Catalytic Ethylene Polymerization in Aqueous Emulsion : Catalyst Tailoring and Synthesis of Very Small Latex Particles

INTRODUCTION Emulsion polymerization yielding polymer latexes, is carried out on a vast scale. The use of water as a dispersion medium offers a unique combination of features, such as effective heat transfer, effective stabilization of polymer particles by surfactants, and environmental friendliness. By contrast to polymerization in solution or in the bulk, in emulsion polymerization a large portion of polymer can be generated in a given volume of reaction mixture without a strong increase in viscosity. The nontoxicity and nonflammability of water are also advantageous. To date, emulsion polymerization is industrially carried out by free radical processes exclusively. This limits the range of accessible polymer microstructures, and correspondingly the attainable materials properties. Therefore, the preparation of polymer latexes by catalytic polymerization of simple olefinic monomers is receiving increasing interest. The synthesis of dispersions from olefins directly obtained from cracking of hydrocarbon feedstocks, without the need for further energy and raw material consuming conversion to other monomers is desirable. To obtain polymer latexes in catalytic emulsion polymerization, water-soluble complexes can be used, in analogy to traditional freeradical polymerization or as an alternative concept aqueous miniemulsions of a solution of a lipophilic catalyst precursor in a small amount of hydrocarbon can be utilized. The latter approach enables the use of the generally more common lipophilic catalysts and in addition somewhat water-sensitive catalysts can also be employed. Attractive and challenging issues are catalysts enabling high polymerization activities, a design of simple and industrially applicable catalytic systems and the control of latex particle size and structure.