Living copolymerization of propylene and ethylene with [tBuNSiMe2Flu]TiMe2 / MAO catalyst system: Effect of MAO/Ti ratio
E O Dare1,* G A Olatunji2, D S Ogunniyi2, S O Oguntoye2 and J T Bamgbose1
1Department
of Chemistry, University of Agriculture, P O Box 28, UNAAB post office, Abeokuta, Nigeria
2
Department of Chemistry, University of Ilorin, P M B 1515, Ilorin, Nigeria
Received 19 December 2005; revised 19 April 2006; accepted
Propylene-co-ethylene polymer (Mn=528,000, Mw/Mn=1.19) was synthesized using [η1: η3-t-BuNSiMe2Flu]TiMe2 (Cat
A) / methylaluminoxane (MAO) system at 0°C. Effect of MAO/Ti ratio on catalyst productivity was investigated and living
nature of copolymer was maintained until the ratio reached 2000. 13C-NMR and GPC have been used to characterize the
copolymer.
Keywords: 13C-NMR, Copolymerization, GPC, Propylene-co-ethylene
IPC Code: C08F38/02
Introduction
Novel copolymers of olefins, especially block-type, having outstanding performance are in great
demand in the commercial world. Copolymerization of ethylene and olefinic monomers has been studied
using homogeneous catalyst systems1-5. Stereoblock ethylene/propylene copolymers are a rubbery
material with the novel properties of a thermoplastic elastomer6. A melt-crystallized copolymer, where
propylene was incorporated in the crystal to an extent depending on the bulkiness of the comonomer, was
reported7.
Homogeneous catalyst, promoting syndiotactic-specific or isotactic-specific polymerization of αolefins and a few copolymers, can be prepared by reaction of methylaluminoxane (MAO) with several
transition metals (Ti, Zr and Hf) and the type of η5-ligand, which are soluble in aromatic hydrocarbon8,9.
The keys to directly controlling the catalyst activity, stereospecificity and molecular weight distribution
(MWD) of the polymers, lie in the titanium complex. Waymouth4 has reported the alternating
copolymerization of ethylene and propylene using iPr(CpFluo)MCl2 (M = Ti, Zr). Preliminary analysis of
the polymers using 13C-NMR spectroscopy revealed that the fluorenyl-based titanocenes, especially [tBuNSiMe2Flu]TiMe2, Cat A (Fig. 1) has been adopted by several authors10-13 to effect syndiospecific
homopolymerization of α-olefins.
t-Bu
N
Me
Me
Si
Fig. 1-Cat A
*Author for correspondence
Tel: 234-804-313-3774
E-mail: dare3160@hotmail.com
Me
Ti
Me
A modified one-pot procedure to synthesize Cat A was reported to give a higher yield 10. This
catalyst produced polypropylene in a living and syndio-specific manner. Also, Cat A has been studied10-13
for homopolymerization of α-olefin. This study reports the results of copolymerization of propylene and
ethylene using Cat A in a series of experiments with systematic variation of MAO/Ti ratio.
Materials and Methods
Materials
All operations were carried out under nitrogen atmosphere using standard Schlenk techniques.
MAO was prepared following a literature procedure14. Cat A was prepared by a one-pot procedure10.
Propylene and ethylene, procured from Mitsubishi Petrochemical, were purified by bubbling through a
NaAlH2(i-Bu)2/1,2,3,4-tetrahydronaphthalene solution. All solvents were obtained from commercial
sources and dried with standard methods.
Copolymerization
Propylene (1.0 mol/l) was measured by a gas flow meter and dissolved in toluene. Thereafter,
prescribed amounts of cocatalyst and catalyst were added to initiate polymerization. After some time,
some samples were taken out with a syringe, and then ethylene (1 atm) was introduced into the reactor.
After addition of ethylene, copolymer samples were taken at regular intervals until 2.5 h when reaction
was quenched by injection of acidified methanol. Copolymer obtained was washed several times with
methanol and dried in vacuum at 60°C for 6h.
Analytical Procedure
1
H-NMR spectra of ligands and Cat A were recorded at room temperature on a Lambda-300
spectrometer operated at 300 MHz in pulse Fourier-Transform mode. Molecular weight and molecular
weight distribution of polymers were determined by Gel permeation chromatography as polystyrene (PS)
standard with universal calibration, by Waters 150 CV at 140°C using O-dichlorobenzene as solvent. 13CNMR spectra were recorded at 130°C on a Lambda-500 spectrometer operated at 125.65 MHz,
respectively, in pulse, Fourier-Transform mode. The spectra were obtained at 130°C in a 5.0 mm o.d.
tube. In 13C-NMR measurements, pulse angle was 45°, and about 10 000 scans were accumulated in 4.9
sec of pulse repetition. Sample solution was prepared in 1,1,2,2-tetrachloroethene-d2 to contain up to 10
percent by weight. The central peak of 1,1,2,2-tetrachloroethene-d2 (74.47 ppm) was used as an internal
reference.
Results and Discussion
Propylene Polymerization in Toluene
Homopolymerization of propylene was first carried out at 0°C in toluene by batch method. Living
polymerization with evidence of narrow MWD (1.08) and syndiospecificity were reestablished in line
with previously reported data10.
Propylene/ethylene Copolymerization
Copolymerization of propylene and ethylene was conducted at 0°C with Cat A /MAO by batch
method (Table 1). When Cat A was added to a solution of MAO and propylene (1.0 M) in toluene,
polypropylene (mol wt, Mn=230,000) and narrow MWD (1.08) was obtained (entry 10h). Addition of
ethylene to the reactor for 2.5 h produced more than two fold higher mol wt (M n=528,000) polymer with
narrow MWD. Neither polypropylene nor polyethylene was produced. Copolymerization of the two
monomers occurred with the same active site11.
Mn value (Fig. 2) increases with increasing polymerization time while still maintaining narrow
MWD. This observation further suggests that living copolymerization proceeded with catalyst system,
and the catalytically active sites are uniform in the course of polymerization. However, unexpected
decrease in productivity was observed during ethylene incorporation. GPC unimodal curve of the
copolymer, which indicates a high block efficiency (Fig. 3) after the copolymerization, shifted to higher
mol wt region. The formation of this copolymer via a uniform active Ti 4+ species by coordination
mechanism was previously well established for propylene homopolymerization.
Effect of the Ratio MAO/Ti on Productivity of Catalyst System
Catalytic activity of Cat A in toluene9 increases with MAO/Ti ratio until a maximum ratio of
1500 (Fig. 4). Thereafter, gross decrease in activity was observed. This suggests that MAO behaves as an
activator towards the initial titanocene dimethyl, and as a deactivating agent towards the titanocene active
species formed. Such catalytic decrease in catalytic activity at high MAO/Ti ratio has been reported with
other metallocenes9,12,13 where it was postulated that MAO might compete with the olefin for the
complexation on the active sites.
When MAO/Ti ratio was 2500, catalytic activity decreases drastically, and mol wt distribution
became broadened to 2.10 (Table 2). MAO15,16 usually contain a significant amount of residual
trialkylaluminium (TMA), which may hinder living polymerization process. Obviously, increase in
MOA/Ti ratio is tantamount to an increase in residual TMA, whose excess may act as poison for the
active species and thereby induce a chain transfer process through a preferential coordination of TMA
with Ti species rather than counterion. This phenomenon has also been established in earlier studies13
where the effect of additives on propylene polymerization was studied. The formation of complexes
between active metallocene species and TMA was observed by NMR spectroscopy16.
Propylene-co-ethylene Copolymer Microstructure
In 13C-NMR spectra of typical P-co-E copolymer (Fig. 5), peaks appearing at 21.78, 30.0 and
46.77 ppm correspond to propylene segments of the copolymer, whereas resonances at 27.96, 28.10 and
37.7 ppm can be assigned to methylene of ethylene segment (EEEEP) of copolymer. The triad analysis
(mm=8%, mr=31% rr=61%) indicates to a large extent the syndiotacticity level of propylene segments.
Furthermore, conspicuous single peaks at 20.5 (Pγγ), 29 (Tββ) and 48.5 (Sαα) signify some stereoregularity
and syndiospecificity in the copolymer.
Conclusions
Propylene-co-ethylene copolymer synthesized from [t-BuNSiMe2Flu]TiMe2/MAO at 0°C shows
that the homogeneous Cat A produces the propylene polymeric segment in a rich syndiospecific form. Cat
A / MAO system gave the copolymer in a living fashion until MAO/Ti ratio exceeded 2000.
Acknowledgements
EOD is thankful to Prof T Shiono of TIT, Japan for providing laboratory facilities. Thanks are
due to Drs Nishi and Matsumae for technical and language assistance. Appreciation also goes to
UNESCO/MONBUSHO for offer of fellowship. Authors thank Prof I C Eromosele, Chemistry
Department, UNAAB, Nigeria for useful suggestions.
References
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2
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6
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copolymer Poymer, 31 (1990) 1999-2006.
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(1989) 2129-2137.
Ishiara N, Kuramoto M & Uoi M, Stereospecific polymerization of styrene giving the syndiotactic polymers,
Macromolecules, 21 (1988) 3356-3361.
Dare E O, Olatunji G A & Ogunniyi D S, Polymerization behavior of propene with [t-BuNSiMe2Flu]TiMe2: Effect of
solvents and cocatalysts, Eur Polym J, 40 (2004) 2333-2341.
Soga K, Chen S, Yoshiharu D, & Shiono T, Polymerization of ethylene and propylene with Cr(C 17H35COO)3/AlEt2Cl/metal
chloride catalysts, Macromolecules, 19 (1986) 2893-2896.
Chien J C W & Sugimoto R J, Kinetic and stereochemical control of propylene polymerization initiated by ethylene
bis(4,5,5,7-tetrahydro-1-indenyl) zirconium dichloride/methyl aluminoxane catalyst, Polym Sci Part A: Polym Chem, 29
(1991) 459-470.
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Effects of Trialkylaluminium as Additive Bull Chem Soc Ethiop, 18 (2004)131-136.
Hassan T, Ioku A, Nishi K, Shiono T & Ikeda T, Syndiospecific living polymerization of propene with [tBuNSiMe2Flu]TiMe2 using MAO as cocatalyst Macromolecules, 34 (2001) 3142-3145.
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Fig captions
Fig. 1. Cat A
Fig.2. Plots of Mn and Mw/Mn as a function of reaction time for propylene-ethylene copolymerization with Cat
A/MAO system at 0°C by sampling method
Fig. 3. GPC curves of copolymer obtained with [t-BuNSiMe2Flu]TiMe2/MAO at 0°C
Fig.4. Influence of MAO/Ti ratio on the catalytic activity of Cat.A/MAO for propylene-co-ethylene polymerization
in toluene at 0°C
Fig. 5. 125 MHz 13C-NMR spectra of polymer obtained with [t-BuNSiMe2Flu]TiMe2/MAO at 0°C
FIGURES AND TABLES
t-Bu
N
Me
Me
Si
Me
Ti
Me
Mw/Mn
Mn( x 10- 4)
Fig. 1. Cat A
Time h
Fig.2. Plots of Mn and Mw/Mn as a function of reaction time for propylene-ethylene copolymerization with Cat
A/MAO system at 0 oC by sampling method
Activity
(Kgpol/molTi.h.[P-co-E])
Fig. 3. GPC curves of copolymer obtained with [t-BuNSiMe2Flu]TiMe2/MAO at 0 oC
45
40
35
30
25
20
15
10
5
0
0
1000
2000
3000
4000
[MAO]/[Ti]
Fig.4. Influence of MAO/Ti ratio on the catalytic activity of Cat.A/MAO for propylene-co-ethylene polymerization
in toluene at 0 oC
Fig. 5. 125 MHz 13C-NMR spectra of polymer obtained with [t-BuNSiMe2Flu]TiMe2/MAO at 0 oC
Table 1: Results of homo- and copolymerization of propene and ethene with [t-BuNSiMe2Flu]TiMe2/
MAOa
Entry
Timeb
Timec
Yield
Activity, kg
Mnd
Mw/Mnd
polymer/mol.Ti.h x 10-4
h
h
g
9e
-
1.2
0.24
10.3
f
f
10g
1.0
-
0.60
30.0
23.0
1.08
11g
-
0.5
0.90
30.0
42.9
1.18
12g
-
1.0
0.96
24.0
46.5
1.21
13g
-
1.5
1.24
24.8
49.3
1.22
14g
-
2.0
1.48
24.7
51.6
1.19
15g
-
2.5
1.42
20.3
52.8
1.19
Polymerization conditions: toluene = 100 ml; Ti = 20 mol; MAO = 8.0 mmol; C3H6
concentration = 1.0 mol/l, C2H4 = 1.0 atm; 0 oC. bPolymerization time of propylene
homopolymerization. cCopolymerization time after the addition of ethylene. dNumber
average molecular weight (Mn) and molecular weight distribution (Mw/Mn) determined by
GPC using universal calibration and polystyrene standards. eHomopolymerization of C2H4.
f
Not determined. gSamples taken at intervals during polymerization.
a
Table 2: Effect of MAO/Ti ratio on productivity and livingness
entry
MAO/Ti
Activity
Mw/Mn
Kgpol/mol.Ti.h.
16
500
28.5
1.21
17
1000
35
1.10
18
1500
40.2
1.27
19
2000
38
1.65
20
2500
25
2.10
21
3000
10
2.16
Journal of Scientific & Industrial Research
VOLUME 65
578
Living copolymerization of propylene
and ethylene with [tBuNSiMe2Flu]TiMe2/MAO catalyst
system: Effect of MAO/Ti ratio
NUMBER 7JULY 2006
Propylene-co-ethylene polymer (Mn=528,000, Mw/Mn=1.19)
was synthesized using [η1: η3-t-BuNSiMe2Flu]TiMe2 (Cat
A)/methylaluminoxane (MAO) system at 0°C. Effect of
MAO/Ti ratio on catalyst productivity was investigated and
living nature of copolymer was maintained until the ratio
reached 2000. 13C-NMR and GPC have been used to
characterize the copolymer.
IPC Code: C08F38/02
E O Dare, G A Olatunji, D S
Ogunniyi,
S O Oguntoye & J T Bamgbose1