Chemistry Reference
In-Depth Information
Tab. 4.1
Polymerization of norbornene and 5-decylnorbornene using catalyst
1
(naked nickel)
with and without 1-decene as a chain transfer agent.
Run
a)
10
3
M
w
Norbornene:
5-decylnorbornene
Decene-1
(mol%)
Conversion
(%)
M
w
/M
n
T
g
1
99.5 : 0.5
1
92
195
2.44
353
2
99.5 : 0.5
1.5
93
124
2.82
351
3
97 : 3
1.5
90
125
2.31
356
4
85 : 15
0.5
87
310
2.40
285
5
95 : 5
2
94
114
2.59
336
6
90 : 10
1.5
90
128
2.37
297
7
99 : 1
0.5
96
331
2.83
375
8
85 : 15
2
92
110
2.20
265
9
b
100
25
100
11
1.83
274
10
c
100
0
87
1250
3.73
370
a)
Runs 1-8 in 1,2-dichloroethane for 1 h at room temperature with a monomer to Ni ratio of
4000 : 1.
b)
Runs 9 and 10 at 2000 : 1 monomer to Ni ratio.
c)
Run 10 for 30 min.
of thallium hexafluorophosphate in the presence of cyclo-1,5-octadiene to yield cat-
alyst
2
[39].
The polymerization of norbornene using catalyst
2
is much more sluggish than
with
1
. Even after 24 h (albeit at a 4000 : 1 molar ratio of norbornene to palla-
dium) in 1,2-dichloroethane, only 66% conversion to poly(norbornene) was
achieved. The difference in rate observed between nickel and palladium was used
to our benefit in determining the initiation event, as explained in the following
section.
4.2.3.4
On the Mechanism of Initiation
Once this new family of catalysts was proven to show excellent polymerization activ-
ity and versatility (see below) in the polymerization of norbornenes, we immediately
set out to understand the mechanism by which the polymerization occurs. The
mechanism is of importance not only in the design of other, and possibly better, cat-
alysts but also in helping to establish the microstructure of the polymers formed.
In an attempt to study the initiation event for norbornene polymerization, the
following NMR tube study was undertaken. The
1
H NMR spectrum of
1
in CDCl
3
was recorded. Approximately 2-3 equiv of norbornene per nickel were added to
this solution and the spectrum was acquired again. No significant change in the
catalyst is evident; only signals due to unreacted
1
and norbornene were observed.
Within 10 min the spectrum was recorded once again. Broad signals due to
poly(norbornene) were observed as well as unreacted
1
and norbornene. These ob-
servations are consistent with a slow initiation rate followed by an extremely rapid
rate of propagation.
Search WWH ::
Custom Search