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In-Depth Information
tion in
aqueous
systems can afford high-molecular-weight aliphatic alternating poly-
ketones at rates similar to the commercial process performed in methanol as a non-
aqueous medium (up to 6
10
4
TO h
-1
). Stable latexes can also be prepared. By prop-
er choice of the olefin component(s), the glass transition temperature can be ad-
justed to values desirable for film formation under ambient conditions. The accessi-
bility of latexes may enable new applications, which also do not require processing of
the polyketones at disadvantageously high temperatures. Ethylene polymerization in
aqueous systems can be carried out with a variety of different catalysts, affording
high-molecular-weight completely amorphous or semicrystalline polymer. In partic-
ular, stable latexes of largely linear polyethylenes have been prepared. Copolymeriza-
tion with various olefinic monomers enables a control of polymer crystallinity. High
catalyst activities have already been achieved in the synthesis of latexes of low-molec-
ular-weight material (up to 2
10
5
TO h
-1
). Such polymerizations can be of interest
as a convenient direct route to wax-dispersions. The accessibility of latexes of high-
molecular-weight polyethylene has also been demonstrated (
M
w
4
10
5
g mol
-1
;
M
w
/
M
n
2 to 4). In olefin polymerization as well as CO copolymerization, a limited con-
version of liquid 1-olefin (co)monomers is yet to be overcome in many cases. As an
example of properties that could find potential application, polyolefins contain a neg-
ligible proportion of double bonds by comparison to styrene-butadiene copolymers, a
hydrocarbon polymer currently prepared by free-radical emulsion polymerization on
a large scale. This can result in a considerably higher stability towards UV-light and
air of polymer films formed from polyolefin latexes.
ROMP in aqueous systems has been investigated intensively. A broad range of
monomers, including many water-soluble norbornene derivatives, can be polymer-
ized to high-molecular-weight hydrophilic or lipophilic materials. Utilization of
well-defined metal carbenes as catalyst precursors respectively initiators enables mo-
lecular weight control and living polymerization, and water-soluble ruthenium car-
benes have recently become available. Whereas, typically, monomer/metal ratios
of 100 : 1 have been applied in reactions of polar-substituted norbornenes, activities
of up to 10
4
TO h
-1
have been observed to date in the synthesis of polynorbornene
latexes. Aqueous ROMP represents a versatile synthetic route to specialty materials,
e.g. resultant water-soluble polymers have been used for biomedical investigations.
Aqueous polymerization of
butadiene
and
norbornenes
, as well as
alkynes
has also
been studied, and stable latexes can also be obtained in various cases.
7.3
Controlled Free-Radical Polymerization in Aqueous Systems with Metal Complexes
7.3.1
General Features of Atom Transfer Radical Polymerization (ATRP)
Radical polymerizations can be controlled by addition of a suitable metal complex,
which reversibly transforms the free radicals into a dormant species. Such is the
case in atom transfer radical polymerization (ATRP) [155, 156]. Contrary to the
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