Chemistry Reference
In-Depth Information
Scheme 6.16
Use of benzaldehyde as polymer end-capping reagent.
The molybdenum catalyst
2
has been used extensively for ADMET polymeriza-
tion. This complex is easier to handle than the tungsten analog and is more toler-
ant of functionality. This complex has allowed the synthesis of polymers contain-
ing esters, carbonates, ethers, sulfides, aromatic amines, boronates, dichlorosi-
lanes, siloxanes, acetals, and conjugated carbon-carbon double bonds [38-45].
Aldehydes, ketones, and protic functionality are not tolerated. The molybdenum
alkylidene will react with aldehydes and ketones, but not esters, in a Wittig fash-
ion [64].
Both of these complexes can be used in ADMET polymerizations at tempera-
tures up to approximately 55
C, although decomposition certainly occurs over the
time scale of a typical ADMET polymerization (days). A structure-reactivity study
was performed on complexes
1
and
2
that revealed a number of features of these
complexes [68]. Notably,
2
will polymerize dienes containing a terminal and a 1,1-
disubstituted olefin, but never produces a tetrasubstituted olefin. One of the sub-
stituents of the 1,1-disubstituted olefin must be a methyl group. In contrast, com-
plex
1
will not react with a 1,1-disubstituted olefin. The tungsten complex is more
reactive towards internal olefins than external olefins [23, 63] indicating that sec-
ondary metathesis, or
trans
-metathesis, probably dominates the catalytic turnovers
in ADMET with complex
1
.
The high
trans
content witnessed in ADMET polymers produced by these cata-
lysts can be explained by the preference of the all-pseudoequatorial configuration
in the metallacyclobutane intermediates, which is suggested by NMR and X-ray
data [23] (Fig. 6.2). The all-equatorial conformation of the metallacyclobutane leads
to the production of
trans
olefins.
Complex
2
is still regarded as the best catalyst for the polymerization of hydro-
carbon ADMET monomers and monomers containing other tolerated functional
groups, if the monomers can be rendered rigorously dry and oxygen-free. The ac-
Fig. 6.2
Structures showing pref-
erence of all-equatorial tungstacy-
clobutanes.
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