Chemistry Reference
In-Depth Information
reaction that has been studied with non-sulfonated triarylphosphines is the aryl-aryl
interchange reaction in [Pd(aryl)IL
2
] complexes (Scheme 7.13) [238].
In a typical organic synthesis by means of Suzuki coupling of monohalogenated
compounds with monoalkyl boronic esters, this reaction results in the formation
of small amounts of side products. In AA-BB step polymerization, however, this
reaction represents a chain termination step or even results in branching
(Scheme 7.13) [239]. Accordingly, with [Pd(aryl)I(PPh
3
)
2
] as a catalyst precursor in
a THF/water mixture, the reaction results in the production of medium molecular
weight polymer (
M
n
=9.7
10
4
g mol
-1
) that is not a rigid rod. This polymer con-
tains a significant amount of phosphorus incorporated into the backbone; thus,
chain termination through phosphine-aryl exchange must have occurred to a sig-
nificant extent. Using a catalyst with more bulky phosphine ligands, [PdI{P(
o
-to-
lyl)
3
}
2
], for which the rate of aryl exchange is significantly lower, a higher molecu-
lar weight polymer is formed (
M
n
=2.0
10
5
g mol
-1
), and no phosphorus can be
detected in the polymer. Conformational analysis of the polymer by light scatter-
ing and viscosimetry indicate that the resulting polymer is close to a rigid rod.
7.5
Conclusions and Outlook
Although the possibility of carrying out catalytic polymerizations in the presence
of water had been known since the 1960s, significant advances in catalytic poly-
merizations in aqueous systems have only been achieved over the past decade. To-
day, (1) various different types of transition metal-catalyzed polymerizations can
be carried out efficiently in aqueous systems. (2) A variety of polymers, ranging
from hydrocarbons to water-soluble polymers, and a scope of polymer architec-
tures are accessible. (3) Polymerization can be carried out in a controlled fashion.
(4) Synthesis of polymer latexes possible in many cases.
One major thrust for these developments are certainly the recent discoveries of
new late transition metal-based polymerization catalysts and initiators. Well-de-
fined organometallic complexes, which do not require water-sensitive co-catalysts,
are now available as catalyst precursors or initiators for olefin polymerization.
Radical polymerization with metal complexes (ATRP) allows for a precise control
of molecular weight in aqueous polymerization. An increased general concern
with environmental issues makes water an attractive reaction medium. With poly-
mer latexes, this advantage is not restricted to the polymerization process but also
applies to the product itself. The general surge of interest in micro- and nanopar-
ticles currently continues, and catalytic polymerization in aqueous emulsion pro-
vides convenient access to dispersions of a range of novel polymer nanoparticles
in the size range of ca. 80 nm to 1
m.
A number of issues and challenges will be of particular interest in the future. Up
to now, little data is available regarding latex properties, such as stability and film
formation, and on the properties of the resulting films (such as adhesion to sub-
strates of varying polarity). In aqueous olefin polymerizations, these properties
Search WWH ::
Custom Search