Transition Metal-Catalyzed Polymerization in Aqueous Systems - Late Transition Metal Polymerization Catalysis - page 251

enable utilization of the commercially available hydrophobic carbene complex 16b
as a catalyst precursor for latex synthesis, the latter was employed as a solution in
toluene/hexadecane miniemulsion droplets (cf. Section 7.2.2.2). Hereby, in addition
to norbornene, cyclooctene and cyclooctadiene, which are unreactive towards the
water-soluble catalysts, can be polymerized to latexes of high molecular weight poly-
mers. Copolymerization of cyclopentene and cyclooctene by 16b in miniemulsion
has also been reported independently by Kühn et al. [127]. Copolymerization of nor-
bornene with poly(ethylene oxide)-substituted norbornene macromonomers by 16b
to afford latex particles has been reported [128]. Although this reaction was carried
out in ethanol/methylene chloride mixtures and thus does not represent an aqueous
polymerization, from the viewpoint of colloid chemistry it is interesting that the con-
cept of polymerizable stabilizers (“surfmers”) [129], well known from free radical po-
lymerization, has been applied to a dispersion-type ROMP.
7.2.4
Recent Progresses in Catalytic Polymerization of Alkynes
7.2.4.1 Introduction
Twenty five years after the first report on catalytic polymerization of acetylene by
Shirikawa, McDiarmid and Heeger [130], the Nobel prize has been awarded to
these researchers for their contribution to the field of conducting polymers. In-
deed, such polymers have promising applications such as low weight/high charge
density batteries, polymer-modified electrodes, or capacitors to name only a few.
Driven by these applications, different classes of polyacetylenes have been synthe-
sized, starting from acetylene itself and from mono- or disubstituted acetylene de-
rivatives.
Alkyne polymerization in organic media has been reviewed [131]. A large vari-
ety of catalysts has been reported to polymerize alkynes in organic media. Similar
to the polymerization of olefins, early transition metal as well as late transition
metal catalysts are effective for this polymerization. Depending on the nature of
the metal, two different mechanisms of polymerization have been suggested: poly-
merization via a metal alkyl intermediate, or via a metal carbene (Scheme 7.9).
With metal alkyl complexes, polymerization proceeds via migratory insertion of
the alkyne into the metal-carbon bond [path (a) in Scheme 7.9] whereas with me-
tal carbenes the mechanism is equivalent to that of metathesis [path (b)].
Scheme 7.9 Mechanisms of alkyne polymerization.