Polymer International
Polym Int 53:1169–1175 (2004)
DOI: 10.1002/pi.1527
Chain structure of polyethylene/polypropylene
in-reactor alloy synthesized in gas phase with
spherical Ziegler–Natta catalyst
Zhisheng Fu, Zhiqiang Fan,∗ Yanzhong Zhang and Junting Xu
The Institute of Polymer Science, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
Abstract: Two polyethylene/polypropylene (PE/PP) in-reactor alloy samples were synthesized by multistage gas-phase polymerization using a spherical Ziegler–Natta catalyst. The alloys show excellent
toughness and stiffness. FTIR, 13 C-NMR and thermal analysis proved that the alloys are mainly composed
of polyethylene, PE-block-PP copolymer and polypropylene. There are also a few percent of ethylenepropylene segmented copolymer with very low crystallinity. The block copolymer fraction accounts for
more than 25 % of the alloy. The role of the block copolymer as compatibilizer between PE and PP is
believed to be the key factor that results in the excellent toughness–stiffness balance of the material.
 2004 Society of Chemical Industry
Keywords: chain structure; PE/PP in-reactor alloy; gas phase; spherical catalyst; mechanical properties
INTRODUCTION
Modification of polypropylene (PP) aimed at improving impact strength is a topic of great significance in
science and industry.1 – 6 Among the ways to toughen
PP, in-reactor blending of PP with other polyolefins
(eg, ethylene-propylene statistical copolymer) by
sequential multi-stage polymerization has been proved
superior both in respect of polymer properties and
production cost.4 – 10 Polypropylene/poly(ethylene-copropylene) (PP/EPR) in-reactor alloy has been industrialized on large scale. However, because there is
more than 10 % of statistical copolymer with very
low modulus in the PP/EPR in-reactor alloy, this
kind of toughened PP suffers from a significant drop
in flexural modulus as compared to PP homopolymer. A possible way to overcome this drawback is
to replace the EPR with polyethylene (PE) in the
alloy, as PE is a crystalline polymer with moderate
rigidity. Though the preparation of PP/PE blends by
multi-stage polymerization was reported many years
ago,11 the toughness–stiffness balance of the material
seems not to be attractive, and commercialization of
this kind of polyolefin is quite limited compared to
PP/EPR in-reactor alloy (so-called block PP or PP-b).
Chain structure and physical properties of propyleneethylene sequential polymerization products, which
were prepared by using TiCl3 -based catalysts, have
been studied by several groups.12 – 16 As the PP and
PE homopolymer chains were not separated from the
block copolymer chains in these studies, the structure
of the components of the studied polyolefins were not
identified on solid experimental bases.
Since the 1990s, the main progress in the production
of PP in-reactor alloy has been the use of a
spherical Ziegler–Natta catalyst.5,6,10,17 PP/EPR alloy
synthesized by spherical catalyst is in the form of
regular spherical granules and shows better mechanical
properties compared to the conventional catalyst
system. The main reason for the better properties
of the spherical in-reactor alloy is the more uniform
dispersion of the second polymer in the alloy, as
the polymer formed in the second stage (ie EPR)
is limited to the tiny pores of the spherical polymer
particles produced in the first stage of polymerization.
The polymerization process also benefits from the
spherical shape of the polymer granules, as risks of
scaling and fouling in the reactor can be reduced.
However, the preparation of PP/PE in-reactor alloy
using spherical Ziegler–Natta catalyst has not been
reported in the literature.
Recently we have studied the chain-structure of
PP/PE in-reactor alloy using a super-active spherical
Ziegler–Natta catalyst.18 It was found that the
in-reactor alloys show both high impact strength
and high flexural modulus. Such good balance
between toughness and stiffness is very important for
applications as high-performance structural materials.
However, when the in-reactor alloy is synthesized by
first polymerizing propylene to form spherical PP
granules in the first stage and then polymerizing
∗ Correspondence to: Zhiqiang Fan, The Institute of Polymer Science, Department of Polymer Science and Engineering, Zhejiang University,
Hangzhou 310027, China
E-mail: fanzq@zju.edu.en
Contract/grant sponsor: Special Funds for Major State Basic Research Projects; contract/grant number: G1999064803
(Received 30 June 2003; revised version received 17 August 2003; accepted 3 February 2004)
Published online 1 June 2004
 2004 Society of Chemical Industry. Polym Int 0959–8103/2004/$30.00
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Z Fu et al
ethylene in the second stage, the particles of the
final product are mostly broken. This phenomenon
actually eliminates the advantages of using a spherical
catalyst. In this paper, we will report on the chain
structure and mechanical properties of a new type of
in-reactor alloy composed of PP and PE, which is
synthesized by polymerizing ethylene in the first stage
and propylene in the second stage in a sequential
reaction mode. To distinguish this type of polyolefin
alloy from the conventional PP/PE alloy reported in
the literature, we name it as PE/PP in-reactor alloy.
Our experiment results show that the granules of the
PE/PP alloy are mostly in the spherical form, and the
toughness–stiffness balance of the alloy is even better
than that of the conventional PP/PE alloy.
EXPERIMENTAL
Synthesis of the PE/PP alloy
The PE/PP in-reactor alloy was synthesized in a threestage polymerization process. In the first stage, or the
pre-polymerization stage, the slurry polymerization of
propylene was conducted in a well-stirred glass reactor
for 30 min. A high-yield spherical Ziegler–Natta
catalyst, TiCl4 /MgCl2 ·ID (ID is internal donor
diisobutyl phthalate), kindly donated by the Beijing
Research Institute of Chemical Industry, was used in
the polymerization. The catalyst has a Ti content
of 3 wt%. Al(C2 H5 )3 (Fluka) was used as the
cocatalyst (with Al/Ti = 60) and Ph2 Si(OCH3 )2 as the
external donor (Al/Si = 25). 30 ml petroleum ether
(bp 60–90 ◦ C) was used as the solvent. Propylene
pressure in the pre-polymerization stage was 1 atm,
and the temperature was 50 ◦ C. A catalyst efficiency
of 15–20 g PP/g catalyst was obtained in this stage.
After the pre-polymerization, the slurry containing the
pre-polymerized catalyst was transferred to a 0.5 l
jacketed Buchiglasuster autoclave. Propylene in the
slurry was removed by evacuating the autoclave to
5 mmHg for 3 s, and ethylene was filled into the
autoclave to 0.6 MPa. Ethylene homopolymerization
was carried out for 1 h. Polymerization temperature for
the high ethylene content sample (HEP) was 70 ◦ C,
and that for the low ethylene content sample (LEP)
was 60 ◦ C. It was found that after about 20 min of
ethylene polymerization, all the petroleum ether in
the reactor was thoroughly absorbed into the polymer
granules, so the polymerization can be regarded as a
gas-phase process. At the end of this stage, ethylene
was removed by evacuating to 5 mmHg for 3 min,
and propylene was filled to the autoclave and then
continuously supplied to the reactor at 0.7 MPa for 2 h.
After the gas-phase propylene polymerization stage,
the reaction was terminated, and the product was
washed with ethanol and dried in vacuum. The total
catalyst efficiency for the three-stage polymerization is
in the range of 3–4 × 103 g polymer/g catalyst.
Fractionation of PE/PP alloy
A modified Kumagawa extractor was used to carry
out temperature-gradient extraction fractionation
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(TGEF) of the polymer.19 n-Octane was used as the
solvent to successively extract the sample at a number
of controlled temperatures (room temperature, 90,
110 and 120 ◦ C). Five fractions were collected at
25, 90, 110, 120 and >120 ◦ C from each alloy, in
which the >120 ◦ C fraction is the residual sample
after the extraction. The fractions were named 25 ◦ C
fraction, 90 ◦ C fraction, 110 ◦ C fraction, 120 ◦ C
fraction and >120 ◦ C fraction. Purified fractions were
obtained after concentrating the extract solutions,
precipitating the polymer, washing and drying the
fractions in vacuum.
Measurements
FTIR spectra of the alloys and the fractions were
recorded on a Bruker Vector-22 spectrometer. Thin
films of polymer prepared by hot pressing were used
as samples.
13
C-NMR spectra of the fractions were measured
on a Bruker AMX400 NMR spectrometer at 100 MHz
o-Dichlorobenzene-d4 was used as solvent to prepare
the polymer solution of 20 wt%. The spectra were
recorded at 120 ◦ C, with hexamethyldisiloxane as
internal reference. Broadband decoupling and a pulse
delay of 5 s were employed. Typically 1000 transients
were collected. Ethylene content of the samples was
determined based on the peak intensity data.
Differential scanning calorimetry (DSC) analysis of
the fractions was performed with a Perkin-Elmer Pyris
1 thermal analyzer under nitrogen atmosphere. About
5 mg sample was sealed in an aluminium sample pan,
heated to 180 ◦ C for 30 min, and annealed at 130, 120,
110, 100, 90, 80, 70 and 60 ◦ C, respectively, each for
12 h. The DSC scan was then recorded at a heating
rate of 5 ◦ C min−1 from 30 ◦ C to 180 ◦ C.
The intrinsic viscosity of polymer fractions was
measured using an Ubbelohde viscometer at 135 ◦ C
with decahydronaphthalene as solvent.
Notched Charpy impact strength of the alloy samples was measured on a Ceast impact strength tester
according to ASTMD 256. The flexural modulus and
flexural strength were measured following ASTMD
709 on a REGER-2000 electronic tester. Sample
plates with sizes of 150 × 150 × 4 mm for mechanical
properties measurements were prepared by compression molding at 180 ◦ C for 5 min under a pressure
of 20 MPa. The samples were then cooled to room
temperature in about 2 h. The sample strips for the
tests were cut from the plates following ASTM.
RESULTS AND DISCUSSION
Structure and properties of the alloys
To investigate the influence of ethylene content in
the PE/PP alloy on its structure and properties, two
samples containing different amounts of ethylene were
synthesized and studied in this work. Figure 1 shows
the morphology of the PE/PP in-reactor alloy granules.
It can be seen that most of the granules have a spherical
shape, and the size distribution of the granules is rather
Polym Int 53:1169–1175 (2004)
Polyethylene/polypropylene chain structure
Figure 1. Morphology of the PE/PP in-reactor alloy granules.
Figure 2. FTIR spectra of HEP and LEP.
Table 1. The chemical composition and mechanical properties of
HEP and LEP
Sample
HEP
LEP
the two samples were fractionated into five fractions
by TGEF.
C2 (wt%)
Impact
strength
(kJ m−2 )
Flexural
strength
(MPa)
Flexural
modulus
(MPa)
82.3
43.4
94
90
19.6
25.7
2007
2634
narrow. This is a feature beneficial to the large-scale
production of PE/PP alloy in an industrial process.
In Table 1 the chemical composition and main
mechanical properties of the two samples are listed.
Though these two samples have very different ethylene
contents, they show quite similar impact strength. The
value of impact strength is much higher than that of a
conventional iPP homopolymer (about 4 kJ m−2 ). The
flexural modulus of the alloys is also higher than that of
conventional iPP (about 1600 MPa). This means that
the PE/PP in-reactor alloy has an excellent balance of
toughness and stiffness.
Figure 2 shows the FTIR spectra of the two
PE/PP samples. It can be seen that the doublet at
720–730 cm−1 is present in both samples, meaning
that both contain crystalline PE chains or segments.
The bands at 998 cm−1 and 841 cm−1 , which represent
isotactic PP chains, are also present in both samples.
Therefore, the PE/PP alloys are mainly composed
of PE and iPP chains or segments. According to
our previous work, there are quite a lot of PEblock-PP copolymer chains present in the PP/PE
in-reactor alloy.18 These block copolymer chains act as
compatibilizer between the PE and PP phases. To see
if such a block copolymer also exists in the PE/PP alloy,
Fractionation results
Table 2 lists the results of TGEF fractionation. The
two samples show very different fraction distributions.
The main difference is reflected in the amount of
the >120 ◦ C fraction. The sample containing lower
ethylene content (LEP) has a markedly higher amount
of >120 ◦ C fraction.
FTIR spectra of the fractions from the two
samples are shown in Fig 3. In all the fractions
collected between 25 and 120 ◦ C the doublet bands
at 720–730 cm−1 are clearly seen. The bands at
998 cm−1 and 841 cm−1 are also detectable in these
fractions. This means that the four fractions extracted
between 25 and 120 ◦ C are all block copolymers of PE
and PP. Differences in the lengths of the PE and PP
segments may have caused the differences in solubility.
On the other hand, the compositions of the >120 ◦ C
fractions from the two samples are quite different. The
>120 ◦ C fraction of sample LEP is nearly pure iPP,
while that of HEP is mainly composed of PE. In
our previous work,18 we found that PE homopolymer
fractions can be eluted at lower than 120 ◦ C in
temperature rising elution fractionation (TREF), and
PP chains of high isotacticity are extracted by TGEF
only at temperatures higher than 120 ◦ C. Therefore,
the fractionation result of HEP is surprising at first
sight. However, when we considered the very small
weight fraction of the >120 ◦ C fraction of HEP, we
began to suspect that the molecular weight may have
influenced the fractionation results. By comparing the
intrinsic viscosity data of the fraction extracted at
Table 2. Fraction distribution and ethylene content of LEP and HEP
LEP
Sample
Fraction (◦ C)
Fraction content (wt%)
C2 (mol%)
C2 (wt%)
HEP
25
90
110
120
>120
25
90
110
120
>120
0.3
32.9
24.6
3.4
58.5
48.4
37.7
95.7
93.7
25.4
33.2
24.9
33.2
0.9
0.6
0.2
87.0
81.7
4.1
59.7
49.7
47.1
98.8
98.2
47.5
77.1
69.2
1.1
92.3
88.9
Polym Int 53:1169–1175 (2004)
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Z Fu et al
Figure 3. FTIR spectra (1400–600 cm−1 ) of the fractions extracted from LEP and HEP at (a) 25 ◦ C; (b) 90 ◦ C; (c) 110 ◦ C; (d) 120 ◦ C; (e) >120 ◦ C.
120 ◦ C with that at >120 ◦ C (Table 3) and supposing
that the two fractions have similar Mark–Houwink
constants (this is reasonable as both fractions are
mainly composed of polyethylene), it is clearly seen
that the >120 ◦ C fraction of sample HEP has a much
higher molecular weight than the 120 ◦ C fraction. On
the other hand, the 120 ◦ C and >120 ◦ C fractions
of LEP have quite similar intrinsic viscosity values.
1172
Therefore, we may say that a small portion of PE
chains can be fractionated according to molecular
weight by TGEF.
Based on the FTIR analysis, we can deduce that
the 25 ◦ C fraction is PP-block-PE in which the PP
segments have low or medium isotacticity. The 110
and 120 ◦ C fractions are PP-block-PE of low and high
PP content, respectively. The >120 ◦ C fraction is PP
Polym Int 53:1169–1175 (2004)
Polyethylene/polypropylene chain structure
Table 3. Intrinsic viscosity of fractions extracted at 120 and >120 ◦ C
from LEP and HEP
LEP
Sample
[η](g/ml)−1
HEP
120 ◦ C
fraction
>120 ◦ C
fraction
120 ◦ C
fraction
>120 ◦ C
fraction
210.2
238.7
270.3
347.3
homopolymer when the PE/PP alloy has a high PP
content, but a small amount of high molecular weight
PE fraction may be found insoluble after extraction at
120 ◦ C when the alloy has a low PP content. According
to the fraction distribution data in Table 2, we can say
that sample HEP is mainly composed of the 110 and
120 ◦ C fractions, which are PE and PE-block-PP or
its mixture with PE, respectively, and LEP contains
large amounts of PP homopolymer, besides the block
copolymer fractions.
NMR analysis
In order to confirm the chain structure of the different
fractions, 13 C-NMR spectra of the 90, 110, 120 and
Figure 4.
13
>120 ◦ C fractions from HEP and LEP were recorded,
and these are shown in Fig 4.
The spectra of the two 90 ◦ C fractions are
quite similar to each other. There is a strong
peak at 28.1 ppm, which is the methylene signal
of long polyethylene sequences. There are several
peaks corresponding to the PPP sequence, such
as Pββ at 20.0 ppm, Tββ at 26.8 ppm and Sαα at
44.6 ppm, meaning that there are long PP segments
in the polymer chain. The peaks at 35.7–36.0 ppm
(Sαδ , Sαγ ), 31.3 ppm (Tδδ ), 25.4 ppm (Sβδ ), 22.9 ppm
(Sββ ) and 18.1 ppm (Pδδ ) indicate that there are also
sequences such as PPEP, PPEE, EPE and PEP in the
chain.20 The peaks at 12.2, 21.0 and 30.3 ppm could
be ascribed to high-boiling-point hydrocarbons that
may come from the TGEF solvent.
The spectra of the 110 ◦ C fractions are characteristic
of PE-block-PP copolymer with very low PP content.
Considering that PP homopolymers are soluble in
n-octane only at a temperature higher than 120 ◦ C,
the possibility that these fractions are PE/PP mixture
can be ruled out. The 110 ◦ C fraction of HEP has
lower content of long PP sequences than that of
LEP. The junction structure between the blocks
C-NMR spectra of the fractions extracted from LEP and HEP at (a) 90 ◦ C; (b) 110 ◦ C; (c) 120 ◦ C; (d) >120 ◦ C.
Polym Int 53:1169–1175 (2004)
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Z Fu et al
represented by the Sαδ , Sβδ and Tβδ peaks were not
detected, which may mean that both blocks are
very long.
The two 120 ◦ C fractions also show 13 C-NMR
signals typical of PE and PP. In the spectrum of
the 120 ◦ C fraction from HEP, trace amount of Sβδ
carbon was detected, which is an indication that
the PE and PP segments are actually connected
to form a block copolymer. Considering that there
is a propylene pre-polymerization stage before the
ethylene polymerization, it is very probable that
there is also some PP-block-PE-block-PP triblock
copolymer in the final product, besides the expected
diblock copolymer. However, the amount of such
a triblock copolymer is very limited, because the
weight of PP produced in the pre-polymerization
stage is only about 20 g/g catalyst, a value much
lower than the estimated total amount of block
copolymers [when the total yield is 3000 g polymer/g
catalyst and 20 wt% of it is the block copolymer (see
Table 2), about 600 g block copolymer/g catalyst will
be formed]. In other words, the triblock copolymer
will account for less than 10 % of the whole block
copolymer component. But it is still unclear whether,
and to what extent, the triblock copolymer chains
contribute to the excellent mechanical properties
of the materials. It is also noteworthy that the
120 ◦ C fraction from HEP has a lower propylene
content than that from LEP. Considering that the
former has a higher intrinsic viscosity value than
the latter, we believe that molecular weight may
also influence the TGEF fractionation of PE/PP inreactor alloy.
The 13 C-NMR spectra of the two >120 ◦ C
fractions are very different from one another.
The >120 ◦ C fraction from LEP is actually pure
iPP, whereas the fraction from HEP is basically
pure PE.
Thermal analysis
To verify the existence of crystallizable PE and PP
segments in the fractions, thermal analyses of annealed
samples of the fractions were made using DSC. Multistep annealing of the samples makes sure that the PE
and PP segments of different lengths form lamellae
of different thickness, thus the DSC melting curve
can reflect the presence of these different lamellae.
As shown in Fig 5, DSC melting curves of the five
fractions from LEP are very different from each
other. The room-temperature fraction is completely
amorphous. Though the FTIR spectrum of this
fraction shows the presence of PE segments with
carbon number larger than 10, these PE segments
are still not long enough to form crystalline lamellae.
On the other hand, the PP segments in this fraction
have too low isotacticity to form crystals. Therefore,
the room-temperature fraction is actually segmented
ethylene-propylene copolymer. Such a low amount
of segmented copolymer could be produced during
switching of monomers, because there will be a little
1174
ethylene left in the reactor (ie ethylene dissolved in
the polymer granules) after switching monomer from
ethylene to propylene.
The 90 ◦ C fraction shows several weak endothermic
peaks in the range 80–120 ◦ C. By considering the
results of 13 C-NMR and FTIR analysis, we believe that
these endotherms are mainly caused by the melting of
PE lamellae of relatively short thickness. This means
that the PE segments in this fraction are long enough
to form crystals, but there are also many short PE
segments, as the peaks are much weaker than a PE
homopolymer.
The DSC curve of the 110 ◦ C fraction shows a
strong peak at 132.9 ◦ C, which is close to the melting
temperature of PE homopolymer. There is also a very
weak endothermic peak at about 140 ◦ C, which may
be caused by the PP crystalline phase. Considering the
13
C-NMR results, we may conclude that this fraction
is composed of PE-block-PP copolymer with very
long PE segments and relatively short PP segments.
However, the existence of pure PE in this fraction
cannot be ruled out. Considering that there is an
ethylene homopolymerization stage in synthesizing the
Figure 5. DSC curves of the five fractions from LEP.
Polym Int 53:1169–1175 (2004)
Polyethylene/polypropylene chain structure
alloy, we believe that pure PE may account for the
main part of this fraction.
The melting curve of the 120 ◦ C fraction shows
two major melting peaks at temperatures similar to
twice of PE and PP, respectively. Moreover, there
is a weak shoulder at 142.6 ◦ C. From the result of
13
C NMR analysis, it can be said that this fraction
is mainly composed of PE-block-PP copolymer with
very long PE and PP segments. The weak endotherm
at 142.6 ◦ C indicates that the crystallization of a part
of the PP segment close to the conjunction point has
been interfered with by the PE phase, resulting in the
formation of an imperfect PP crystalline phase. Finally,
the >120 ◦ C fraction shows only one strong endotherm
at 165 ◦ C, meaning that it is nearly pure PP.
By combining the results of FTIR, NMR and
DSC analysis, we can get a clear map of the chain
structure and structure distribution of the PE/PP
alloy. In both samples, PE homopolymer is mainly
found in the 110 ◦ C fraction, and PP homopolymer
exists in the >120 ◦ C fraction. The 120 ◦ C fraction
is mainly composed of block copolymer with very
long PE and PP segments. These three fractions
constitute more than 95 % of the alloy. Results
presented in this paper also show that TGEF is very
similar to TREF18,21 in terms of the fractionation
mechanism. The extraction temperature, similar to the
elution temperature in TREF, is mainly determined
by the chemical composition and crystallinity of
the fraction. It is the higher extraction temperature
of crystalline PP segments than the PE segments
that makes the extraction temperature of PE-blockPP lower than that of PP but higher than that of
PE. Molecular weight of the fractions can influence
their solubility to a certain extent, and this will
add unexpected complexity to the explanation of the
fractionation results.
As shown in Table 2, both samples have plenty of
ethylene in the 110 ◦ C and 120 ◦ C fractions, but the
HEP sample has a much lower propylene content
than LEP. This difference in composition may explain
the difference in the mechanical properties of the two
samples. The higher flexural strength and flexural
modulus of LEP compared with HEP can mainly
be attributed to the higher propylene content of
the former. However, some kind of synergistic effect
between the PP and PE phases may also play an
important role, as the flexural modulus of the alloy is
even higher than that of a PP homopolymer, and the
impact strength of the alloy is also very high. The role
of the block copolymer fractions as the compatibilizer
between PE and PP should be the key factor to form
this kind of synergistic effect.
CONCLUSIONS
Two PE/PP in-reactor alloy samples were synthesized
by multi-stage gas-phase polymerization using a
Polym Int 53:1169–1175 (2004)
spherical Ziegler–Natta catalyst. The alloys show
excellent toughness–stiffness balance. Both samples
can be fractionated by temperature-gradient extraction
fractionation according to the chain structure of
the fractions. The 110, 120 and >120 ◦ C fractions
constitute more than 95 % of the alloy with high
propylene content, while the alloy with low propylene
content has almost no fraction extracted at higher
than 120 ◦ C. By FTIR, 13 C-NMR and thermal
analysis, it is proved that the alloy is mainly
composed of polyethylene, PE-block-PP copolymer
and polypropylene. There are also a few percent
of ethylene-propylene segmented copolymer with
very low crystallinity. The block copolymer fraction
accounts for more than 25 % of the alloy. The role of
the block copolymer as compatibilizer between PE and
PP is believed to be the key factor that results in the
excellent toughness–stiffness balance of the material.
ACKNOWLEDGEMENTS
Support by the Special Funds for Major State
Basic Research Projects (grant no. G1999064803)
is gratefully acknowledged.
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