Chemistry Reference
In-Depth Information
Table 5.
13
C-NMR pentad distribution (mol%) for iPPs.
Samples
mmmm
mmmr
rmmr
mmrr
mmrm
รพ
rrmr
mrmr
rrrr
mrrr
mrrm
iPP52
51.5
13.3
1.7
12.2
5.6
1.4
5.2
4.1
5.0
iPP76
76.3
9.8
0.0
7.0
2.0
0.0
1.8
0.6
2.5
iPP87
87.0
6.2
0.7
3.2
0.7
0.1
0.6
0.4
1.1
iPP91
90.6
6.5
0.4
1.4
0.3
0.1
0.1
0.1
0.5
entirely for such slow decrease in isospe-
cificity.
The application of TREF visually eluci-
dates the changes in the distribution of
isospecific active sites shown in Figure 3
and 4. There exist two remarkable peaks
in these TREF diagrams. The higher tem-
perature (ca. 115
8
C) peak corresponds to a
higher isospecific active site, and the lower
(ca. 108
8
C) to lower isospecific site. The
intensity of the higher peak gradually
decreases with increasing extraction time.
Interestingly, the lower peak grows with
increasing extraction time of the internal
donor. The growing suggests that the higher
isospecific active site is converted to the
lower one by the extraction of the internal
donor. In order to study this phenomenon
in detail, the iPPs were fractionated into
two fractions (denoted as Fraction I and
Fraction II) in two temperature ranges of
90
respectively, permiting the estimation of
the [C
] for each site type. The character-
ization and the kinetic data are summarized
in Table 2 and 3. It appears that the [C
]of
the lower isospecific site increases as the
[C
] for the higher isospecific site decreases.
In particular, this tendency is remarkable
in the iPP series made with Cat-A, in good
agreement with the changes in the TREF
profiles. The TREF results demonstrate
that the higher isospecific active site is con-
verted into the lower one by the extraction
of the internal donor.
In conclusion, TREF has been proved to
be quite effective to evaluate the change of
stereospecificity of active sites.
TREF Application to Polymer
Degradation Behavior
PP is very vulnerable against oxidation.
This process is called ''degradation'' and it
is a very important practical issue. Although
many researchers have been involved in
developing industrial polypropylene resins,
C, respectively.
Fractions I and II are produced with the
lower and higher isospecific active site,
112
8
C and 112
135
8
Figure 6.
Thermogravimetric analysis results of fractionated iPPs.
