Transition Metal-Catalyzed Polymerization in Aqueous Systems - Late Transition Metal Polymerization Catalysis - page 267

can be controlled by copolymerizations of common monomers, including polar-
functionalized olefins. Increases in catalyst efficiency regarding comonomer incor-
poration in ethylene polymerization and stability towards polar monomers are desir-
able. For instance, polyolefin latexes with film-forming properties could be useful for
UV-stable coatings. The possibility of preparing a variety of different polymer archi-
tectures in aqueous polymerizations by ATRP, such as highly branched polymers,
amphiphilic copolymers or surface-grafted inorganic or organic polymer particles,
will be of strong future interest. Molecular weight control in the preparation of
water-soluble polymers, such as polyacrylamide, is particularly attractive. Albeit very
high catalyst activities have already been reported in some cases, for all of the poly-
merizations discussed further improvements in catalyst activities are desirable. For
instance, in a polymerization aimed at preparing a latex it is desirable to leave the
catalyst in the product in order to maintain the intriguing simplicity of this direct
route to a polymer dispersion. To achieve negligible contents of more or less pre-
cious and potentially harmful metals and ligands, high efficiencies are necessary.
The relevance of metal and ligand residues will obviously depend on the application
sought and the catalyst used. The mechanisms of particle formation and polymeriza-
tion kinetics are of fundamental interest in their own right, but an understanding is
also desirable for a rational control of polymerization and to increase catalyst effi-
ciencies in these multiphase systems in general. For instance, the location of the
catalytically active metal centers during the different stages of the reaction (aque-
ous phase, inside monomer droplets and the respective polymer particles, or on
the interface) is largely unclarified to date. The scope of particle morphologies acces-
sible, e.g. core-shell particles with domains of different glass transition tempera-
tures, remains to be explored.
In summary, recent advances in aqueous catalytic polymerizations have afforded
a range of new materials, particularly polymer latexes previously inaccessible and
new water-soluble polymers. Various attractive topics for fundamental research
have in turn emerged, and potential applications can be envisioned. The attractive-
ness and versatility of this field results from the overlap and combination of poly-
mer chemistry, organometallic chemistry, catalysis, and colloid science.

7.6
References

1 Emulsion Polymerization and Emulsion
Polymers, P. A. Lovell, M. S. El-Aasser
(eds), Wiley, Chichester 1997.
2 G. Lagaly, O. Schulz, R. Zimehl, Dis-
persionen und Emulsionen , Steinkopff,
Darmstadt 1997.
3 Wäßrige Polymerdispersionen, D. Distler
(ed.), Wiley-VCH, Weinheim 1999.

5 P. L. Tang, E.D. Sudol, M. E. Adams,

C.A. Silebi, M. S. El-Aasser in: Polymer
latices. ACS Symp. Series 492, 1992 ,
pp. 72-98.

6 E.D. Sudol, M. S. El-Aasser in Ref. [1],

pp. 699-722.

7 K. Landfester, Macromol. Rapid Com-

mun. 2001 , 22 , 896-936.

4 R.M. Fitch, Polymer Colloids: a Compre-

hensive Introduction , Academic Press, San
Diego 1997.

8 D. F. Evans, H. Wennerstroem, The Col-

loidal Domain: Where Physics, Chemistry,
Biology, and Technology Meet , VCH, Wein-
heim, 2 nd edn., 1999, pp. 66-67.