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Catalytic conversions in aqueous environments by transition metal complexes have become a well-established field over the past two decades. However, the vast majority of investigations have focussed on small-molecule synthesis. This may appear somewhat surprising as water is a particularly attractive reaction medium, especially for polymerization reactions. For example, aqueous emulsion and suspension polymerization is carried out today on a large scale by noncatalytic free-radical routes. Polymer latices can be obtained as a product, that is, stable aqueous dispersions of polymer particles in the size range of 50 to 1000 nm. Such latices possess a unique property profile. Amongst other advantages, the use of water as a dispersing medium is particularly environmentally friendly. In comparison to these free-radical reactions, aqueous catalytic polymerizations of olefinic monomers have received less attention. However, considerable advances and an increased awareness of this field have emerged during the past few years. A variety of high molecular weight polymers ranging from amorphous or semicrystalline polyolefins to polar-substituted hydrophilic materials have now been prepared by catalytic polymerization of olefinic monomers in water. Polymer latices based on a number of readily available monomers are accessible and catalytic activities as high as 105 turnovers per hour have already been reported. As another example, materials prepared by aqueous catalytic polymerization have been investigated as protein inhibitors. A versatile field spanning colloids, polymer, and coordination chemistry has emerged.

Alternating copolymerization of carbon monoxide with ethylene or 1-olefins in aqueous emulsion by water-insoluble palladium(II) complexes is reported. Latices of aliphatic polyketones (1-olefin/CO copolymers and ethylene/undec-10-enoic acid/CO terpolymers), prepared by catalytic polymerization, are described for the first time. An in situ catalyst system [{R2P(CH2)(3)PR2}Pd(OAc)(2)]/ strong acid (R = Ph or (CH2)(13)CH3) or well-defined complexes [{Ph2P(CH2)(3)PPh2}PdMe(NCCH3)]Y-+(-) = [B{3,5-(F3C)(2)C6H3}(4)](-) or SbF6-) were used in the form of a solution of the palladium(II) complex in miniemulsion droplets of a hydrocarbon dispersed in the continuous aqueous phase. Catalyst activities of up to 5 x 10(3) TO h(-1) slightly exceed those of nonaqueous polymerizations in methanol with the same catalysts. Polymer molecular weights (GPC vs PMMA standards) are typically M-w 2 x 10(5) (ethylene copolymers) respectively M-w 2 x 10(4) (1-olefin copolymers) with M-w/M-n 2-4. The 1-olefin copolymers exhibit glass transition temperatures of T-g = +10 to -55 degreesC, which is in the range desirable for latex applications.

Übergangsmetall-katalysierte Umsetzungen in wässerigen Medien sind in den vergangenen zwei Jahrzehnten intensiv untersucht worden und haben sich zu einem umfangreichen Gebiet entwickelt. Die große Mehrzahl der Arbeiten befasste sich mit der Synthese niedermolekularer Verbindungen. Dabei ist Wasser insbesondere für Polymerisationsreaktionen ein attraktives Reaktionsmedium. Zum Beispiel werden Emulsions- und Suspensionspolymerisationen in Wasser heute im großen Maßstab mittels (nichtkatalytischer) radikalischer Verfahren durchgeführt. Als Produkt können Polymerlatices erhalten werden, d. h. stabile wässerige Dispersionen von Polymerpartikeln mit Größen von 50 bis 1000 nm. Solche Latices haben ein außergewöhnliches Eigenschaftsprofil. Die Verwendung von Wasser als Dispersionsmedium ist zudem besonders umweltfreundlich. Im Vergleich zu diesen radikalischen Reaktionen haben katalytische Polymerisationen von Olefinen in Wasser weniger Aufmerksamkeit gefunden. In jüngster Zeit wurden jedoch erhebliche Fortschritte erzielt. Verschiedenste hochmolekulare Polymere, von amorphen oder teilkristallinen Polyolefinen bis zu polar substituierten hydrophilen Materialien, wurden mittels katalytischer Polymerisation olefinischer Monomere in Wasser hergestellt. Polymerlatices sind ausgehend von gut verfügbaren Monomeren zugänglich. Bei ihrer Synthese wurden bereits katalytische Aktivitäten bis 105 mol umgesetztes Substrat je mol Metall und Stunde beobachtet. Auch als Proteininhibitoren wurden Materialien untersucht, die durch katalytische Polymerisation in Wasser hergestellt wurden. Es hat sich ein vielseitiges Arbeitsgebiet herauskristallisiert, das Kolloid-, Polymer- und Koordinationschemie vereint.

Ethylene is polymerized in water as a reaction medium by Pd(II) and Ni(II) complexes to afford branched or linear homopolymer.

The coordination polymerization of ethylene in water as a reaction medium was studied. Rubbery amorphous branched polyethylene was obtained when a known cationic diimine-substituted methyl complex was employed as a catalyst precursor. High rates of up to 900 TOh(-1) (turnover frequency) were observed. In contrast to solution polymerization in an organic solvent, the rate of suspension polymerization in water increases greatly with ethylene pressure in the range up to 20 bar; this indicates control of the polymerization rate by the concentration of the olefin monomer at the catalytically active site. The effect and mode of mass transfer phenomena were studied. A high catalyst stability in the aqueous coordination polymerization was observed. It was found to be due to an "encapsulation" of the water-insoluble catalyst precursor in the hydrophobic amorphous polymer during the polymerization reaction, and this resulted in strongly restricted accessibility for the aqueous phase. Surprisingly, exposure of the water-stable catalyst precursor to ethylene monomer in solution in the presence of water resulted in immediate decomposition. Polymer microstructure, and thermal and mechanical properties were investigated. The different degree of branching, molecular weight, and corresponding macroscopic properties of the polymers obtained in water as a reaction medium versus solution polymerization in methylene chloride under the same conditions are due to the different phase behavior during polymerization (suspension vs. solution), as opposed to an effect of water on the catalytically active centers.