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.

Ethylene is polymerized in water as a reaction medium by Pd(II) and Ni(II) complexes to afford branched or linear homopolymer.Water possesses unique properties as a reaction medium. It is highly polar and immiscible with most organic compounds, has a high heat capacity and also features a strong propensity for micelle formation. In addition, water is an ideal medium from an environmental and safety perspective. Thus, emulsion and suspension polymerization of olefinic monomers is employed on a vast scale, e.g. for the direct production of water-based lattices, used for coatings and paints. In contrast to these free radical polymerizations, transition metal catalyzed coordination polymerization reactions in water have received less attention, as the early transition metal catalysts 1 used predominately are extremely sensitive to moisture.Late transition metal complexes are generally less sensitive to polar media due to their less oxophilic nature. Due to the propensity of late transition metal alkyl complexes for bhydride elimination, dimers or oligomers are usually obtained in C-C linkage of ethylene. 2 Only a limited number of catalysts for polymerization to high molecular weight products are known. Most of them are based either on neutral Ni(II) complexes 3,14a of formally monoanionic bidentate ligands or on cationic Fe, Co, Ni or Pd complexes 4 of neutral multidentate ligands with bulky substituted N donor atoms. 5 The aforementioned stability of late transition metal complexes is demonstrated by the tolerance of some of these polymerization catalysts towards polar functionalized comonomers 3d,4b,c and polar organic solvents. 3c,d,4c,6 In the context of possible side reactions in transition metal catalysis in aqueous media 7 (such as hydrolysis of metal alkyl species, attack of water on coordinated substrates or coordination of water to the metal center as a ligand), regarding polymerization reactions a conceivable effect of water on chain transfer 8 is of specific interest as small (absolute) changes in the overall chain transfer rate will strongly influence product molecular weight.A very slow (ca. 1 turnover per day) coordination polymerization of ethylene in water catalyzed by a Rh complex has previously been investigated. 9 We now report on the homopolymerization of ethylene in water by neutral Ni(II) and cationic Pd(II) complexes.

The coordination polymerization of ethylene in water as a reaction medium was studied. Rubbery amorphous branched polyethylene was obtained when a known cationic diiminesubstituted methyl complex was employed as a catalyst precursor. High rates of up to 900 TO h À1 (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 ªencapsulationº of the waterinsoluble 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.

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)-) 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 × 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 Mw 2 × 10 5 (ethylene copolymers) respectively Mw 2 × 10 4 (1-olefin copolymers) with Mw/Mn 2-4. The 1-olefin copolymers exhibit glass transition temperatures of Tg ) +10 to -55°C, which is in the range desirable for latex applications.

Die Durchf¸hrung katalytischer Polymerisationen in w‰sserigen Systemen (w‰sseriger katalytischer Polymerisationen) mag auf den ersten Blick als ein Widerspruch erscheinen. Schlie˚lich ist die extreme Empfindlichkeit der in der technischen Olefinpolymerisation eingesetzten Ziegler-oder Phillips-Katalysatoren gegen Feuchtigkeit hinl‰nglich bekannt. Wasser bietet jedoch als Medium f¸r Polymerisationsreaktionen eine Kombination von au˚ergewˆhnlichen Eigenschaften: * Die hohe W‰rmekapazit‰t ermˆglicht ein effektives Ab-f¸hren der Polymerisationsw‰rme. * Die hohe Polarit‰t f¸hrt zu einem vˆllig anderen Mischungsverhalten mit vielen Monomeren und Polymeren als bei organischen Lˆsungsmitteln. * Mit Tensiden kˆnnen Dispersionen hydrophober Polymerpartikel in Wasser gegen Aggregation gesch¸tzt werden. * Wasser ist unbrennbar und nicht toxisch. Die vielseitigen aus diesem Eigenschaftsprofil resultierenden Mˆglichkeiten kˆnnen an den etablierten nichtkatalytischen Polymerisationsverfahren veranschaulicht werden.