Chemistry Reference
In-Depth Information
Most early transition metal complexes are considered to catalyze the polymeriza-
tion through metal alkyl type intermediates. A typical example of such a catalyst,
affording a high molecular weight polyacetylene [132], is Ti(O
n
Bu)
4
/Et
3
Al [133].
The alkyl aluminum is thought to alkylate the titanium center, thus generating a
titanium ethyl complex, in which migratory insertion occurs. As in 1-olefin poly-
merization, chain transfer proceeds via
-hydride elimination, generating a tita-
nium hydride complex. Other catalytic systems include [V(mmh)
3
]/Et
3
Al [134],
[Co(NO
3
)
2
]/NaBH
4
[135, 136] or [Fe(acac)
3
]/Et
3
Al [137] (mmh=2-methylbutane-2,3-
dionato; acac = acetylacetonate). The necessity of activating most of these com-
plexes with aluminum alkyl co-catalysts is disadvantageous with regard to poly-
merizations in aqueous systems.
Although several rhodium carbenes (used in cyclopropanation of alkenes) are
known, alkyne polymerization catalysts based on rhodium are thought to propa-
gate in a similar fashion as early transition metal catalysts. To our knowledge, the
first rhodium-based catalyst system was RhCl
3
/LiBH
4
[138]. Subsequently, rho-
dium complexes
20
[139],
21
[139] and
22
[140] bearing diene ligands were then
used for the synthesis of stereoregular
cis-transoidal
polyphenylacetylene with high
polymer yield.
Usually, initiation of chain growth with rhodium catalysts does not require an al-
kylating agent. Kern has postulated the formation of an active species via oxidative
addition of the terminal alkyne to rhodium, so-called hydroethynylation [138]. This
reaction has also been reported by Werner et al. [141]: when contacting
[RhCl(P
i
Pr
3
)
2
] with phenylacetylene in the presence of pyridine, the alkyne hydride
complex
23
is generated (Scheme 7.10). In the presence of a base such as cyclopen-
tadienyl sodium, a tetracoordinated Rh
I
complex
24
was obtained and isolated.
Metals of Groups 5 and 6 (Nb, Ta, Mo and W) are known to form carbene com-
plexes and are widely used in olefin metathesis [99, 100, 111]. Therefore, the poly-
merization of substituted alkynes with catalysts based on these metals is assumed
Scheme 7.10
Oxidative addition of phenylacetylene (py=pyridine).
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