Chemistry Reference
In-Depth Information
Several investigations have pointed out
that the formation and stabilization of
zirconium cations (Zr
รพ
) may be easier
when support materials have surface acidic
properties.
[3,31]
In this work, we have
studied this phenomenon through the use
of synthesized MCM-41 materials with
different silicon to aluminium molar ratios.
Table 3 shows that the acidity of the support
does not have a significant influence on
the properties of the copolymers, apart
from those related with the incorporation
of higher amounts of the
a
-olefin. When
MCM-41 is used, the MWD continues to be
narrow, but the CCD broadens significantly
with increasing 1-hexene content and even
becomes bimodal for certain 1-hexene con-
centrations, as shown in Figure 3. This may
be related to the presence of two catalyst
site types on the surface of the support.
Therefore, the Si/Al ratio can be used to
control the shape of the CCD. Interestingly,
lower temperature peaks become more
prominent with decreasing Si/Al ratios.
Another important property affecting
the behaviour of support materials for
metallocenes is pore size, since the support
has to be able to anchor the catalyst
precursor, the cocatalyst, and permit easy
access to the active sites for the mono-
mers.
[18]
The geometrical constraints of the
parallel hexagonal channel structure of the
SBA-15 materials may affect the pattern
of monomer insertion and chain growth
process, which offers a way to control the
polymer chain structure and morphology
during the polymerization.
[32]
To study this
effect, we have synthesized three SBA-15
materials with different pore sizes using
several swelling agents, as described above.
Table 4 summarizes the properties of
the poly(ethylene-co-1-hexene) made with
(nBuCp)
2
ZrCl
2
/MAO supported on SBA-15
materials. The MWD is still narrow, but the
CCD can also become broad and bimodal,
as depicted in Figure 4. Larger pore sizes
seem to favour 1-hexene incorporation
slightly, which may be related in part to
intraparticle mass transfer resistances.
Finally, we have calculated the overall
mol% of 1-hexene for the copolymer sam-
ples obtained with (nBuCp)
2
ZrCl
2
/MAO
supported on both MCM-41 and SBA-
15 using the calibration curve shown in
Figure 2. These results are reported in
Table 5, together with overall 1-hexene
mol% measured with
13
C NMR. The good
agreement between the two techniques
indicates that our calibration curve also
works well for the bimodal CCD copoly-
mers made with (nBuCp)
2
ZrCl
2
/MAO sup-
ported on both MCM-41 and SBA-15.
Table 3.
Characterization of the poly(ethylene-co-1-hexene) made with (nBuCp)
2
ZrCl
2
/MAO immobilized on supports
synthesized with different Si/Al ratios.
Support
mL 1-hexene
0
5
25
50
75
MCM-41-15
M
n
(
g/
mol)
62358
37521
44796
63216
54288
M
w
=
M
n
2.95
2.85
2.94
3.3
3.58
T
m
(
C)
136
124
109
103
100
8
T
c
(
8
C)
113
110
95
95
82
Cr
ystallinity (%)
66
54
36
32
26
MCM-41-60
M
n
(g
/m
ol)
60915
38927
46271
60234
52746
2.9
2.7
2.75
3.09
3.85
M
w
=
M
n
T
m
(
8
C)
133
123
110
110
110
T
c
(
8
C)
116
110
91
94
86
Cr
ystallinity (%)
62
53
36
32
31
MCM-41-infinite
M
n
(g
/m
ol)
67674
40961
46713
40065
51420
M
w
=
M
n
3.21
3.07
3.07
4.25
4
T
m
(
C)
133
124
110
111
108
8
T
c
(
8
C)
116
110
91
93
102
Crystallinity (%)
65
52
37
34
32
