Transition Metal-Catalyzed Polymerization in Aqueous Systems - Late Transition Metal Polymerization Catalysis - page 233

Scheme 7.4 Schematic representation of the chain growth mechanisms of various metal-
mediated polymerizations (R and Ar/Ar =growing polymer chain. X= halogen).
Whereas early transition metal (that is metals of Group 4 to 6) centers generally
strongly bind water, coordination to relatively less oxophilic late transition metal
complexes can be comparably weak and does not pose a severe problem in many
cases. Other possible side reactions are attack of water on coordinated olefin
monomer or other ligands [18].
In addition to the reactivity of the catalytically active metal centers, a sensitivity
of added co-catalysts towards water can obviously also be detrimental. For in-
stance, Ziegler catalysts or metallocenes are most often employed with aluminum
alkyls as co-catalysts. For polymerization in aqueous systems, catalysts which do
not require water-sensitive co-catalysts are desirable.
This contribution reviews aqueous transition metal catalyzed polymerizations of
olefinic monomers. Consequently, classical free-radical polymerizations employing
metal-containing redox-systems as initiators, for example, are not considered.
To enable a comparison of polymerization of different monomers and of cata-
lysts based on different metals, activities are usually given as turnovers per hour,
TO h -1 (TO=mol substrate converted per mol of metal). When comparing activ-
ities determined by different authors under strongly varying conditions the values
should be taken as an indication of the order of magnitude. The same holds true
for polymer molecular weights.