Chemistry Reference
In-Depth Information
Table 2.
Physical properties of ethylene-1-hexene copolymers.
M
w
10
5
1
st
Heating
2
nd
Heating
Catalyst
[1-Hex]
C
6
(g/mol)
M
w
/M
n
(M)
mol-(%)
T
m
X
c
T
m
X
c
(
8
C)
(%)
(
8
C)
(%)
0.3
3.3
18.7
28
136.7
59
130.2
48
TpTiCl
2
(OEt)
0.4
2.2
14.8
27
137.2
59
130.7
47
0.5
6.1
10.4
25
134.2
60
126.8
49
TpTiCl
2
(OiPr)
0.5
1.2
6.2
21
135.7
58
128.8
47
TpTiCl
2
(OnBu)
0.5
3.4
16.4
33
137.0
64
129.5
47
presence of 1-hexene in the copolymer
was analyzed by
13
C NMR. Figure 5 shows
an spectrum of a copolymer made with
TpTiCl
2
(OEt) which is representative of
those made with TpTiCl
2
(OR) complexes.
According to the ASTM X70-8605-2
method, all the spectra showed character-
istic signals of ethylene/1-hexene copoly-
mers.
As seen in Table 2, the 1-hexene con-
tent in the copolymers synthesized with
TpTiCl
2
(OEt) for the established comono-
mer concentrations showed a higher value
at 0.5 M (6.1 mol %). The lower 1-hexene
content (1.2 mol %) was obtained with
TpTiCl
2
(O
the melting temperature drops down. This
behavior might indicate that when the
1-hexene content in the copolymer incre-
ases, the side chains hinder the growth of
crystal lamellae, bringing about a drop
of melting temperature. The copolymers
showed melting temperatures ranging from
126 to 131
8
C and crystallinities between 47
to 49 %. Additionally, the effect of alkoxyl
group type on thermal properties was not
significant. All these properties allowed to
infer that the obtained copolymers showed
high molecular weights (around 10
10
5
g/
mol) with low comonomer incorporation
(up to 6 mol-%).
A difference of 15% was observed
when the thermal properties of the nascent
powders (1
st
heating) and the melt-
crystallized sample (2
nd
heating) of the
copolymers were compared. A similar
behavior was found in the homopolymers
(Table 1), which showed a difference of
30% in its crystallinities. The lower differ-
ence observed in the copolymers case
can be a consequence of the entangle-
ment density reduction as a result of lower
molecular weights present in the copo-
lymers. This behavior allows higher chain
mobility during the crystallization process.
For the TpTiCl
2
(OR)/MAO/ethylene/
H
2
system, the homopolymers were char-
acterized by
13
C NMR, GPC and DSC. The
results are summarized in Table 3.
The
13
C NMR analysis of the polyethy-
lene samples showed a single signal of
methylene group at 29.5 ppm, which shows
that all polymers were linear. It is worthy
mentioning that the weight average molec-
ular weights (M
w
) were strongly influenced
Pr), which was attributed to
catalyst steric effects. The average molec-
ular weights of the copolymers made with
TpTiCl
2
(OEt) decreases from 18.7
10
5
to
10.4
i
10
5
g/mol as the 1-hexene concentra-
tion in the feed was increased (0.3-0.5 M).
These results are in agreement with the
propagation rate of comonomer insertion
and 1-hexene concentration. The copoly-
mers showed wide molecular weight dis-
tributions, as can be noted in Table 2, in
agreement with the results obtained by
Nakazawa et al.,
[27]
Jordan et al.,
[28-30]
and
our previous study
[18]
on ethylene poly-
merization. This fact is an evidence that
more of one active species is present in the
catalyst.
On the other hand, Table 2 shows the
1-hexene concentration effect on the 2
nd
melting temperature. The melting tempera-
ture did not vary significantly when the
1-hexene concentration in the feed was
between 0.3-0.4. However, at the maximum
1-hexene concentration in the feed (0.5),
