Chemistry Reference
In-Depth Information
syn-1-cis
Scheme 5.9
Reaction of
syn-
and
anti-
rotamers in ROMP.
Ar =2,6-i-Pr
2
-C
6
H
3
, 2,6-Me
2
-C
6
H
3
.R=
t
-Bu, CMe(CF
3
)
2
, etc.
sion. The high dependency of the
cis
-
trans
contents of a polymer on the temperature
as found for the polymerization of NBDF6 or DCMNBD with Mo(N-2-
t
-Bu-
C
6
H
4
)(CHCMe
2
Ph){2,2
-
t
-Bu
4
](C
6
H
2
)
2
O
2
} again underlined the importance
of
syn
-
anti
conversion with respect to the time-scale of the polymerization [101]. Thus,
the careful choice of an alkoxide in these systems offers an attractive access to poly-
mers with either
cis-
or
trans-
configured double bonds as well as to highly tactic poly-
mers [102]. While polymers prepared fromMo-based initiators containing non-fluori-
nated alkoxides such as
t
-butoxide are all-
trans
and highly tactic, polymers based on
cis
-vinylene units prepared from fluorinated analogues are usually only 70-75% tac-
tic. In order to be capable of preparing polymers which are >99%
cis
and >99% tactic,
the use of chiral alkoxide ligands was elaborated [103-105]. By this approach, poly-
merizations were forced to proceed exclusively via the
syn
-rotamer by an adjustment
of the accessibility of the
trans
-rotamer. This was achieved by a subtle combination of
the steric bulks of the phenoxide and arylimido ligands. Tab. 5.1 gives an overview
over the corresponding chiral ligands, the investigated monomers and the polymer
properties.
7-Oxanorbornene and 7-oxanorbornadiene-derivatives were found to form ob-
servable and even isolable molybdametallacycles. Thus, 7-oxa-2,3-(bistrifluoro-
methyl)norbornadiene reacts with Mo(N-2,6-i-Pr
2
-C
6
H
3
)(CHCMe
3
)(OCMe
3
)
2
to
form a remarkably stable metallacycle [106]. A similar tungstametallacycle was ob-
served in the reaction of 2,3-bis(trifluoromethyl)norbornadiene with W(N-2,6-i-Pr
2
-
C
6
H
3
)(CHCMe
3
)(OCMe
3
)
2
[98].
-[4,4
,6,6
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