High Molecular Mass Polyethylene Aqueous Latexes by Catalytic Polymerization

Emulsion polymerization of olefinic monomers is one of the most important and also versatile polymerization processes. Polymer latexes are obtained, that is, stable aqueous dispersions of surfactant-stabilized polymer microparticles in the range of 50 to 1000 nm diameter. Numerous applications of latexes (for example, in coatings) involve the formation of films upon evaporation of the dispersing medium, and the environmental friendliness and nonflammability of water is particularly advantageous. To date, polymer latexes are produced by free-radical processes exclusively. Transition metal catalyzed coordination polymerization has received comparatively little attention, as the early transition metal catalysts used commercially for polyolefin production are extremely sensitive to moisture. Carrying out such reactions in water is a highly attractive goal, however, as many polymer microstructures are not available by means other than catalytic polymerization. We and others have recently reported the successful polymerization of ethylene in water by neutral nickel(II) complexes. However, dispersions consisting of low molecular mass material with a degree of polymerization of less than 100 (Mn values typically 1000 gmol-1) were obtained. The formation of higher molecular mass polymers (with chain length considerably above the entanglement limit) is a crucial prerequisite to fully exploit the unique property profile of latexes. At the same time, the complex steps of microparticle formation during polymerization must afford stable dispersions of these high molecular mass polymers