectra of the Pd catalysts in the crosslinked PVA
alone (cPVA) and five types of IPNs with different contents concen-
trations of PVA and PAM (labeled as the molar ratios of the vinyl
alcohol to acrylamide repeating units VA:AM). The spectra illus-
trate that a strong band present near 1096 cmꢀ1 in Fig. 1A and B,
due to the stretching vibration of the acetalized O–C–O group from
the cPVA, gradually disappears in the IPNs with increasing content
of PAM (see Fig. 1E and F). Meanwhile, a band observed near
1657 cmꢀ1 from the amide I stretching vibration from the cross-
linked PAM gradually became the strongest band in the IPNs with
increasing content of PAM (see Fig. 1E and F). These spectral
changes verified the existence of two types of polymers in the
catalysts.
trate that the Pd particles (shown as black dots in Fig. 2) are well
dispersed without aggregates in the IPN supports (the polymers
may show multiphase domains in the TEM).
As shown in Fig. 2, the majority of the nanoparticles are non-
spherical and contain a large number of sharp corners, defects,
and edge sites. The surface atoms at these sites are physically
unstable and chemically active. The particle sizes measured from
the micrographs and sorted into population percentages are shown
as histograms in Fig. 3 for Pd@cPVA and three types of Pd@IPNs
with different polymer repeating unit ratios. The standard devia-
tions of the particle sizes reflect the widths of the size distributions
and are listed in Table 1. The Pd particles in cPVA have an average
size of d = 27.6 nm (Feret’s diameter), with a standard deviation of
12.5 nm (counted from 217 particles), whereas those in the IPNs
have slightly larger average sizes ranging from 32 to 43 nm (see
exact values listed in Table 1). As shown in the histograms in
Fig. 3, the largest population of the Pd particles in cPVA appears
in the range of 20–30 nm, whereas in the IPNs the largest popula-
tions of the Pd particles shift to larger size ranges, corresponding to
an increase in the average particle sizes by 18–50%. Therefore, the
introduction of a PAM network into cPVA has moderately increased
the average particle sizes.
The moderate increase in the average particle sizes from the
crosslinked PVA to PVA–PAM IPNs are likely caused by Ostwald
ripening sintering during the crosslinking of PVA and PAM chains.
This phenomenon occurs by the detachment of the Pd atoms from
smaller nanoparticles and their subsequent transfer to larger parti-
cles [7,35]. As a result, the larger particles grow in size while the
smaller particles shrink or dissolve gradually. This occurs because
of the lower average coordination numbers of the atoms at the
smaller particle surface and the relative ease of their removal;
therefore, the large particles become larger at the expense of the
smaller particles.
The PVA colloidal dispersions were stable enough to withstand
the potential aggregation or precipitation of the Pd particles during
the crosslinking process of PVA and the polymerization of acrylam-
ide. Fig. 2A–D shows transmission electron micrographs of Pd in
cPVA and three types of PVA–PAM IPN networks with different
polymer repeating unit ratios (VA:AM ratios). Further magnified
TEM micrographs are shown in Fig. 2E and F. The TEM images illus-
Mesh sizes of the crosslinked polymers can be analyzed by the
swelling method [32–34,36]. The results from the swelling tests in
water were first used to calculate the equilibrium swelling ratio,
which is defined as:
(A) cPVA
(B) PVA:PAM=3.304
(C) PVA:PAM=1.652
(D) PVA:PAM=0.826
(E) PVA:PAM=0.413
(F) PVA:PAM=0.207
3500
3000
2500
2000
1500
1000
Wavenumber (cm-1)
Fig. 1. FTIR spectra of Pd catalysts in cPVA alone and in five types of IPNs containing different contents of PVA and PAM (labeled as the molar ratios of the VA to AM repeating
units).

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K. Zhan et al. / Reactive & Functional Polymers 71 (2011) 756–765
Fig. 2. Transmission electron micrographs of unused Pd particles (A) in cPVA, (B) in the IPN network with VA:AM = 0.826:1, (C) in the IPN network with VA:AM = 1.652:1, and
(D) in the IPN network with VA:AM = 0.413:1. Scale bars in (A) to (D) are 500 nm. Higher magnification transmission electron micrographs of unused Pd in the IPN network
with VA:AM = 1.652:1 (E) and in the IPN network with VA:AM = 0.413:1 (F). Scale bars in (E) and (F) are 100 nm.
"
!#
1=2
1
v2;s
2Mc
Q ¼
ð1Þ
ꢀ1=3
2;s
n ¼
v
Cn
l ¼ Q1=3ðCn=XÞ1=2l
ð3Þ
Mr
where
v
2,s is the volume fraction of the polymer in the swollen state.
where Cn is the Flory characteristic ratio, and l is the carbon–carbon
bond length (0.154 nm). The value of Cn is 8.3 for PVA and 8.5 for
PAM in water [37].
A theoretical molecular weight between crosslinks Mc can be
determined from the nominal crosslinking ratio X and the molecu-
lar weight of the repeating unit Mr of PVA (Mr = 44) or PAM
(Mr = 71) as follows:
By applying Eq. (3), a mesh size n = 3.9 nm in water was ob-
tained for the Pd@cPVA. For the Pd@IPNs in water, mesh sizes
n1 = 3.9–4.3 nm were obtained for the PVA network, while mesh
sizes n2 = 2.5–2.8 nm were obtained for the PAM network (see Ta-
ble 1). The mesh size values of the polymers were one order of
magnitude smaller than the average sizes of the Pd particles ob-
tained from the TEM measurements. Water is a good solvent for
both PVA and PAM, whereas DMF is a poor solvent for PVA and a
nonsolvent for PAM; therefore, the swelling ratios of the IPNs in
Mc ¼ Mr=2X
ð2Þ
For neutral homopolymers, experimental values of Mc are the
same as the theoretical values. The mesh size, n, defines the linear
distance between consecutive crosslinks, which indirectly indi-
cates the diffusional space available for solute transport and can
be calculated using following equation [32–34,36]:

K. Zhan et al. / Reactive & Functional Polymers 71 (2011) 756–765
761
35
30
25
20
15
10
5
20
18
(B)
(A)
16
14
12
10
8
6
4
2
0
0
0
10 20 30 40 50 60 70 80 90 100 110 120
Size/nm
0
10 20 30 40 50 60 70 80 90 100 110 120
Size/nm
35
30
25
20
15
10
5
30
25
20
15
10
5
(C)
(D)
0
0
0
0
10 20 30 40 50 60 70 80 90 100 110
Size/nm
10 20 30 40 50 60 70 80 90 100 110 120
Size/nm
Fig. 3. Size distributions of unused Pd particles (A) in cPVA, (B) in IPN with VA:AM = 1.625:1, (C) in IPN with VA:AM = 0.826:1, and (D) in IPN with VA:AM = 0.413:1.
Table 1
Pd particle sizes (average particle size d standard deviation
a
), polymer equilibrium swelling ratios Q, mesh sizes n, and catalysis yields from Heck coupling reaction between
a
iodobenzene and acrylic acid
.
b,c
Catalyst type
VA:AM ratio
d
r
(nm)
Q in water
n in water (nm)
Q in DMF
n in DMF (nm)
Catalysis yield
(%)
PVA
PAM
PVA
PAM
PVA
Pure VA
1.652:1
0.826:1
0.413:1
27.6 12.5
42.5 22.0
32.9 10.8
36.8 17.9
2.17
2.14
2.36
3.01
3.9
3.9
4.0
4.3
–
1.83
1.41
1.15
1.12
3.7
3.4
3.1
3.1
–
85
81
86
75
IPN #1
IPN #2
IPN #3
2.5
2.6
2.8
2.1
2.0
2.0
a
b
c
See reaction conditions in Section 2.
The yields refer to isolated products based on iodobenzene.
All products gave satisfactory 1H NMR, FTIR, and melting points which are provided in Supplementary data at the end of the manuscript.
DMF were reduced appreciably in comparison with their counter-
parts swollen in water (see Table 1). This led to even smaller mesh
sizes of the PVA and PAM in DMF. Thus, the covalent bonding
meshes in the IPN networks are small enough to encage the Pd
nanoparticles but large enough for reactants to diffuse to the par-
ticle surfaces in water and DMF.
The accuracy of the mesh sizes obtained from the swelling tests
can be further validated by fabrication of two types of Pd@cPVA with
Pd particle sizes of d = 4.5 nm 1.4 nm and d = 5.8 nm 2.4 nm
(average Feret’s diameter standard deviation). Unfortunately,
both types of Pd particles lost their contents significantly in the poly-
mers and leached into their solutions quickly in only two consecu-
tive runs of the catalytic tests. Evidently, this can be ascribed to
their particle sizes being so close to the mesh size of cPVA (3.7 nm
in DMF) (Table 1).
recycling each catalyst for Heck coupling reactions between iodo-
benzene and acrylic acid (see Scheme 2, Y = COOH) is shown in
Fig. 4 (black and grey bars). Note that the yields refer to real iso-
lated products based on iodoarenes. The Pd@IPN catalyst could
be reused in the DMF solvent at least 21 times without a significant
change in the reaction yields. This kind of long-term stability of Pd
nanoparticles has rarely been referenced in the literature. The Pd
nanocatalysts used in the Heck coupling reactions reported thus
far could be recycled only about 5–10 times [5–9,14–29] and even
fewer times when using more open polymer support structures
(e.g., dendrimers or porous resins) with quick Pd leaching [9,11–
17]. Palladium salts supported on crosslinked PAM alone could
be reused only 6 times [31a], and similarly, Konjac glucoman-
nan-(a type of natural polysaccharide) supported palladium cata-
lysts could be recycled only five times [38]. Thus, the recycling
lifetime of the Pd@IPN system is extraordinary. The methods of
immobilizing the Pd metal to polymer supports described in the
literature often involve the use of ionic bonding or coordination
bonding to phosphines, N-heterocyclic carbenes, palladacycles, or
pincer ligands. The catalytic activities in the presence of such li-
gands are sometimes quite controversial because some of the com-
plexes immobilized in the Pd2+ oxidation state might undergo a
Pd2+/Pd4+ catalytic cycle and complicate the mechanism of the cou-
pling reaction [28].
3.2. Recyclability of palladium nanoparticles in Heck coupling
reactions
Tight fastening of the Pd metals in the covalent bonding struc-
tures of the IPNs imparts robust thermal stability against direct
aggregation and suppresses the loss of Pd metals to a negligible le-
vel. A comparison of the catalytic yields of the Pd@IPN catalyst
(VA:AM = 0.826:1) to that of the Pd@cPVA catalyst obtained after

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K. Zhan et al. / Reactive & Functional Polymers 71 (2011) 756–765
Y
Pd(0)
I
+
Y
Y= COOH, Ph, etc.
Scheme 2. Heck coupling reaction catalyzed by Pd nanoparticles in IPN and cPVA.
By comparison, the activity of the Pd@cPVA catalyst was reduced
vent. As shown in the first entry of Table 2, the coupling reaction in
water between iodobenzene and styrene proceeded smoothly in
the presence of the Pd@IPN catalyst and a phase transfer agent
(hexadecyltrimethylammonium bromide), producing (E)-stilbene
at a high yield of 90.5%. For the coupling reaction between iodo-
benzene and acrylic acid (entry 2, Table 2), the product (E)-cin-
namic acid was obtained with a yield of 95.9%. Apparently, the
highly polar and hydrophilic nature of the component polymers
rendered the reactions functional in water. The ability to use a non-
toxic, environmentally benign, water-only solvent is of ecological
and economical importance [39,40]. The use of the phase transfer
agent promotes the migration of the base from the aqueous phase
to the organic reactants to neutralize the protonated Pd2+ complex
species generated in the reaction, facilitating the regeneration of
the active zerovalent Pd catalyst in the reaction cycle and carrying
the resulting salt into the aqueous phase.
appreciably after the 9th repetitive use, delivering a yield of only
22% (see Fig. 4). Because the amount of the Pd metal in the IPN is
the same as that in the cPVA, the fact that the Pd@IPN catalyst
has a lifetime twice as long as that of the cPVA can be primarily ex-
plained by the firmer confinement of the Pd nanoparticles by the
covalent chains of PAM with smaller mesh sizes. The amide side
chains in the PAM are not believed to be strong ligands for the Pd
metals; however, the decreased mesh size of the IPN structure did
not affect the reactants ability to reach the Pd particle surfaces. This
is also demonstrated by the comparison of the Pd@IPN using differ-
ent molar ratios of the VA and AM repeating units for the Heck cou-
pling reaction between iodobenzene and acrylic acid. All of the
catalysts demonstrated similar high yields of (E)-cinnamic acid
after isolation and almost identical activities for the reaction (see
Table 1). Obviously, the presence of the PAM network chains does
not affect either diffusion of the reactants to the Pd particle surfaces
or the yields significantly, even though the nominal crosslinking de-
gree of the PAM network is higher than that of the PVA network.
For comparative purposes, catalysis using an unsupported PdCl2
catalyst was also performed. The Pd salt could catalyze the Heck
reaction for acrylic acid and iodobenzene with a high yield near
99–100%; however, the isolation and reuse of the Pd salt from
the reaction product solutions containing the base (triethylamine)
and by-products became very complicated. Therefore, a significant
loss of the Pd metals occurred during the work-up, and the reaction
solution recovered from the first run of catalysis did not have en-
ough activity for consecutive runs to be performed.
The recyclability of the Pd@IPN catalyst in water is also dis-
played in Fig. 4 (open bars). The catalyst can be reused 13 times
for the coupling reaction between iodobenzene and acrylic acid
in water. This means that the recyclability of the Pd@IPN in the
water solvent is very good and comparable to that reported in a re-
cent study, which demonstrated that Pd nanoparticles immobi-
lized on fluorous silica gel could be reused 11–15 times [40a].
Traditionally, Heck coupling reactions are conducted in polar
aprotic organic solvents, such as DMF or DMSO, with a Pd(0) com-
plex or a Pd2+ salt (which is reduced in situ to Pd(0) during the cat-
alytic redox cycle), preferably in the presence of a strong ligand
(e.g., a toxic ligand phosphine), which stabilizes the Pd(0) atoms
as complexes and avoids the fast precipitation of the Pd(0) from
the solution. The Pd@IPN system requires no such ligand, as the
Pd nanoparticles are prevented from coalescence and are firmly
stabilized by the polar IPN structures. The catalytic activity of the
Pd@IPN in terms of turnover frequencies (TOFs), which are defined
as mol product/mol catalyst/hour, which is calculated from the iso-
lated yield, the amount of Pd used, and the reaction time, are about
50 times larger than those of the recently reported polyelectrolyte
complex stabilized palladium nanoparticles [31b]; however, the
Pd@IPN has much longer recycling performance than the latter.
For applications in aqueous conditions, it is quite reasonable
that several types of hydrophilic polymers other than PVA and
PAM can be used to form different IPN structures and examine
the polymer effect on the catalytic activity. This will form a large
family of IPNs suitable for support materials. On the other hand,
the choice of the polymers also depends on the optimal condition
of the Heck reaction; therefore, not all hydrophilic polymers can
serve as components of the IPN for the Heck coupling reaction.
Non-ionizable hydrophilic polymers have an advantage that the
pH of the Heck reaction can be easily controlled by an external
base added; therefore, PVA and PAM are suitable materials for
the Heck coupling reactions, as demonstrated in this study.
The selectivity of the catalysts was also examined for a series of
Heck coupling reactions in DMF solvents by using derivatives of
iodoarenes and acrylic acid or styrene. The results are listed in Ta-
ble 2. In all cases, the products were identified to be either exclu-
sively an (E)-cinnamic acid derivative or an (E)-stilbene. Other
types of isomeric products were not detectable by HPLC or 1H
NMR analysis. As shown in Table 2, the yields of the isolated
3.3. Catalysis for Heck coupling reactions in aqueous and organic
media
The second feature of the Pd@IPN described here is its capability
to catalyze Heck reaction in neat water without addition of a cosol-
120%
100%
80%
60%
40%
20%
0%
1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
recycle times
Fig. 4. Comparison of the recyclability of the Pd@IPN catalyst in the coupling
reaction of iodobenzene and acrylic acid in DMF solvent (solid black bars) and in
water (open bars). Also shown is the recyclability of the Pd@cPVA catalyst in DMF
solvent (grey bars) under the same experimental conditions.

K. Zhan et al. / Reactive & Functional Polymers 71 (2011) 756–765
763
Table 2
Results of various Heck coupling reactions catalyzed by Pd@cPVA and Pd@IPN.
c,d
Entrya
Reactants
Catalyst
Solvent
Yield (%)
TOF
Aryl halide (mmol)
Alkene (mmol)
1
2
3
4
5
6
7
8
9
Iodobenzene (10)
Iodobenzene (10)
Styrene (15)
IPNb
IPNb
IPNb
IPNb
IPNb
IPNb
IPNb
cPVA
cPVA
cPVA
Water
Water
Water
DMF
DMF
DMF
DMF
DMF
DMF
DMF
91
96
35
92
63
41
69
69
85
80
302
321
98
256
78
114
192
192
237
223
Acrylic acid (20)
Acrylic acid (20)
Acrylic acid (20)
Acrylic acid (20)
Acrylic acid (20)
Methyl acrylate (20)
Styrene (20)
Bromobenzene (10)
4-Iodoanisole (10)
p-Fluoroiodobenzene (4.5)
2-Iodothiophene (10)
4,40-Diiododiphenyl (5)
4-Iodoanisole (10)
4-Iodoanisole (10)
4-Iodoanisole (10)
Methyl acrylate (20)
Acrylic acid (20)
10
a
b
c
See Section 2 for reaction conditions.
The IPN used contain VA:AM = 0.826:1.
The yields refer to isolated products based on Ar–X.
d
All products gave satisfactory 1H NMR, 13C NMR, FTIR spectra, and melting points which are provided in the Supplementary data at the end of the manuscript.
products from the Pd@IPN catalyst range from 92.0% (for a phenyl
ring with a methoxy-substituent) to 68.8% (for a double coupling
reaction) to 41% (for a relatively difficult substrate with a thio-
phene ring). The yields were primarily dependent on the nature
of the substituent at the para-position of the ring and the substitu-
ent on the olefin. The catalytic activities are higher than those pre-
viously reported for Pd nanoparticles encapsulated in several other
types of polymers [12,31b,41]. Furthermore, for the coupling reac-
tion between bromobenzene and acrylic acid (entry 3, Table 2), the
TOF (98) and yield (35%) are also much higher than the recently
reported TOF and yield given by charged polymer complex stabi-
lized Pd nanoparticles (only about 0.25 and 5%, respectively) [31b].
3.4. The role of the Pd nanoparticles in the coupling reaction
Fig. 5A and B shows a representative TEM image of the nanopar-
ticles in the IPN with VA:AM = 0.826:1 after 21 cycles of catalysis
in DMF solvent and the corresponding size distribution of the
nanoparticles. At this stage, the Pd@IPN still showed very signifi-
cant catalytic activity; however, statistical analysis of 192 particles
from the TEM images showed that the average size of the particles
increased to 49.7 nm while their standard deviation also increased
appreciably to 33.9 nm as compared to the average particle size of
32.9 nm with a standard deviation of 10.8 nm before catalysis. If
the size increase was simply caused by the leaching of small parti-
30
(BB))
25
20
15
10
5
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140
Size/nm
Fig. 5. (A) Transmission electron micrograph (scale bar = 500 nm) and (B) size distribution of the used Pd particles in the IPN with VA:AM = 0.826:1. Higher magnification
transmission electron micrographs of the used Pd in the IPN (C and D) (scale bars = 100 nm).

764
K. Zhan et al. / Reactive & Functional Polymers 71 (2011) 756–765
cles into the reaction solutions and the retention of the large par-
ticles in the IPN, a narrower size distribution would be expected.
The increase in the average particle diameter and the observation
of a wider distribution of the particles after the reactions, particu-
larly concerning the appearance of particles larger than 100 nm
(Fig. 5B as compared to Fig. 3C), is best explained by the Ostwald
ripening effect, which involves the process of Pd dissolution from
smaller particles and re-deposition to larger particle surfaces dur-
ing the catalytic reaction [7,35]. In addition, a large number of the
used particles formed well-defined polyhedron shapes with
straight edges, as shown in the higher magnification TEM images
(Fig. 5C and D). This further illustrates that the active Pd species
were re-deposited during the reaction, forming the polyhedron
particles. The appearance of a large population (about 27%) of the
particles in the size range of 10–20 nm in Fig. 5B could be due to
new particles generated in the catalytic reaction. It is reasonable
to conclude that the detached Pd atoms generated in the Ostwald
ripening process were involved in the catalytic redox cycle, reacted
with the iodoarenes, and then re-deposited on larger particles
which ultimately led to the increase in the average particle size
and the distribution width, as well as the appearance of well-
defined particle shapes. In other words, the nanoparticles play a
catalytic role in the Heck reaction through a homogeneous mech-
anism involving the participation of dissolved Pd atoms, which is
in accordance with recent studies [42–44].
heterogeneous catalysis) and activity (like a classical homogeneous
catalysis). We have shown here that palladium nanoparticles in
properly designed hydrophilic IPN structures can achieve the goal
of superior durability and can be used for Heck coupling reactions
not only in organic solvents but also in neat water. The reaction in-
volves a homogeneous mechanism and also bears the unique
advantages of a heterogeneous catalysis; that is, the catalysts can
be recovered easily by simple filtration and regenerated to full
activity. As a result, the highly stable catalysts show negligible me-
tal leaching and can be reused more than 20 times in DMF and 13
times in water. Such robust character bridges the gap between
homogeneous and heterogeneous catalysis and combines the
advantages of both types of reactions, which is quite valuable for
reducing the cost of the use of noble metals, minimizing the con-
tamination of the metals in the products, and simplifying the
experimental process. The results reported in this study demon-
strate an improvement in the use of Pd nanoparticles for C–C
cross-coupling reactions when compared with Pd nanoparticles
dispersed in other hydrophilic structures, e.g., poly(N-isopropylac-
rylamide)-grafted Pd nanoparticles [47], polymethacrylic acid
copolymer bead-supported Pd nanoparticles [48], polyion com-
plexes composed of polyacrylic acid and poly[4-chloromethylsty-
rene-co-(4-vinylbenzyl)tributylammoniumchloride] [31b], and
chitosan-supported Pd particles [41a] in which low activities and
a quick decrease in the catalytic activity was observed in the recy-
cling of the catalyst. The combined benefits of the ability to use
water as solvent for the coupling reactions and the increased recy-
clability indicate that the Pd-encaged IPNs are green sustainable
catalyst reactors and are suitable for applications requiring contin-
uous processing conditions.
Analysis of the Pd metal concentration of the final reaction solu-
tions by the ICP–AES method revealed extremely low amounts of
palladium leaching from the IPNs. Only 0.0506 and 0.140 lmol of
Pd were found in the DMF solution phase after the first and 23rd
cycles of the reaction, respectively. These corresponded to only
0.177% and 0.488% of the total amount of Pd used in the catalytic
cycle. Such a level of leaching is one order of magnitude lower than
those ‘‘leach-proof’’ Pd nanoparticles reported very recently for
C–C coupling reactions [21a,24] and should not significantly con-
tribute to the increase in the average particle size and distribution
width of the Pd in the IPN. In the aqueous reaction, only 0.16 and
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (No. 20974063), Natural Science Foundation of
Zhejiang Province (No. Y4090504), State Key Laboratory of Coordi-
nation Chemistry (Nanjing University), Department of Education,
Zhejiang Province (No. 20070495), Department of Science and
Technology of Zhejiang Province (No. 2009R10040), Zhejiang Xin-
miao Project, and Shaoxing University (No. 08LG1005).
0.125 lmol of Pd were found in the solution phase after the first
and 13th cycle of the reaction, respectively, which corresponded
to only 1.25% and 0.98% of the total amount of Pd used in the
catalytic cycle. This indicates that the Pd leaching in the water
solution reactions is slightly higher than in the DMF solvent condi-
tion. In addition, we found that the trace amount of soluble Pd ions
and/or atoms in the solution phase could not catalyze the reaction
effectively after the Pd@IPN was removed from the reaction system
and all the new reagents were added. It is reasonable to conclude
that the majority of the soluble Pd ion and/or atoms, once gener-
ated inside the IPN, are re-deposited to the large nanoparticle sur-
faces in the reduction step of the catalytic cycle or that they
precipitate quickly inside the IPN pores and form new particles.
The re-deposition of palladium active species on the surfaces of
support materials and palladium particles was reported in some
studies [45]. It seems that the very fast and dynamic Pd metal dis-
solution–deposition process driven by the particle ripening effect
inside the IPN pores has the remarkable feature of minimizing
the amount of Pd leached into the solution phase. This means that
the catalysis is essentially confined inside the IPN pores and the
Pd@IPN catalysts can be recovered as a ‘‘heterogeneous’’ reactor.
Appendix A. Supplementary material
Supplementary information including NMR and FTIR spectro-
scopic data of the reaction products and their assignments, can
be found in the online version of this manuscript. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.reactfunctpolym.2011.04.007.
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