Chemistry Reference
In-Depth Information
Polymerization of the water-insoluble monomer phenylacetylene with the same
catalysts was investigated in both homogeneous (THF) and biphasic systems (tol-
uene/water 5 : 1). In the biphasic mixture, [RhCl(cod)(tppms)]·H
2
O afforded a
polymer with a low activity (ca. TO 10 h
-1
), whereas all other catalysts only dimer-
ized or trimerized the monomer. By comparison, in refluxing THF solution,
[RhCl(cod)(tppms)]·H
2
O yielded only low oligomers with a very low activity
(TO=1.5 h
-1
). The reasons for this apparent enhancement in propagation rate in
the presence of water are unclear.
Subsequently, Tang et al. studied the polymerization of phenylacetylene and of
para-substituted phenylacetylenes with lipophilic and water-soluble catalysts [148].
With [Rh(nbd)Cl]
2
(
22
; nbd = norbornadiene), a strong influence of the nature of
the solvent on the polyphenylacetylene yield was observed. Whereas almost no
polymer is obtained in toluene, 58% conversion is observed in water within 30
min (corresponding to 264 turnovers). This effect is even enhanced with (
p
-
methylphenyl)acetylene as a monomer. The polymer molecular weight also in-
creased on going from toluene to water, the polyphenylacetylene obtained in the
aqueous system being insoluble even in THF. These two effects are intriguing as
neither the catalyst nor the monomer have been noted to be soluble in the reac-
tion medium. Similar results in terms of polymer yield and of molecular weight
were obtained with [Rh(cod)Cl]
2
(
21
) in water. The polymer microstructure is
highly stereoregular with a
cis
content of 94% as determined by the method of Si-
mionescu and Percec [149]. Unfortunately, polymerization in pure phenylacetylene
monomer has not been reported and therefore it is not possible to conclude
whether water is only a dispersion medium or if it interacts with the catalyst.
Aqueous polymerizations of phenylacetylene have been carried out with several
other rhodium complexes of the general formula [RhCl(cod)(L)], L representing a
nitrogen-based ligand, such as NH
3
,
t
BuNH
2
, piperidine or
N
-methylimidazole
[148]. These catalysts also afford high polymer yields, with polymer molecular
weights between 7
10
3
and 2.3
10
4
g mol
-1
(
M
w
/
M
n
around 2) with a high
cis
content (around 85%).
Besides this work with rather lipophilic catalyst precursors, two water-soluble
analogues have been investigated: [Rh(tos)(cod)(H
2
O)] (
30
; tos= tosylate) and the
similar compound [Rh(tos)(nbd)(H
2
O)] (
31
) [148]. The authors have verified that
water is still the best reaction medium for polymerization of phenylacetylene com-
pared to toluene, THF and neat monomer, thus indicating a possible involvement
of water in the formation of the active catalyst (830 TO in water). With catalyst
precursor
30
, low polydispersities and a
cis
content of 90% are found, whereas cat-
alyst precursor
31
affords a larger polydispersity but a higher cis content (100%).
Interestingly, these catalysts proved to be tolerant to air since similar polymeriza-
tion results are obtained with tap water without exclusion of air.
Joo et al. utilized the highly water-soluble rhodium complex [RhCl(CO)(TPPTS)
2
]
for the polymerization of terminal alkynes (phenylacetylene and (4-methylphe-
nyl)acetylene; for the structure of TPPTS cf. Section 7.2.2.3) [150]. This catalyst se-
lectively produces
cis
-transoid polymers at room temperature in homogeneous solu-
tion in water/methanol mixtures, as well as in biphasic mixtures of water and chloro-
Search WWH ::
Custom Search