Chemistry Reference
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13
C NMR spectrum of dimers
4
and
5
before (a) and after separation (b and c).
Fig. 4.17
NMR assignments for trimer
6
and trimer
7
are shown in Fig. 4.19. Thus the tri-
mers are enchained in a 2,3-fashion just as for the dimers and only differ with re-
gard to the enchainment of the third (or last) norbornene unit adjacent to the vi-
nyl terminus. Trimer
6
consists of a
rm
triad of norbornene units, while trimer
7
is an
mm
triad. Presumably, the same is true for the two trimer peaks observed
with
m
/
z
338 (ethyl “head” group and a vinyl “tail”); one consists of the
rm
triad
of norbornenes and the other the
mm
.
Given the oligomer results, a modified catalytic cycle can be constructed for the
dimerization and trimerization of norbornene using the nickel catalyst in the pres-
ence of ethylene (see Fig. 4.20).
A cationic nickel-hydride is the proposed catalytically active species. Insertion of
norbornene results in the nickel-norbornyl cationic intermediate, where R is H
originating from the nickel-hydride. Under the conditions employed, ethylene in-
sertion is competitive with the first norbornene insertion, followed by norbornene
insertion to yield the nickel-norbornyl intermediate where R is ethyl. The second
norbornene insertion is stereoselective; about 96% of the time a meso enchain-
ment results. Insertion of ethylene, followed by
-hydrogen elimination forms di-
mers
4
and
5
(as well as 4% of the isomeric
rac
dimers). Insertion of the third
norbornene is non-stereoselective or random; the trimeric product of
meso
or
rac
insertion is isolated after ethylene insertion and
-hydrogen elimination (trimers
6
and
7
, where R=H, along with the trimers where R=ethyl). Extrapolation of the
random insertion of norbornene observed in the trimers to the high polymer pre-
dicts that the nickel-based poly(norbornene) is best described as atactic which
should result in a more soluble polymer than a more regular poly(norbornene).
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