Emulsion polymerization is usually confined to radical polymerization in aqueous medium.
Only a few examples of anionic, cationic and catalytic polymerizations have been performed in aqueous emulsion. This is of course due to to the reactivity of water toward carbocations, carbanions and organometallic complexes. We have been interested in performing catalytic polymerizations in aqueous emulsion. Polyolefins such as polyethylene and polypropylene are commodity polymers which are prepared on a very large scale (approx 100 million tons per year). We could envisage that the preparation of aqueous emulsions of polyolefins could open the way to a new class of materials. We discovered by serendipity a polymerization catalyst which has a homopolymerization activity greater than 2,000,000 g/g/h. This catalyst is a neutral Nickel phospholyide which belongs to the family of Shop catalysts unrabelled by Prof. Keim. We initially used this catalyst to prepare polyolefins in aqueous emulsion.
Using this catalyst, it is possible to polymerize ethylene in water. As the reaction occurs in water, the activity is much lower than in organic medium, and reaches at best 10,000 g/g/h (P = 20 bars, T = 55 oC). Even in the presence of surfactant, the product is not colloidaly stable. Here is a picture of such product (ie a dispersion which is not colloidaly stable).
By using a miniemulsion process, it is however possible to increase colloidal stability and to generate a stable latex of polyolefin. On the right, this is a picture of a HDPE latex, and on the left, the transmission electron microscopy (TEM) picture of this latex. Note how the surface of these nanoparticles is irregular: this is due to the high crystallinity of HDPE.
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Left: Latex of HDPE
Right : TEM of the latex particles(average size : 30
x 200 nm)
the rugged shape is ascribed to the crystallinity |
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Several monomers, polar and non-polar, have been copolymerized with ethylene in emulsion. The insertion of such comonomers results in a decrease of the crystallitiy.
The particles become spherical and tacky. The colloidal stability is also greatly improved. Due to the confinement of the comonomer within the growing polymer particles (compartimentalization effect) the insertion of the comonomer is favored. Thus, using Pd catalysts, polar monomers such as acrylates have been copolymerized with ethylene in emulsion, leading to the formation of very interesting latexes. They have a low crystallinity, and therefore they easily form films, they adhere to many surfaces, and they are very hydrophobic.
These latexes can be used to prepare films, in the same manner that acrylic latexes are used in aqueous paints. These very hydrophobic latexes have promising properties for the protection against corrosion, as shown below with a standard corrosion test performed with a Qfog salt box. The aging of the coating was followed by electrochemical impedance spectroscopy.
Unique polyethylene nanoparticles can be prepared. This is, for example, the case for poly(ethylene-co-acrylic acid) nanoparticles. In collaboration with
Prof. Françoise Winnik, from University of Montreal, we have shown that these nanoparticles are thermoreversible.
In order to explain why these nanoparticles are thermoreversible, a modeling work has been performed in collaboration with
Prof.
Armand Soldera. This work has highlighted the pivotal role played by the surface tension energy of lateral faces in the formation of polyethylene nanocrystals dispersed in water.
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