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to proceed via a metathesis-type mechanism. For catalysts prepared
in situ
, metal
chlorides and metal carbonyls are the two most important classes of precursors. Al-
kyne polymerizations with metal chlorides (MoCl
5
, WCl
6
, NbCl
5
and TaCl
5
) have
been investigated by Masuda et al. [131]. MoCl
5
, without any additional co-catalyst,
has been used for the synthesis of polyphenylacetylene [142] but the catalyst effi-
ciency is greatly increased upon addition of a co-catalyst (such as
n
Bu
4
Sn, Et
3
SiH,
Ph
3
Sb or Ph
3
Bi) [143]. In addition to these ill-defined catalyst systems, alkylidene
complexes based on tungsten (
25-27
[144] and
28
[145]), molybdenum [145] (
28
)
and tantalum [146] (
29
) have been synthesized and isolated prior to polymeriza-
tion. These well-defined catalyst precursors do not require a co-catalyst for polymer-
ization. Despite their advantages, these metals of Groups 5 and 6 are highly electro-
philic, which is disadvantageous for their use in aqueous polymerizations.
7.2.4.2
Catalytic Polymerization of Alkynes in Aqueous Systems
As for other coordination polymerizations, the utilization of water as a reaction
medium raises issues such as catalyst stability towards water, and miscibility of
the monomers, polymers and the catalyst with water. Concerning the catalyst sta-
bility towards water, to date polymerizations of alkynes in aqueous systems have
only been reported with rhodium and iridium catalysts. Unfortunately, the mecha-
nism of polymerization with these complexes in non-aqueous, organic reaction
media has not been completely elucidated (
vide supra
).
Blum, Schumann and co-workers reported the first study on alkyne polymerization
with water-soluble rhodium complexes in homogeneous or biphasic aqueous systems
[147]. Several
complexes were utilized: RhCl
3
·3H
2
O,
[RhCl(tppms)
3
]·4H
2
O
(tppms =3-Ph
2
PC
6
H
4
SO
3
Na),
[RhCl(cod)(tppms)]·H
2
O (cod=1,5-cyclooctadiene),
[Rh(
-SPh)(3-Ph
2
PC
6
H
4
CO
2
H)(CO)]
2
and the cluster
[Rh
3
O(OAc)
6
-(H
2
O)
3
]OAc. Trimerization of propynoic acid and oligomerization of
phenylacetylene were investigated with these catalyst precursors.
In the cyclotrimerization of propynoic acid, trimellitic (1,2,4-C
6
H
3
(CO
2
H)
3
) and
trimesic acid (1,3,5-C
6
H
3
(CO
2
H)
3
) are obtained as the product of head-to-head or
head-to-tail linkage, respectively. For the sake of comparison, reactions were con-
ducted both in neat water and in neat THF. In all cases, the yield of the trimer
was higher in the aqueous system, evidencing the high tolerance of the rhodium
catalyst towards water.
-3-Ph
2
PC
6
H
4
SO
3
)(cod)]
2
, [Rh(
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