Chemistry Reference
In-Depth Information
7.2
Catalytic Olefin and Alkyne Polymerization in Aqueous Systems
7.2.1
Historical Development
The reaction of ethylene with carbon monoxide in the presence of potassium tet-
racyanonickelate(II) can be regarded as the first example of an aqueous polymer-
ization. In a patent filed 1948, Reppe and Magin described the formation of oligo-
mers (n=1, 2) and higher “polyketones” [Eq. (3)] [19]. Albeit based on the analyti-
cal data provided it is unlikely that a true higher-molecular weight polymer was
formed; without doubt several repeat units were incorporated per product mole-
cule, a mechanistic feature which also applies to polymerization reactions.
3
In the 1960s a number of publications on aqueous polymerizations of different
monomers appeared. Rinehart et al. and Canale and co-workers independently re-
ported aqueous polymerization of butadiene catalyzed by rhodium salts. Utilizing
Rh
III
Cl
3
·3H
2
O as a catalyst precursor, semicrystalline
trans
-1,4-polybutadiene was
obtained stereoselectively [Eq. (4); >99%
trans
] [20, 21].
4
By comparison, free-radical butadiene polymerization in aqueous emulsion typi-
cally yields polymers with 60%
trans
-incorporated units under similar conditions
[22]. Polymer molecular weights of 2.6
10
4
g mol
-1
were determined by light scat-
tering [21].
A free-radical polymerization mechanism can be excluded on the basis of the
polymer microstructure and experiments with radical inhibitors. Rhodium(I)-spe-
cies, formed by reduction of Rh
III
salts used as catalyst precursors by butadiene
monomer, have been suggested as the active species. The catalyst is stable during
the aqueous polymerization for over 30 h [23]. Catalyst activities are moderate
with up to ca. 2
10
3
TO h
-1
[24, 25]. By contrast to industrially important free-
radical copolymerization, styrene is not incorporated in the rhodium-catalyzed bu-
tadiene polymerization [26]. Only scarce data is available regarding the stability
and other properties of the polymer dispersions obtained. Precipitation of consid-
erable portions of the polymer has been mentioned at high conversions in buta-
diene polymerization [23, 27].
Under similar conditions as those described by Rinehart for butadiene polymer-
ization, Natta et al. investigated the polymerization of cyclobutene. With low conver-
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