4
8
I. Kim, C.-S. Ha / Journal of Molecular Catalysis A: Chemical 210 (2004) 47–52
Polymerization grade of ethylene (SK Co., Korea) was
purified by passing it through columns of Fisher RIDOX
catalyst and molecular sieve 5 Å/13X. Trimethylaluminum
3. Results and discussion
3.1. Effect of AlR on the ethylene polymerization
3
(AlMe3), triethylaluminum (AlEt3), diethylaluminum chlo-
ride (AlEt2Cl), triisobutylaluminum (Al(i-Bu)3), and di-
isobutylaluminum hydride (Al(i-Bu)2H) donated by Korea
Petrochemical Co. and were used without further pu-
rification. MAO was purchased from Akzo Chemical as
Metallocene catalysts for the polymerization of ethylene
are almost as old as Ziegler–Natta catalysis itself. In 1957,
Natta et al. [8] and independently Breslow and Newburg [3]
reported on a soluble, crystalline and isolobal complex from
8
.4 wt.% total Al (Al in MAO is composed of 81.5 wt.%
2 2 3
Cp TiCl /AlEt catalyst which was active towards ethylene
from MAO and 18.5 wt.% from AlMe3) solution in toluene.
Solvents were distilled from Na/benzophenone and stored
over molecular sieves (4 Å). Literature procedures [7] were
polymerization but much slower than a comparative het-
erogeneous Ziegler–Natta catalyst, e.g. TiCl /AlEt . Even
4
3
though there have been tremendous research about and
developments on the metallocene catalyst systems, MAO
invented by Kaminsky et al. [9] remains the most effective
cocatalyst for the metallocene complexes. We revisited the
effectiveness of AlR3 as a cocatalyst for the metallocene
amide compound. Common alkylaluminums such as AlMe3,
AlEt3, AlEt2Cl, Ai(i-Bu)3, and Al(i-Bu)2H, were tested as
∗
employed to synthesize Cp Zr(NMe2)2 complexes.
2
2
.1. Polymerization of ethylene
Ethylene polymerizations were performed in a 250 ml
round-bottom flask equipped with a magnetic stirrer and a
thermometer. In a dry box, the reactor was charged with
toluene (80 ml) and a prescribed amount of MAO or alky-
laluminum cocatalyst. The reactor was immersed in a con-
stant temperature bath previously set to desired temperature.
When the reactor temperature had been equilibrated to the
bath temperature, ethylene was introduced into the reactor
after removing argon gas under vacuum. When absorption
of ethylene into toluene ceased, a prescribed amount of met-
allocene catalyst dissolved in toluene was injected into the
reactor and then the polymerization was started. Polymer-
ization kinetics was determined at every 0.01 s from the rate
of consumption, measured by a hot-wire flowmeter (model
∗
cocatalysts for Cp Zr(NMe2)2 complex. The rate of poly-
2
∗
merization (Rp) obtained by using Cp Zr(NMe2)2/AlMe3
and Cp Zr(NMe2)2/AlEt2Cl catalysts were too low to
2
∗
2
detect by our rate monitoring system, as reported earlier.
However, AlEt3, Al(i-Bu)3, and Al(i-Bu)2H cocatalyzed
systems showed unexpectedly high activity in the ethylene
polymerizations. Figs. 1–3 illustrate rate profiles of ethylene
polymerization obtained by using AlEt3, Al(i-Bu)3, and
Al(i-Bu)2H as cocatalysts, respectively. Polymerization rate
∗
profile obtained by Cp Zr(NMe2)2/MAO (Al/Zr = 2000)
2
catalyst is also shown for the comparison in Fig. 1. The
Al(i-Bu)3 and Al(i-Bu)2H cocatalyzed systems are charac-
terized by both higher activity and better stability than the
5
850 D from Brooks Instrument Div.) connected to a per-
sonal computer through an ac/dc converter. Polymerization
was quenched by the addition of methanol containing HCl
(5 vol.%) and then the unreacted monomer was vented. The
polymer was washed with an excess amount of methanol
◦
and dried in vacuo at 50 C.
2
.2. Characterization techniques
Thermal analysis of polymer was carried out by using a
dupont differential scanning calorimeter (DSC, model 910)
◦
at 10 C/min heating rate under nitrogen atmosphere. The
results of the second scan are reported to eliminate dif-
ferences in sample history. Infrared spectra were obtained
with an ATI Mattson Genesis Series FTIR. Polymer films of
1
00 m thickness were prepared for IR examination by us-
◦
ing a hot press (Graseby Specac Film Maker) at 150 C for
3
0 s. Molecular weight and its distribution (Mw/Mn) were
determined by gel permeation chromatography (GPC) on a
◦
Waters 150-C at 135 C in 1,2-dichlorobenzene with a data
1
processor, equipped with polystyrene gel columns. H NMR
∗
Fig. 1. Rp vs. time curve obtained by Cp Zr(NMe2)2/AlEt3 catalyst. Poly-
2
spectra of compounds were recorded on a Bruker WM-300
spectrometer using tetramethylsilane as an internal standard
CD2Cl2 solvent. Wilmad NMR tube with J Young valve was
used for a NMR-scale reaction.
◦
merization conditions: Tp = 30 C, PC H = 1.3 atm, [Zr] = 1.275 M,
2
4
toluene = 80 ml; and (a) [Al]/[Zr] = 9; (b) 12; (c) 18; and (d) 27.
MAO cocatalyzed system polymerized at the same conditions except
[Al]/[Zr] = 2000.