Synthesis of Very Small Polymer Particles by Catalytic Polymerization in Aqueous Systems

In general terms, submicron particles can be prepared either by disintegration of larger entities, or by synthesis from small building blocks such as atoms, ions or molecules. By comparison to disintegration techniques, the latter approach provides efficient access to particles down to the nanometer size regime; and it can allow for the controlled synthesis of particles in terms of size, size distribution and morphology. Examples are the synthesis of noble metal colloids by reduction of solutions of metal ions, or the synthesis of polymer dispersions by emulsion polymerization. Classical emulsion polymerization provides particles with sizes in the range of ca. 1 mm down to 50 nm. Smaller particles are accessible in some cases by microemulsion polymerization as a special technique, as outlined below. By comparison to these well established free-radical polymerizations, catalytic synthesis of polymer dispersions offers a number of unique features. Polymer microstructures, and thus morphologies of particles, and of materials prepared thereof, can be controlled by the catalyst. Catalytic polymerization is complementary to radical polymerization to a large extent in terms of the monomers suited, and correspondingly the polymers accessible. In the past few years, we and the group of Spitz and Claverie have studied the synthesis of polyethylene dispersions by catalytic polymerization in some detail.