Synthesis and Application of Hyperbranched Polyester-Grafted Polyethylene (HBPE-g-PE) Containing Palladium Nanoparticles as Efficient Nanocatalyst

Article abstract of DOI:10.1007/s10562-016-1720-y

In nowadays, palladium-catalyzed reactions are of utmost importance in synthetic organic chemistry. Due to advantages of heterogeneous catalysts such as the easy handling and good recyclability, usually these types of catalysts are first choice in constructing a new catalytic system. Here we report a facile approach to prepare hyperbranched polyester-grafted-polyethylene supported palladium nanoparticles as an efficient catalyst in Suzuki reaction and solvent-free aerobic oxidation of alcohols. The reactions were carried out in different conditions with various reagents and the efficiency was determined. Because of the amplification effect of hyperbranched polyester, high loading capacities (around 0.276 mmol g^-1 for Pd) was achieved. The catalyst was characterized with X-ray powder diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, differential scanning calorimetry and thermogravimetric analysis. This metal nanocatalyst has a very low leaching loss, high activity and selectivity, excellent thermal stability and can be reused at least five times without deactivation of catalytic activity.

Full text of DOI:10.1007/s10562-016-1720-y

Catal Lett
DOI 10.1007/s10562-016-1720-y
Synthesis and Application of Hyperbranched Polyester-Grafted
Polyethylene (HBPE-g-PE) Containing Palladium Nanoparticles
as Efficient Nanocatalyst
Hossein Mahdavi1 Razieh Sahraei1
•
Received: 19 January 2016 / Accepted: 22 February 2016
Ó Springer Science+Business Media New York 2016
Abstract In nowadays, palladium-catalyzed reactions are
of utmost importance in synthetic organic chemistry. Due
to advantages of heterogeneous catalysts such as the easy
handling and good recyclability, usually these types of
catalysts are first choice in constructing a new catalytic
system. Here we report a facile approach to prepare
hyperbranched polyester-grafted-polyethylene supported
palladium nanoparticles as an efficient catalyst in Suzuki
reaction and solvent-free aerobic oxidation of alcohols. The
reactions were carried out in different conditions with
various reagents and the efficiency was determined.
Because of the amplification effect of hyperbranched
polyester, high loading capacities (around 0.276 mmol g-1
for Pd) was achieved. The catalyst was characterized with
X-ray powder diffraction, scanning electron microscopy,
energy-dispersive X-ray spectroscopy, transmission elec-
tron microscopy, differential scanning calorimetry and
thermogravimetric analysis. This metal nanocatalyst has a
very low leaching loss, high activity and selectivity,
excellent thermal stability and can be reused at least five
times without deactivation of catalytic activity.
catalyst was carefully examined for Suzuki reaction and
solvent-free oxidation of alcohols.
Keywords Polyethylene-support Á Hyperbranched
polyester Á Palladium nanoparticles Á Suzuki reaction Á
Oxidation of alcohols
1 Introduction
Graphical Abstract In this work, a hyperbranched
polyester based on 2,2-bis (methylol) propionic acid (bis-
MPA) was synthesized and grafted onto carboxylic acid
functionalized polyethylene powder. The final nanocatalyst
was prepared via solution loading of PdCl2 and reduction
to palladium nanoparticles on the HBPE-g-PE. The activity
and stability of prepared polymer-supported heterogeneous
Transition-metal catalyzed reactions are fundamental
transformations in organic synthesis. Palladium is one of
the most important metals, which has been used as a cat-
alyst in synthesis of a wide variety of organic compound
[1]. The most well-known palladium-catalyzed reactions
are coupling of aryl halides with organoboronic acids
(Suzuki–Miyaura reaction), coupling of aryl halides with
alkenes (Mizoroki–Heck reaction), oxidation of alcohols,
etc. These types of reactions have a great industrial
potential for the synthesis of pharmaceuticals, functional
materials, and natural products [2–5].
& Hossein Mahdavi
hmahdavi@khayam.ut.ac.ir
1
Both heterogeneous and homogeneous systems can be
used to carry out these reactions. Despite the excellent
School of Chemistry, College of Science, University of
Tehran, P.O. Box 14155–6455, Tehran, Iran
123

H. Mahdavi,R. Sahraei
activity and selectivity of homogeneous catalysis, they
have several drawbacks such as difficult separation and
recycling of the catalyst. In contrast, the heterogeneous
catalysis could be recycled by simple filtration or cen-
trifugation, which in turn decrease environmental pollution
caused by residual toxic palladium metal in waste. In
addition, they have other excellent features including ease
of handling and good reusability. Therefore, heterogeneous
catalyst is much affordable in catalytic reactions [6, 7]. In
recent years, significant efforts have been made for
heterogenization of catalysts to solid supports. Many
organic and inorganic supports such as polymers [8], zeo-
lites [6], silica [9], carbon nanofiber [10], clay [11], metal
oxides [12], partially reduced graphene oxide (PRGO) [13],
and alumina [14] have been used in preparation of efficient
and environmental-friendly heterogeneous catalytic sys-
tems. Immobilizing palladium nanoparticles onto the sup-
port prevents aggregation of metal particles and increases
surface area and activity of catalyst [15]. In comparison to
inorganic supports, better control on catalytic activity was
obtained with polymer supports [16]. In the past decade,
there has been increasing interest on the preparation of
polymer supported palladium nanoparticles with narrow
size distribution for Pd-catalyzed transformations due to
their much enhanced catalytic activity and selectivity.
Oberhauser and co-workers employed carboxylic acid end-
functionalized poly(lactic acid) (PLA)-based stereocom-
plexes containing PdNPs as heterogeneous catalyst for
chemoselective hydrogenation of the C=C double bond in
cinnamaldehyde. For 3-phenylpropanal, 90 % chemose-
lectivity at 88 % conversion was obtained [17]. Siyo et al.
prepared different sized PVP stabilized Pd NPs with a
mean diameter between 1.8 and 4.4 nm for the aerobic
oxidation of 5-hydroxymethylfurfural into 2,5-furandicar-
boxylic acid in an alkaline aqueous solution. The influence
of the particle size on conversion and product yield in the
presence of different sized Pd NPs was investigated [18].
Raspolli Galletti et al. employed polyketone as a support
and stabilizing agent for Pd nanoparticles in order to
examine its catalytic performance in the selective hydro-
genation of cinnamaldehyde to hydrocinnamaldehyde. The
sample with smaller nanoparticle size and a narrower metal
particle size distribution show the highest selectivity
(87.7 %) [19].
substrates having polar surfaces can be provided in pres-
ence of polar functional groups, such as –OH, –COOH,
–NH2, and halides onto the surface of inert polyolefins [20,
21]. Surface properties of polyolefins such as wetability,
hydrophilicity, and biocompatibility are improved via
modification approaches. These functional groups provide
the selective sites for subsequent polymer grafting [21].
There are three strategies for modification of the surfaces
and preparing hyper-graft polymers [22]. These strategies
are called ‘‘grafting onto’’, ‘‘grafting from’’, and ‘‘grafting
through’’. In all grafting routes, we need proper functional
groups onto the surface of the solid support. Various
chemical reactions such as oxidation can provide these
functions onto the surface. Several functional polymer side
chains can be created onto the polyolefin backbone by
grafting methods, which offers various practical advan-
tages [20, 21].
Metal nanoparticles can be stabilized with polymers
containing heteroatoms. Dendrimers and hyperbranched
polymers are excellent model systems, which act as cap-
ping agents and stabilize metal nanoparticles [23]. Hyper-
branched polymers are more easily made by simple one-
step polymerizations in contrast to dendrimers. Hyper-
branched polymers are highly branched macromolecules
with dendritic architecture, which contain numerous func-
tional end groups [24]. High density of functional groups
can increase the loading capacity of the metal nanoparticles
and improve catalytic activity of the catalyst. High dis-
persion and narrow size distribution of metal particles play
a key role in improvement of catalytic performance [23].
Therefore, hyperbranched grafting onto the PE-support
provides numerous functional groups, which increase
loading capacity of palladium nanoparticles and stabilizes
the reduced form of Pd-NPs.
In the present work, we synthesized a hyperbranched
polyester based on 2,2-bis (methylol) propionic acid (bis-
MPA) and then, this hyperbranched polyester grafted onto
carboxylic acid functionalized polyethylene powder. The
final nanocatalyst was prepared via solution loading of
PdCl2 and reduction to palladium nanoparticles on the
HBPE-g-PE. The activity, selectivity, and stability of pre-
pared polymer-supported heterogeneous catalyst were
carefully examined for Suzuki reaction and solvent-free
oxidation of alcohols.
Polyolefins like polyethylene (PE) and polypropylene
(PP) are good choices for solid supports among commercial
polymers. These types of polymers have excellent chemical
and physical properties along with low cost, good recy-
clability, and simple process-ability. Despite these superior
features, lack of the reactive groups has limited the inter-
action of polyolefins with other materials and polymers,
which leads to solubility restrictions in common solvents
[20]. However, strong interactions between polyolefins and
2 Experimental
2.1 Materials
Granular low density polyethylene LH 0075 (melt flow
index: 0.75 g/10 min) was purchased from Bandar Imam
Petrochemical Company. 2,2-Bis (methylol) propionic acid
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Synthesis and Application of Hyperbranched Polyester-Grafted Polyethylene (HBPE-g-PE)…
(Bis-MPA), 2-Ethyl-2-(hydroxymethyl)-1,3-propanediol
(TMP) and p-Toluenesulfonic acid were supplied from
Merck. Palladium (II) chloride (C99.9 %) was purchased
from Sigma-Aldrich. All solvents and other chemical
reagents were purchased from Merck and Sigma-Aldrich
and used without any further purification.
and p-TSA (0.056 mg) into the reaction mixture. Argon
passing and connecting to the vacuum steps were repeated
in this stage, too. Synthesized polymer was dissolved in
acetone for 24 h and then precipitated in n-hexane [25].
Fourth generation of poly (Bis-MPA) was prepared by
this methodology with Mn = 6620 g mol-1
.
2.2 Characterization
2.4 Catalyst Preparation
The amount of palladium nanoparticles supported on
HBPE-g-PE was analyzed on a Varian VISTA-MPX
inductively coupled plasma-optical emission spectrometry
(ICP-OES) instrument.
2.4.1 Chemical Modification of Polyethylene Powder
Low density polyethylene (LH0075) (2.0 g) were dissolved
in hot xylene (20 ml) at 110–120 °C under stirring. PE
powder was collected by precipitation into methanol and
then was dried in the vacuum. The obtained white powder
was extracted with methylene chloride in a soxhlet appa-
ratus at 40–50 °C to remove any commercial additives. In a
100 mL round-bottomed flask, Potassium dichromate
(5.0 g) was dissolved in distilled water (30 ml) and sulfuric
acid (%98, 21 ml). Then, 2.0 g of PE powder were added
into the mixture. The reaction mixture was heated under
reflux and stirring on an oil bath at 90 °C for 4 h. Yellow
oxidized polyethylene (PE–COOH) was collected by fil-
tration and washed thoroughly with distilled water and
dried at 50–60 °C for 24 h under vacuum.
In order to converting prepared carboxylic acid groups
on the PE surface to the corresponding acid chloride,
thionyl chloride (15 ml), a drop of dimethylformamide,
and PE–COOH (2.0 g) were mixed in 50 ml round-bot-
tomed flask. The mixture was heated at 60–70 °C for 14 h
under reflux and N2 purge. To minimize hydrolysis, the
powder was collected and washed with THF and trans-
ferred to the subsequent reaction solution immediately.
X-ray diffraction (XRD) pattern of the catalyst was
recorded on a Philips analytical X-ray diffractometer
(XPert MPD) with monochromatized Cu/Ka radiation
(k = 0.15424 nm). Diffraction data were collected from
10° to 90° (2h).
Scanning electron microscopy (SEM) images of the
samples were obtained on AIS2100 scanning electron
microscope and (TESCAN-VEGA3 XMU) field emission
scanning electron microscope equipped with EDX and
elemental mapping accessories. Transmission electron
microscopy (TEM) observations were performed on a
Philips CM30 electron microscope at an accelerating
voltage of 200.00 kV in order to obtain the mean particle
size of Pd nanoparticles.
Thermal properties of the samples were analyzed by
Thermogravimetric (TG) and Differential scanning calori-
metric (DSC) analysis using TGA Q50 V6.3 Build 189 and
DSC Q100 V9.0 Build 275 instruments, respectively. The
analyses were performed under nitrogen atmosphere and a
heating rate of 10 °C min-1
.
The yields of catalytic reactions were determined by gas
chromatography using Shimadzu gas chromatograph.
ATR-FTIR spectra of the samples were run on EQUT-
NOX55 BRUKER spectrometer with DTGS detector.
2.4.2 Grafting of Hyperbranched Polyester
onto the Modified Polyethylene
2.3 Preparation of Hyperbranched Polyester
In a 25 mL round-bottomed flask, HBPE (G4) (0.6 g) was
dissolved in dioxane (12 ml). A drop of pyridine as an acid
scavenger was added into the mixture and then 2.0 g of
PE–COCl was added and the mixture was placed in an oil
bath under reflux and stirring at 60–70 °C for 24 h. The
prepared HBPE-g-PE was washed with acetone and dried
under vacuum.
Bis-MPA (6.71 g), TMP (0.745 g), and p-TSA (33.6 mg)
were mixed in a three-necked flask equipped with an argon
inlet, a drying tube, and a stirrer. The reaction was carried
out in preheated oil bath in 140 °C under a stream of argon
(for removing of water formed during the reaction). After
2 h, the argon stream was removed and the flask connected
to a vacuum for 1 h. For synthesis of third generation of
hyperbranched polyester, Bis-MPA (8.94 g) and p-TSA
(44.8 mg) were added to the mixture and the argon stream
was turned on for 2 h. Then the argon stream was removed
and the vacuum was applied for 1 h. Forth generation of
hyperbranched polyester (HBPEG4) was synthesized by
removing the vacuum and addition of Bis-MPA (17.875 g)
The percentage of hyperbranched grafting was calcu-
lated by following equation:
Wg À Wo
Graftingð%Þ ¼
 100
Wo
where Wg is the weight of hyperbranched-grafted poly-
ethylene and Wo is the weight of polyethylene before
hypergrafting.
123

H. Mahdavi,R. Sahraei
To achieve the more effective functional groups to
complex with Pd ions, the surface hydroxyl groups were
transformed into carboxylic acid functionality. Therefore,
HBPE-g-PE (2.0 g) was added to a flask containing
dimethylformamide (30 ml), succinic anhydride (1.6 g),
and triethylamine (2.8 ml). The reaction mixture was stir-
red at 90 °C for 6 h under reflux. The collected powder was
washed with methanol and dried under vacuum.
Oxygen flow was bubbled at a flow-rate of 20 ml/min.
After the allowed reaction time duration, the catalyst was
removed from the mixture, and the products were analyzed
using gas chromatography.
2.7 Reusability of the Catalyst
After completion of the reaction in the first run, the catalyst
was removed by simple filtration and washed with acetone.
It was dried under vacuum and then was added to another
vessel containing the starting materials. The sequential
reaction was carried out under the same conditions as
discussed before. The recycling was repeated successfully
for five cycles without significant loss of catalytic activity.
2.4.3 Formation of Pd Nanoparticles on the Synthesized
Polymer Support
A mixture of palladium chloride (10 mg) and sodium
chloride (66.2 mg) in deionized water (15 ml) was heated
to achieve the dark-red aqueous solution of Na2PdCl4.
Then, 2.0 g of supported polymer was added to the solution
and in order to better dispersion, the mixture was placed
under ultrasonic waves for 5 min and then was stirred at
room temperature for 48 h. The yellow powder was col-
lected and washed with deionized water three times to
remove any residual palladium ions from the surface of
support. To determine the Pd content, 1.0 g of solid was
treated with mixture of concentrated HCl and H2SO4 (3/
1)(8 ml) at 50–60 °C. The filtrate was diluted to 250 ml
with distilled water and analyzed with ICP spectroscopy
using calibration curve method. The loading amount of Pd
(II) is calculated to be 0.276 mmol/g of polymer support.
1.0 g of collected solid was redispersed in aqueous
solution (25 ml) of freshly prepared NaBH4 (1.0 M) and
stirred for 60 min at room temperature. The yellow powder
became black due to reduction of Pd (II) to Pd (0).
3 Results and Discussion
3.1 Preparation and Characterization
of the Catalyst
Contrary to the commercial application of polyolefins, the
lack of reactive groups has restricted many uses of these
kinds of polymers, including interaction with other mate-
rials. Chemical modification of polyolefins, introduces a
controlled amount of functional groups onto the polyolefin
backbone, which improve some properties such as solu-
bility, hydrophilicity, and ability of metal loading [20].
One of the most useful ways for creating appropriate
functional groups on the surface of inert polymers is
grafting of dendrimers or hyperbranched polymers [21].
Hyperbranched grafting provides useful functional groups
on the polyethylene backbone.
2.5 General Procedure for Suzuki Reaction
One of the most employed methods for surface modifi-
cation of polyolefins is the oxidation reaction using various
soluble manganese and chromium oxidants, such as
potassium dichromate or chromium trioxide in sulfuric
acid. With this manner, carboxylic acid and ketones are the
most functional groups which be formed on the polymer
surface [26, 27]. These functional groups provide the
selective sites for the subsequent hyperbranched grafting.
Thus, we focused on chemical modification of poly-
ethylene and then hyperbranched grafting to provide suf-
ficient functional groups for loading of palladium. The
hyperbranched polyester-grafted-polyethylene (HBPE-g-
PE) supported palladium catalyst was synthesized follow-
ing the simple procedure shown in Scheme 1.
HBPE-g-PE containing Pd nanoparticles (7.0 mg), aryl
halide (1.0 mmol), phenylboronic acid (1.5 mmol), and
K2CO3 (2.0 mmol) were added to mixture of H2O/DMF
(1:1) (2.5 ml). The reaction mixture was stirred in a flask at
90 °C under reflux for the indicated time. The reaction was
monitored by TLC. On completion of the reaction, the
catalyst was separated by simple filtration. Ethyl acetate
was added to the mixture and decanted. After evaporation
of solvent, the products were subjected to GC analyzer to
obtain yields of the reactions.
2.6 General Procedure for Solvent-Free Aerobic
Oxidation of Alcohols
Oxidized-PE powder with carboxylic acid functional
groups (PE–COOH) was obtained by oxidation of PE with
mixture of K2Cr2O7/H2SO4/H2O as the oxidant. The con-
tent of created carboxylic acid by oxidation reaction is low
for immobilizing high amount of palladium on the surface.
Therefore, hyperbranched polyester with highly amounts of
Oxidation of alcohols with palladium nanoparticles sup-
ported on PE-HBPE was performed using molecular oxy-
gen in the absence of solvent. For this purpose, 5.0 mg of
the catalyst was dispersed in alcohol (5.0 ml) in a three-
necked round flask with a reflux condenser under stirring.
123

Synthesis and Application of Hyperbranched Polyester-Grafted Polyethylene (HBPE-g-PE)…
Scheme 1 Preparation of
hyperbranched polyester-g-
polyethylene supported
palladium
terminal hydroxyl groups was used to enhance loading
capacity of palladium. For this aim, hyperbranched
polyester as the palladium nanoparticles template was
attached to modified polyethylene. Esterification reaction
between carboxylic acid of oxidized-PE and hydroxyl
group of hyperbranched polyester is slow and needs high
temperature. Hence, the carboxylic acid groups of oxi-
dized-PE were converted to acid chloride functionality
using thionyl chloride to accelerate grafting process. In
order to prevent hydrolysis of acid chloride groups with
moisture, the prepared powder was isolated and employed
in next step immediately.
terminal hydroxyl groups was synthesized via the melt
polymerization of 2,2-Bis (methylol) propionic acid (AB2
monomer) and 2-Ethyl-2-(hydroxymethyl)-1,3-propanediol
(B3 monomer, as the core moiety) using p-Toluenesulfonic
acid as the catalyst in a pseudo-one-step reaction [25] and
then was covalently linked to the modified PE-support
through ‘‘grafting to’’ approach. Degree of grafting was
estimated to be 33.3 % by weighting the powder before
(2.0 g) and after (2.66 g) grafting. After grafting, in order
to achieve better chelating of Pd (II), terminal hydroxyl
groups of HBPEG4 were converted to carboxylic acid
functionality in presence of succinic anhydride.
Fourth generation of hyperbranched polyester
(HBPEG4), with a well-defined structure and numerous
For preparation of Pd nanoparticles supported system, at
first the Pd salt were coordinated with functional groups of
123

H. Mahdavi,R. Sahraei
the support and then reduced to Pd particles by reducing
agent. Palladium deposition onto the HBPE-g-PE support
was accomplished by immersing the powder in Na2PdCl4
aqueous solution, which leads to homogeneous dispersion
of palladium on the surface of the support through
chelating of Pd (II) with hyperbranched polyester end
groups. NaBH4 is a very convenient reducing agent, which
reduces Pd (II) to Pd (0) easily at room temperature.
Reduction of Pd (II) to Pd (0) causes the yellow powder
turned to brown and then dark after 1 h. There is no need to
use stabilizers in the reaction mixture because it seems that
highly numerous functional groups of hyperbranched
polyester are capable of stabilizing the Pd nanoparticles
and prevent aggregation, which increases surface area and
activity of the catalyst. The palladium loading content was
determined to be 0.276 mmol/g by ICP analysis.
carboxylic acid. The broad band at around 3390 cm-1 is
attributed to carboxylic acid O–H stretch and the C–O
stretch appears in the region 1100–1200 cm-1. Therefore,
oxidation of polyethylene and creating carboxylic acid
functionality has occurred successfully after 4 h. The ATR-
FTIR spectrum of HBPE-g-PE shows the absence of
absorption band corresponding to carbonyl stretch of car-
boxylic acid at 1710 cm-1 and the presence of the ester-
carbonyl band at 1729 cm-1 corresponding to hyper-
branched polyester. The peaks at 1100–1200 cm-1 and a
wide band in the region 3300–3400 cm-1 due to C–O and
terminal hydroxyl groups, respectively, are implying the
successful grafting of hyperbranched polyester onto the
surface of modified PE.
The surface morphology of prepared material was stud-
ied using scanning electron microscopy (SEM). Figure 2
presents the SEM images for pure PE, PE-COOH, HBPE-g-
PE and HBPE-g-PE containing palladium nanoparticles.
Surface morphology of polyethylene was clearly changed
by creation of different functionalities. The surface of pure
polyethylene is very smooth and flat. After the oxidation
process, some functional groups are generated in the surface
and change the surface morphology. Examination of Fig. 2c
shows that the stiffness of surface is more increased after
grafting and creation of highly functional groups related to
hyperbranched polyester. Comparison of Fig. 2a–c shows
increasing trend of surface stiffness along oxidation and
grafting of hyperbranched polyester.
Figure 1 displays the ATR-FTIR spectra of synthesized
hyperbranched polyester, neat PE, oxidized-PE, and
HBPE-g-PE. For hyperbranched polyester, the absorption
frequencies of carbonyl and C–O appear at 1722 and
1100–1200 cm-1 respectively and
a broad peak is
observed at around 3370 cm-1 related to terminal hydroxyl
groups. For polyethylene, the observed absorbance at
1467 cm-1 corresponds to C–C stretching and C–H
stretching peaks are observed at 2849 and 2916 cm-1. For
oxidized-PE containing carboxylic acid groups, new peak
is observed at 1710 cm-1 due to the carbonyl stretch of
The elemental distribution of C, O and Pd within HBPE-
g-PE containing palladium nanoparticles and EDX spec-
trum of HBPE-g-PE supported Pd catalyst are presented in
Fig. 3a, b. Typical carbon, oxygen and palladium peaks are
detected by EDX analysis, which prove Pd is successfully
loaded into HBPE-g-PE support. Alternatively, elemental
maps further confirm the successful loading of Pd into the
support with homogeneous dispersion and uniform size
distribution. Histogram of the Pd particles size distribution
(Fig. 3c) was determined by counting about 350 particles.
It could be observed that Pd particles are in diameters from
2 to 19 nm, and the average particle size is * 11.5 nm.
Then it is possible to conclude that the palladium particles
are well dispersed within the catalyst in nanometer scale.
Figure 4 shows the TEM images of the HBPE-g-PE
containing palladium nanoparticles and the histogram of
the PdNPs size. The dark spots are the Pd nanoparticles
with a size in the range of 2.0–16 nm. The size distribution
diagram estimated from the TEM image shows that the
mean size of Pd is *7.0 nm, which is in agreement with
results obtained from elemental maps. These observations
revealed good stabilizing properties of hyperbranched
polyester (HBPE), which prevent the aggregation of
nanoparticles. These relatively small particles with narrow
size distribution and high surface area can perform
Fig. 1 ATR-FTIR spectra of HBPEG4, PE, PE-COOH, and HPBE-g-
PE
123

Synthesis and Application of Hyperbranched Polyester-Grafted Polyethylene (HBPE-g-PE)…
Fig. 2 SEM images of (a) neat
polyethylene, (b) oxidized-
polyethylene, (c) HBPE-g-PE
and (d) HBPE-g-PE containing
palladium nanoparticles
Fig. 3 (a) SEM mapping photograph of C, O and Pd in HBPE-g-PE containing palladium nanoparticles; (b) EDX pattern of the catalyst; and
(c) Histogram of the Pd nanoparticles size
catalytic reactions in high activity and selectivity. Thus,
HBPE with numerous functional groups is an attractive
supporting agent for palladium catalysts.
an acceptable dispersion in polar solvents. Low density
polyethylene has semi-crystalline structure and consists of
carbon and hydrogen chains with no functionality. Thus,
solubility of LDPE is very low in existing solvents at the
ambient temperature. Polar carboxylic acid groups caused
The presence of highly hydroxyl groups increases sur-
face hydrophilicity and therefore, HBPE-g-PE should have
123

H. Mahdavi,R. Sahraei
Fig. 4 (a) Low and high
magnification TEM images of
HBPE-g-PE containing
palladium nanoparticles and
(b) histogram of the Pd
nanoparticles immobilized on
HBPE-g-PE
better dispersion of PE-COOH in polar solvents. After
grafting of hyperbranched polyester on the surface of
polyethylene and creating highly amounts of terminal
hydroxyl groups, solubility of powder was further
increased. Figure 5 illustrates enhancement of dispersion
of polyethylene in DMF as polar solvent at room temper-
ature after surface modification and grafting process. As
expected, increasing surface functional groups enhances
dispersing ability of polyethylene in polar solvents.
X-ray powder diffraction (XRD) pattern of HBPE-g-PE
containing palladium nanoparticles is shown in Fig. 6.
Observed diffraction peaks at 43.58, 49.38, 70.62, and
84.41° correspond to (111), (200), (220), and (311) crys-
tallographic planes of Pd (0) nanoparticles, respectively.
The sharp peaks are appeared at 21.66 and 23.83° corre-
spond to (110) and (200) planes of (CH2)n chains of
polyethylene, respectively. The crystallite size of
nanoparticles was calculated by using Scherrer equation
from XRD pattern data to be 10.6 nm which is in a good
agreement with the mean particle size observed from TEM
and SEM mapping images.
Fig. 5 Dispersion ability of (a) polyethylene (b) oxidized-polyethy-
lene (c) HBPE-g-PE in DMF as polar solvent at room temperature
Thermal studies were conducted to determine the
applicability of catalyst in reactions operating at the high
temperatures. Thermal stability of neat polyethylene was
compared to HBPE-g-PE and HBPE-g-PE containing
123

Synthesis and Application of Hyperbranched Polyester-Grafted Polyethylene (HBPE-g-PE)…
Fig. 6 XRD patterns of hyperbranched polyester-g-polyethylene
containing palladium nanoparticles
Fig. 7 TGA curves of neat polyethylene, HBPE-g-PE, and HBPE-g-
PE containing palladium nanoparticles (HBPE-g-PE/PdNPs)
Fig. 8 DSC thermograms of (a) neat polyethylene; (b) hyperbranched
polyester-g-polyethylene containing palladium nanoparticles
palladium nanoparticles (HBPE-g-PE/PdNPs) by means of
TGA upon heating the samples at the rate of 10 °C min-1
from 25 to 600 °C. Breaking the C–C bonds of poly-
ethylene backbone is accomplished at high temperatures
and requires high energy. As shown in Fig. 7 for PE, the
weight loss due to decomposition starts above 420 °C
which indicating excellent stability of polyethylene at
elevated temperatures. Thermal decomposition of hyper-
branched polyester starts at about 250–275 °C [28].
Grafting of HBPE onto the polyethylene surface induces
that decomposition starts at lower temperatures (310 °C).
Comparing the HBPE-g-PE and HBPE-g-PE/PdNPs curves
shows that the weight loss of HBPE-g-PE/PdNPs occurs at
a higher temperature (360 °C) which may be attributed to
the presence of Pd nanoparticles in the polymer matrix.
Therefore, the prepared catalyst shows excellent thermal
stability and preserves its activity and structure at high
temperatures. Therefore, using the catalyst in catalytic
reactions conditions has no limitation.
transition temperature of polyethylene after grafting and
immobilizing of Pd nanoparticles. LDPE is a semi-crys-
talline polymer with low glass transition temperature
(-47.09 °C) and grafting of hyperbranched polyester
increases amorphous segments in polymer backbone.
Hydrogen-bonding of hydroxyl end groups of hyper-
branched polyester that are not coordinated with palladium,
increases Tg of the support. In addition partial loss of the
conformational freedom of hyperbranched polyester seg-
ments due to complexation with palladium enhances glass
transition temperature.
On the other hand, the presence of palladium between
polymer chains facilitates segmental motions of chains and
decreases glass transition temperature slightly. All these
reasons cause the glass transition temperature of HBPE-g-
PE to be observed at 9.45 °C. Melting point of pure PE and
HBPE-g-PE supported palladium nanoparticles are 110.28
and 109.74 °C, respectively. Due to grafting is performed
in amorphous areas of polyethylene and crystalline areas of
DSC analyses of pure PE and HBPE-g-PE containing
palladium nanoparticles are shown in Fig. 8. Comparison
of the diagrams indicated a significant increase in glass
123

H. Mahdavi,R. Sahraei
PE remain intact, melting point of polyethylene will not
change remarkably.
selectivity toward substituted biphenyl products have been
achieved for all substrates. Examination of Table 1 indi-
cates that 86 % cross-coupling yield can be achieved by
low content of catalyst for the reaction of iodobenzene with
phenylboronic acid within just 1 h.
3.2 Catalytic Activity Study of the Catalyst
in Suzuki–Miyaura Reaction
A product yield of 78 % was observed when the sub-
strates were 1-iodo-4-methylbenzene and phenylboronic
acid (entry 7). With the aryl halide substrate changed to
4-iodonitrobenzene, the best yield (96 %) was obtained at
90 °C and within 1 h.
Nowadays, the Suzuki reaction is one of the most popular
coupling reactions in organic synthesis. Here, the catalytic
property of the prepared catalyst was initially examined in
Suzuki reaction. The coupling reaction between aryl halide
and arylboronic acid with electron-donating and electron-
withdrawing substituents proceeded effectively to obtain
corresponding products in good to excellent yields. The
coupling of iodobenzene with phenylboronic acid was
studied initially as a model reaction. The HBPE-g-PE
supported catalyst acted as a heterogeneous catalyst in this
catalytic reaction successfully. To optimize reaction con-
dition, the catalytic reactions were carried out with dif-
ferent solvents, bases and at different temperatures.
Optimization studies determine how solvent, base, and
temperature influence the coupling reaction. Tetrabuty-
lammonium bromide as a phase transfer agent was added to
enhance the performance of the catalyst in the aqueous
medium. The Results that are given in Table 1 summarize
Suzuki coupling of aryl halide and arylboronic acid con-
taining various substituents. Isolated yields of the products
were good to excellent within short reaction times and high
A moderately good yield of 76 % was observed for
idodobenzene and 4-bromophenylboronic acid substrates.
Compared to iodobenzene, the reaction took longer when
the substrate was bromobenzene. Decrease in activity was
observed when the halide substituent was changed to
chloride. Chlorobenzene was generally less reactive toward
Suzuki reaction under the same condition. The best result
was observed when the reaction was performed in DMF/
H2O (1:1). Further catalytic reactions in H2O/DMF (1:1)
using different bases were carried out, and it was found that
K2CO3 act as the best bases (entries 12–14). Other bases
such as K3PO4, and NaOH were slightly less effective to
obtain high product yield. When the reaction was carried
out without a significant content of base, no activity was
observed even at a prolonged reaction time. Therefore, the
presence of base as the activator of organo-boron com-
pounds is necessary in the reaction mixture. Influence of
Table 1 Suzuki coupling of aromatic halides with different Arylboronic acids
Entry
Aryl halide
Arylboronic acid
Time (h)
Temperature
(°C)
Solvent
Base
Yield (%)
Selectivity toward
related biphenyl
product (%)
1
C6H5I
C6H5Br
C6H5Cl
C6H5I
C6H5I
IC6H4NO2
C7H7I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
CH3C6H4B(OH)2
BrC6H4B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
1
4
6
1
3
1
1
2
2
2
1
3
2
2
1
1
90
90
90
90
90
90
90
90
90
90
90
90
90
90
70
100
DMF/H2O (1:1)
DMF/H2O (1:1)
DMF/H2O (1:1)
DMF/H2O (1:1)
DMF/H2O (1:1)
DMF/H2O (1:1)
DMF/H2O (1:1)
H2O/Dioxane
Dioxane
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
None
86
81
72
92
76
96
78
75
62
77
78
–
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
H2O
DMF
DMF/H2O (1:1)
DMF/H2O (1:1)
DMF/H2O (1:1)
DMF/H2O (1:1)
DMF/H2O (1:1)
K3PO4
NaOH
K2CO3
K2CO3
78
73
72
78
123

Synthesis and Application of Hyperbranched Polyester-Grafted Polyethylene (HBPE-g-PE)…
temperature was investigated by performing the reaction at
70, 90, and 100 °C. Efficiency was increased with
increasing temperature from 70 to 90 °C. Partial decrease
was observed at 100 °C which can be attributed to the
agglomeration of palladium nanoparticles and then
decreasing the available number of active sites. Thus, the
best system for this reaction was 2.5 ml of H2O/DMF (1:1)
as solvent and K2CO3 as base (2.0 mmol) at 90 °C. The
reactions were performed well under this optimized con-
dition, and the appropriate products were obtained in good
to excellent yields within 1–4 h.
boiling point of final products. As presented in Table 2,
conversion yield increases at elevated temperatures, but
products selectivity is still high (95–100 %) at lower
temperature. Performing the reaction under solvent-free
condition has several advantages such as saving costs,
reducing environmental pollution, removing solvent sepa-
ration step, and raising purity of oxidized alcohols.
3.3 Performance of the Catalyst in Solvent-Free
Aerobic Oxidation of Alcohols
Nowadays, oxidation of alcohols is performed using stoi-
chiometric oxygen donors such as permanganate or chro-
mate in a commercial scale. Recently, some attempts were
conducted to replace molecular oxygen for oxidation of
alcohols to reduce environmental concerns [29]. One of the
most useful transformations among Pd-catalyzed reactions
is the selective oxidation of alcohols to corresponding
carbonyl compounds including aldehydes and ketones.
Palladium catalysts can oxidize alcohols using molecular
oxygen without any use of base and under mild solvent-
free reaction conditions [30].
In this work, we chose a number of primary or sec-
ondary alcohols to perform oxidation reaction and prepare
relevant aldehydes or ketones under solvent-free condition.
The low content of prepared catalyst (5.0 mg) revealed
good activity and high selectivity toward corresponding
aldehydes or ketones in aerobic oxidation of various
alcohols. Various alcohols oxidize to aldehydes or ketones
with different boiling temperature. Considering that, tem-
perature of the reactions was changed according to the
Fig. 9 Recycling experiments of supported Pd catalyst in (a) Suzuki
coupling of iodobenzene and phenylboronic acid; and (b) aerobic
oxidation of benzyl alcohol
Table 2 Solvent-free aerobic oxidation of various alcohols
Entry
Alcohols
Temperature (°C)
Time (h)
Yield (%)
Selectivitya (%)
1
2
3
4
a
Benzyl alcohol
Cyclohexanol
1-Butanol
90
90
65
40
2
4
5
6
80
74
78
71
95
100
98
2-Propanol
100
Selectivity refers to the corresponding aldehyde or ketone; for benzyl alcohol, by-products are toluene and benzoic acid; for 1-butanol, by-
product is butyric acid
123

H. Mahdavi,R. Sahraei
Table 3 Suzuki coupling reaction of iodobenzene with phenylboronic acid catalyzed by different heterogeneous Pd-based catalysts
Entry
Catalyst
Condition
Time
Yield (%)
Ref.
1
SMNPs-Salen Pd (II)
DMF/H2O, K2CO3, 100 °C
H2O/EtOH, Na2CO3, 40 °C
DMF, K3PO4, reflux temperature
Xylene, K2CO3, 130 °C
1 h
100
99
[1]
2
Pd/MIL-53(Al)-NH2
30 min
20 min
6 h
[2]
3
FemSILP-NHC-Pd complex
Chitosan–2(diphenylphosphino)imine–Pd
Pd/g-C3N4
52
[3]
4
34
[4]
5
PEG/H2O, K2CO3, r.t.
30 min
6 h
99.9
98
[5]
6
Pd-1/FSG
MeOH/H2O, K2CO3, 80 °C
Ethanol, K2CO3, reflux temperature
Xylene, K2CO3, 143 °C
[6]
7
Pd–ZnFe2O4 MNP
4 h
92
[7]
8
Chitosan-pyridyl imine Pd complex
8.64 %Pd(B)-MCM41
Polymer-anchored Pd(II) complex
Pd@g2Amino-Si(HIPE)P2
HBPE-g-PE/Pd NPs
1 h
85
[8]
9
DMF/H2O, Et3N, 80 °C
2 h
56
[9]
10
11
12
DMF/H2O, K2CO3, 80 °C
Dioxane, K2CO3, 115 °C
DMF/H2O, K2CO3, 90 °C
5 h
100
98
[10]
[11]
This work
72 h
1 h
86
Table 4 Comparison of the catalytic performance of resulting HBPE-g-PE/PdNPs catalyst with different reported heterogeneous Pd-based
catalyst employed in solvent-free aerobic oxidation of benzyl alcohol
Entry
Catalyst
Temperature (°C)
Time (h)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
9
10
Pd-GC
110
140
160
140
120
100
80
6
3
1
1
4
4
7
1
1
2
72.5
80
[12]
2.5 % Au ? 2.5 % Pd/TiO2DP2
1Pd/MPTMS-TUD
1Au5Pd/APS-S16
2 %Pd/SBA-15-900 N
Pd/NaX
[13]
26.3
22.3
70
[14]
[15]
[16]
66
[17]
Pd@C-GluA-550
Pd/2.5MnOx/CNT
Pd/CeO2
70
[18]
160
120
90
15.4
95.7
80
[19]
[20]
HBPE-g-PE/Pd NPs
This work
Effect of O2 as co-oxidant and hydrogen acceptor [30]
was evaluated by carrying out the oxidation of 2-propanol
in presence and absence of oxygen. Presence of oxygen
enhances catalytic performance of the catalyst and facili-
tates the oxidation process.
could be used as a heterogeneous catalyst in catalytic
reactions.
3.5 Recycling of the Catalyst
3.4 Hot Filtration Test
Recycling of the catalyst was studied in Suzuki coupling of
iodobenzene and phenylboronic acid, and aerobic oxidation
of benzyl alcohol as the model reactions. As presented in
Fig. 9, after five consecutive runs, the catalytic activity of
collected powder was still high and the catalyst required a
bit longer time to achieve high conversion.
Hot filtration test is a way to show the catalyst acts
heterogeneously in the reaction condition. Metal leaching
is a common problem for the heterogeneous catalysts [3].
To investigate content of palladium leached into the reac-
tion mixture, the catalyst was separated by simple filtration
after completion of catalytic reactions, and the filtrate was
analyzed by ICP. No palladium leaching was observed by
ICP analysis of the filtrate after the end of the reaction.
Thus, the immobilized palladium was very stable in reac-
tion condition and the obtained products were free of
contamination with palladium, and the synthesized catalyst
Slight loss of activity in third run can be attributed to
aggregation of nanoparticles and poisoning of the active
sites. Thus, the recovery can be successfully accomplished
up to five cycles without significant loss of activity.
Reusability of the catalyst leads to using low toxic and
expensive palladium and reducing the cost of products in
large-scale processes.
123

Synthesis and Application of Hyperbranched Polyester-Grafted Polyethylene (HBPE-g-PE)…
5. Mauger C, Mignani G (2005) The Synthesis of important phar-
A comparison of the activity of various heterogeneous
Pd-based catalysts with HBPE-g-PE/PdNPs in the Suzuki
coupling reaction and solvent-free aerobic oxidation of
alcohols published in the literature is listed in Tables 3 and
4 [30–49].
maceutical building blocks by palladium-catalyzed coupling
reaction: access to various arylhydrazines. Adv Synth Catal
347(6):773–782
6. Yin L, Liebscher J (2007) Carbon-carbon coupling reactions
catalyzed by heterogeneous palladium catalysts. Chem Rev
107(1):133–173
7. Garrett CE, Prasad K (2004) The art of meeting palladium
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Some of listed heterogeneous catalysts require higher
temperatures or prolonged reaction times in order to obtain
excellent yields for both catalytic reactions (e.g.,
Pd@g2Amino-Si(HIPE)P2 for Suzuki reaction and Pd-GC
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4 Conclusions
In summary, we have prepared an efficient palladium cat-
alyst supported hyperbranched polyester-grafted-poly-
ethylene, which was used for Suzuki reaction and solvent-
free aerobic oxidation of alcohols. Hyperbranched polye-
ster grafted on the modified PE support acts as both sta-
bilizer and well-defined template for high loading of
palladium nanoparticles with homogeneous distribution in
this process. Prepared catalyst displayed a good conversion
and excellent selectivity in these catalytic reactions, sug-
gesting the effective catalytic activity within short reaction
times. The catalyst could be recovered by simple filtration
and reused for five successive runs without significant loss
of activity. No palladium leaching was observed during
reactions. The hyperbranched polyester has a high ability to
entrap palladium without remarkable leaching of the cat-
alyst into the reaction mixture. The TGA results confirmed
the excellent thermal stability of the catalyst at high tem-
peratures, which has no restriction for carrying out the
reactions at evaluated temperatures. Therefore, short
reaction times, high product yields, excellent selectivity,
simple separation, good reusability, excellent thermal sta-
bility, low cost, and no Pd leaching which lead to low
contamination of isolated products are main features of the
prepared catalyst.
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