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Title

Effects of various poisoning compounds on theactivity and stereospecificity of heterogeneous
Ziegler‑Natta catalyst

Author(s)Tangjituabun, Kitti; Kim, Sang Yull; Hiraoka,
Yuichi; Taniike, Toshiaki; Terano, Minoru;
Jongsomjit, Bunjerd; Praserthdam, Piyasan

Citation

Science and Technology of Advanced Materials, 9:024402‑1‑024402‑4

Issue Date

2008‑07‑07

Type

Journal Article

Text version

publisher

URL

http://hdl.handle.net/10119/8822

RightsCopyright (C) Institute of Physics and IOP
Publishing Limited 2008. This is the author‑
created version of Institute of Physics and IOP
Publishing Limited, Kitti Tangjituabun, Sang Yull
Kim, Yuichi Hiraoka, Toshiaki Taniike, Minoru
Terano, Bunjerd Jongsomjit and Piyasan
Praserthdam, Science and Technology of Advanced
Materials, 9, 2008, 024402‑1‑024402‑4.
http://dx.doi.org/10.1088/1468‑6996/9/2/024402

Description

Japan Advanced Institute of Science and Technology

IOP PUBLISHINGSCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS

Sci. Technol. Adv. Mater. 9 (2008) 024402 (4pp)

doi:10.1088/1468-6996/9/2/024402

Effects of various poisoning compounds
on the activity and stereospecificityof heterogeneous Ziegler–Natta catalyst
Kitti Tangjituabun1 , Sang Yull Kim2 , Yuichi Hiraoka3 , Toshiaki Taniike4 ,Minoru Terano4 , Bunjerd Jongsomjit1 and Piyasan Praserthdam1
1Center of Excellence on Catalysis and Catalytic Reaction Engineering,
Department of Chemical Engineering, Chulalongkorn University, Bangkok 10330, Thailand
2
Catalyst Research Division, Samsung Total Petrochemicals Co., Ltd., Chung Nam 356-711, Korea
3
Tokyo Research Laboratory, Yuki Gosei Kogyo Co., Ltd., Tokyo 174-0043, Japan
4
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi,
Ishikawa 923-1292, Japan
E-mail: terano@jaist.ac.jp

Received 16 January 2008
Accepted for publication 13 March 2008
Published 7 July 2008
Online at stacks.iop.org/STAM/9/024402
Abstract
A TiCl4 /ethylbenzoate/MgCl2 Ziegler–Natta catalyst was pretreated with chemically different
poisoning compounds to investigate their effects on the catalyst activity and stereospecificity
for propylene polymerization. The poisoning power on the activity was in the order ofmethanol > acetone > ethyl acetate. A kinetic analysis using the stopped-flow method
revealed that addition of the poisoning materials decreased the activity through the reductionof the number of active sites, whereas the catalyst isospecificity was hardly affected by thesematerials.
Keywords: poisoning material, Ziegler–Natta catalyst, stopped-flow propylene polymerization(Some figures in this article are in colour only in the electronic version)accompanied by an increase in the chain propagation rate, forwhich we reported that the chain propagation rate constant ofisospecific sites was several times higher than that of aspecificsites [6, 7]. Thus, the coordination of the electron donors canbe regarded as affecting the catalyst activity in the directionsof both activation and deactivation (normally the deactivation
effect by the donors is stronger than the activation effect,leading to a decrease in the total activity). However, up to now,the fundamental mechanisms concerning the activation anddeactivation behavior of the active sites have not yet been fully
elucidated and are still open to discussion as a consequenceof the complexity of the catalytic system, particularly in theabsence of precise kinetic studies on the number of active sites
and their chain propagation rate constants.
Recently, the stopped-flow technique has been
demonstrated to be one of the most powerful methods forstudying the kinetic and stereospecific natures of the active

1. Introduction
Highly active MgCl2 -based Ziegler–Natta catalysts have beenused for the majority of global polyolefin production during
the past several decades since their discovery in the 1950s.
Because of commercial use, much effort has devoted toensuring a continuous improvement in the catalyst activity
and stereospecificity through the discovery of increasingly
suitable electron donors [1]. The function of such donors in
the MgCl2 -supported catalysts is known to induce electronicand steric effects on active species by coordinating on the
catalyst surface in the vicinity of the active species, leading
to a marked increase in the stereospecificity of the catalyst
through the selective poisoning of nonstereospecific sites or
the transformation of aspecific sites to isospecific ones [2–5].
The selective poisoning leads to a decrease in the numberof active centers. On the other hand, the transformation is1468-6996/08/024402+04$30.00

1© 2008 National Institute for Materials Science

Printed in the UKSci. Technol. Adv. Mater. 9 (2008) 024402

K Tangjituabun et al

2.3. Slurry propylene polymerizationsites on the heterogeneous Ziegler–Natta catalysts used for
propylene polymerization [8]. The polymerization reaction
proceeds within an extremely short period (ca. 0.2 s),
exhibits quasi-living characteristics, where the states of
the active sites are kept constant without time-dependentchanges, and no side reactions including chain transfer and thedeactivation of the active sites occur during polymerization.
Because of these outstanding properties, direct information on
the nature of active sites and the polymerization mechanism
can be obtained [6, 9–11].
In the present study, a TiCl4 /ethylbenzoate/MgCl2
catalyst prepared via a chemical route from magnesium
ethoxide was pretreated by several chemically different
poisoning materials such as an alcohol, ketone and ester.
Simple and sterically small molecules were selected as
examples of poisoning compounds to clarify the mechanismof site deactivation without notable steric effects from these
poisoning materials, which may not generate isospecific sites
or even transforming aspecific sites into isospecific ones. A
precise kinetic study on the effect of the poisoning materials
was carried out using the stopped-flow polymerization
combined with gel permeation chromatography (GPC)
analysis.

Slurry polymerization was conducted to obtain preliminary
information on the deactivating effect of the poisoning
materials. It was performed in a 300 ml round-bottom glass
reactor equipped with a magnetic stirrer. The required
amounts of heptane (200 ml) and catalyst (0.095 mmol)
were added to the reactor and then heated to 30 ◦ C alongwith saturated propylene at atmospheric pressure. A specificamount of a poisoning material was introduced into thecatalyst vessel and reacted with the catalyst for 1 min followedby the injection of TIBA (Al/Ti molar ratio: 30) to startpolymerization. After 10 min, the reaction was terminated byventing the reactor and by adding acidic ethanol. The productwas washed with water several times and then evaporated anddried under vacuum at 60 ◦ C for 6 h.
2.4. Stopped-flow polymerization
The stopped-flow polymerization of propylene and thedetermination of the kinetic parameters were carried out inaccordance with the method reported previously [6, 8, 12–15].
The propylene polymerization was typically performed with acatalyst (ca. 1.0 g) and TIBA (14 mmol, Al/Ti molar ratio: 30)
in heptane at 30 ◦ C for 0.1, 0.15 and 0.18 s. A heptane slurry(100 ml) of the catalyst and a solution of TIBA in heptane(100 ml) saturated with 1 atm of propylene were placed into
two vessels. Methanol as a poisoning material was introduced
into the catalyst vessel and reacted with the catalyst for 1 min
before starting polymerization (poisoning material/Ti molar
ratio = 0.1).
The propagation rate constant kp and active-siteconcentration [C∗ ] were determined by the following
equations [15];2. Experimental
2.1. Chemicals
Research-grade propylene of (donated by Chisso Corp.)
was used without further purification. Methanol, acetone
and ethyl acetate were used as the representative alcohol,
ketone and ester, respectively, which have the least sterichindrance to minimize steric effects from the poisoning
materials. Ethyl benzoate (EB) as an internal donor, andanhydrous-grade methanol, acetone and ethyl acetate wereused as the poisoning materials (purchased from Wako Pure
Chemical Industries, Ltd.) after dehydration using a molecularsieve. Anhydrous magnesium ethoxide (Mg(OEt)2 ), titanium
tetrachloride (TiCl4 ) (both donated by Toho Titanium Co.,
Ltd.), triisobutylaluminium (TIBA, donated by Tosoh Akzo
Corp.) and nitrogen (purchased from Uno Sanso Co.) wereused without further purification. Heptane and toluene wereused as solvents and were purified by passing them throughthe 13X column of a molecular sieve.

kp · [M] · t,
1 + ktr · t(1)

Y = kp · [M] · [C∗ ] · t,(2)

M̄ n = M0 ·where M̄ n , M0 [M], t, ktr and Y are the number-averagemolecular weight of the polymer, the molecular weight ofthe monomer, the monomer concentration, the polymerization
time, the chain-transfer rate constant and the polymer yield,
respectively.
2.5. GPC characterization
The molecular weight and its distribution for thepolypropylene obtained in this study were determined by gel
permeation chromatography (GPC, Senshu SSC-7100) with
polystyrene gel columns (Tosoh TSK-GEL G3000HHR and
TSK-GEL G5000HHR) at 140 ◦ C using o-dichlorobenzene(ODCB) as a solvent.

2.2. Catalyst preparation
In the current study, a monoester-type catalyst TiCl4 /EB/
MgCl2 was prepared by a chemical reaction without
mechanical treatment (ball milling). TiCl4 was reacted witha mixture of (Mg(OEt)2 ) and EB in the following procedure:(Mg(OEt)2 ) (40 g) and TiCl4 (80 ml) in toluene (320 ml) were
heated to 90 ◦ C, mixed with EB (12 ml) and reacted at 90 ◦ C
for 2 h. The mixture was washed with toluene and treatedagain with TiCl4 (80 ml) in toluene (320 ml) at 90 ◦ C for 2 h.
Finally, the catalyst obtained was washed with heptane several
times and then used as to form a heptane slurry. The Ti content
was determined by titration to be 3 wt.%.

2.6. 13 C NMR spectroscopy
13

C NMR spectra were recorded on a Varian Gemini
300 spectrometer at 120 ◦ C using 20% (w/v) solutions in
1,2,4-trichlorobenzene. 10% (v/v) benzene- d6 was added foruse as the internal lock, and hexamethyldisiloxane was usedas the reference (internal chemical shift).
2

Sci. Technol. Adv. Mater. 9 (2008) 024402

K Tangjituabun et al

Table 1. Activity and tacticity of the PP without and with poisoning
materials a .
Poisoning material

1
2
3
4

None
Methanol
AcetoneEthyl Acetate

Yield (kg/mmole-Ti)

mmmm

95.3
58.9
73.9
91.0

83.1
82.6
82.0
82.1

None
Methanol

100
Yield (g-PP mol-Ti–1)

Entry

120

a
Slurry propylene polymerization was carried out with TIBA
(Al/Ti = 30) in heptane at 30 ◦ C after the pretreatment of thecatalyst with the poisoning materials for 1 min (poisoning
material/Ti = 0.1).

80
60
40

3. Results and discussion
20
In the first series of propylene polymerizations, slurry
polymerizations were conducted after pretreatment of thecatalyst with the poisoning materials for 1 min. The results
are listed in table 1. At a poisoning material/Ti molar ratio of
0.1 (entries 1–4), the catalyst activity decreased substantiallyin the following order: ethyl acetate > acetone > methanol,
while there was no significant change in the mmmm pentad.
The results clearly demonstrated that the catalytic activity
was greatly affected by the chemical nature of the poisoning
materials, and that all the materials poisoned the active sites
independently of their stereospecificity.
In the case of methanol, it is generally accepted thatTiCl4 readily reacts with alcohol to produce HCl and alkoxide
complexes that are inactive for propylene polymerization [16].
In addition, HCl reacts with TIBA to reduce its activation
power, and the titanium chloride alkoxides produced on the
catalyst surface might have a negative effect on the catalyst
activity [16]. Ethyl acetate, the donor might coordinate on
the MgCl2 support though its oxygen atom, similarly tomonoester-type donors such as EB. Therefore, the catalytic
activity was found to be little affected. Acetone might interact
with the catalyst surface in a similar fashion to ethyl acetate,
and it also reacted with the Ti-alkyl or Ti-polymer to formbulky alkoxy-Ti chlorides [17], resulting in an apparent
decrease in catalytic activity.
In the second series of polymerizations, kinetic studies
were carried out using a combination of the stopped-flow
technique and GPC analysis with the aim of quantifying theeffect of the poisoning materials in terms of the numberof active sites and the chain propagation rate constant.
Again, the catalysts were pretreated with methanol for 1 min
before starting the polymerization because methanol showed
the most prominent deactivation effect. Figure 1 shows thedependence of the polymer yields on the polymerization timewith and without methanol. A linear correlation was observed
within 0.2 s indicating that the catalyst possessed constantactivity without deactivation and late activation even after
adding methanol, suggesting that the reaction of methanol
with the catalyst was completed within the 1 min pretreatment.
From figure 2, the two molecular weights are proportional
to the polymerization time, which confirms the quasi-living
nature of the polymerization with and without methanol.
Furthermore, the molecular weight was unchanged after the
pretreatment with methanol.

0
0.00

0.05

0.10
0.15
0.20
Polymerization time (s)0.25

Figure 1. Yield of the polypropylene plotted for the polymerization
time without and with a methanol pretreatment.

25 000
20 000None
MethanolMn

15 000
10 000
5000
0
0.00

0.05
0.10
0.15
0.20
Polymerization time (s)0.25

Figure 2. Number-average molecular weight ( M̄ n ) of thepolypropylene without and with methanol pretreatment plottedagainst polymerization time.
Table 2. Kinetic parameters for propylene polymerization obtained
without and with methanola .
Entry

Poisoning material

1
2

None
Methanolkp (l/mol · s)b[C∗ ] (mol.%)b

4,800
4,600

0.40
0.28

a

Stopped-flow propylene polymerization was performed withTIBA (Al/Ti = 30) in heptane at 30 ◦ C after the pretreatment of
the catalyst with methanol for 1 min (methanol/Ti = 0.1).
b
kp and [C∗ ] were calculated from equations (1) and (2).

From the kinetic parameters as reported in table 2, thevalue of kp was hardly changed while the number of [C∗ ]markedly decreased from 0.40 to 0.28 mol.%, i.e., there was3

Sci. Technol. Adv. Mater. 9 (2008) 024402

K Tangjituabun et ala 30% reduction compared with the reference polymerization
without the pretreatment. It is well-known that heterogeneous
Ziegler–Natta catalysts contain a great variety of active sites
and the chain propagation rate constant of different typesof active sites significantly increases as their isospecificity
increases [18, 19]. In this regard, the value of kp could
have only changed in the case that there was some selective
poisoning of the different types of active sites. For example,
the selective poisoning of aspecific or isospecific sites must
lead to a higher or lower kp , respectively. Thus, a constantvalue of kp was indicative of nonselective poisoning, which
was supported by the nearly constant mmmm pentad in theslurry polymerization, as shown in table 1. Using the methodof the stopped-flow polymerization, we were able to attributethe poisoning effect of methanol on the catalyst performance
to only the decrease in the number of active centers.
Finally, the result of stopped-flow polymerizationsuggested that the active-site concentration was sensitive tothe methanol. The reaction taking place at a methanol/Ti
molar ratio of 0.1 would result in the reduction of the activity
by about 10% if TiCl3 OCH3 , the product of the reaction, was
not active in the polymerization. However, the actual observed
decrease in the activity was as large as 30%, which might be
explained by the same reason as for the corresponding resultof the slurry polymerization, i.e., the negative effect of thebulky alkoxy-Ti chlorides [17].

Acknowledgments
The authors gratefully acknowledge the Thailand Research
Fund (TRF) and the Royal Golden Jubilee program for thefinancial support. The authors also thank Toho Titanium
Co., Ltd., Chisso Corp., Asahi Chemical Industry Co., Ltd.,
Mitsubishi Chemical Corp. and Tosoh Akzo Corp., for theirsupport.

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Rapid Commun. 10 503

4. Conclusions
We examined the effects of poisoning materials on the
catalytic performance of a MgCl2 -based Ziegler–Nattacatalyst using methanol, acetone, and ethyl acetate as theleast bulky poisoning agents. Methanol exhibited the strongest
deactivation power followed by acetone and ethyl acetate
due to its highest reactivity in forming titanium chloride
alkoxide, which was inactive for polymerization. A precise
kinetic study on the effect of the methanol pretreatment using
the stopped-flow method revealed that the deactivation was
only due to the decrease in the number of active centerscaused by methanol, was insensitive to the type of active sites,
and maintained a constant kp value and the stereoregularityof the obtained polymers both without and with methanol
pretreatment.

4