Emulsion Polymerization

Keywords
emulsion, soap

The definition for emulsion is: "A stable colloidal suspension as milk, consisting of an immiscible liquid dispersed and held in another liquid by a substance called an emulsifier." What this means is that something (in this case a monomer and its polymer) are suspended as very tiny droplets in a liquid that they aren't soluble in.

In order to understand emulsion polymerization and emulsifiers, we’ve got to understand how soap works. Yes, soap! If you already know how soap works and have Grandma’s recipe for lye soap, click here to get to the part on emulsion polymerization. If you’re curious, then read on!

Soap's Dirty Job

Soap molecules suffer from a multiple personality disorder, but their dual personalities are always apparent. A soap, or surfactant as it is referred to in emulsion polymerization, has two ends of different solubility. One end, termed the tail, is a long hydrocarbon that is soluble in nonpolar, organic compounds. The other, the head, is often a sodium or potassium salt, which is water soluble. The water soluble part can be the salt of a carboxylic acid or sulfonic acid but not phosphates. Turns out, phospherous acids and salts in all their forms are nutrients that make algae grow- not so good if you flush your soapy water into a lake, river or ocean where algae blooms can happen. The technical term for the chemical display of "dual personalities" is amphipathic.

One soap molecule isn’t much good to you. But when you get a whole bunch of ‘em together, neat stuff starts to happen. At a certain concentration in water, soap molecules congregate and form micelles. Scientists have an apt (if not original) name for this called the critical micelle concentration, or CMC for short. Don’t let the scientists fool you; these molecules are really doin’ the hokey pokey, tails inward.

Any dirt, grease, or grime that you happen to have on your hands is most likely organic and looks like this:

When you wash your hands with soapy water, the hokey pokey party really gets goin’. The insoluble dirt particle jumps right in the middle of the soapy micelle where it's pretty happy. It doesn't want to get out so it stays dissolved in the organic tails of the soap molecules.

Now while the dirt is dissolved in the micelle, the micelle is dissolved in water, and...Voila! With copious amounts of water, you can wash everything down the drain.

Now a question for you: Why is it difficult to take a bath in the ocean?

Finally...Emulsion Polymerization

In an emulsion polymerization, the soap, or surfactant, is dissolved in water until the critical micelle concentration (CMC) is reached. The interior of the micelle provides the site necessary for polymerization. A monomer (like styrene or methyl methacrylate) and a water soluble free radical initiator are added and the whole batch is shaken or stirred (sorry, James Bond). Emulsion polymerizations are always performed free radically. Anionic and cationic chain ends would be rapidly quenched by the water. The product of an emulsion polymerization is called a latex; does the term "latex paint" ring a bell? Latex paint, in fact, is what you get if you do the emulsion polymerization at high concentrations under just the right conditions, and then, brush it on and use water clean up for your brushes.

Location, Location, Location

Once everything is thrown in the pot, the monomer can be found in three different places. First, it can be in large monomer droplets floating around aimlessly in the water doing nothing. Second, some of the monomer may be dissolved in the water, but this is unlikely since it makes the process work not so well; you choose monomers that just aren't water soluble for this type of polymerization. Remember, organic monomers like styrene and methyl methacrylate are hydrophobic, so they work just fine. Lastly, the monomer may be found in micelles in small amounts, which is exactly where we want it. Now look back at the definition at the beginning of this page. The immiscible liquid is the hydrophobic monomer, the mother liquor is water, and the emulsifier is soap.

Initiation and Polymerization

Initiation takes place when an initiator fragment migrates into a micelle and reacts with a monomer molecule. Water soluble initiators, such as peroxides and persulfates, are commonly used that are just a tiny bit soluble inside the micelles. Because they're water soluble, they don't dissolve in the monomer outside the micelles and that prevents polymerization in the big monomer droplets.

Once polymerization starts, the micelle is referred to as a particle. Polymer molecules in these particles can grow to extremely high molecular weights, especially if the initiator concentration is low. That makes the radical concentration and the rate of termination low as well- all good. Sometimes a chain transfer agent is added to the mix to keep the molecular weight from getting too high.

Propagation

As the polymerization taking place inside the micelles uses up the monomer there, more monomer migrates from the large monomer droplets into the micelles to sustain polymerization. On average, there is one radical per micelle. Because of this, there isn't much competition for monomer between the growing chains in the particles, so they grow to nearly identical molecular weights and the polydispersity is low.

Another advantage of emulsion polymerization is that practically all the monomer is consumed in when the reaction is done, meaning the latex can be used without purification since there's no left over monomer to smell things up. This is important for paints and coatings. Just add some color to the latex, pour it into a can, and it's ready to use.

Here's the neat aspect of emulsion polymerization: each micelle can be considered as a mini bulk polymerization. Unlike traditional bulk polymerizations, there is no unreacted monomer leftover, and no thermal "hot spots" form. In bulk polymerizations (no solvent, just monomer and initiator), thermal hot spots cause run-away polymerization that generates even more heat, leading to degradation, discoloration and chain transfer that broadens the molecular weight distribution. An increase in temperature sometimes even cause the rate of polymerization to increase explosively. The water here acts as a heat sink for all those mini reactors and keeps them from blowing up! Pyromaniacs don't do emulsion polymerizations.

Molecular Weight

Now this is cool, too: The rate of polymerization is the same as the rate of disappearance of monomer and monomer disappears faster when there are more particles. In order to have more particles there must be more micelles. If the soap suds concentration is increased, this ought to give us more micelles. Now suppose the concentration of initiator is left the same. This will give us more particles and fewer radicals. What this means is the number of radicals per micelle drops below one. In other words, the rate of termination will be low since there are so few radicals that can find each other to terminate. WHEW!

So the end result is this: decreasing the initiator concentration increases molecular weight and rate of polymerization! This is completely opposite from bulk and solution polymerization. To increase the rate of polymerization for those, you have to heat up the reaction or increase the initiator concentration, both of which increase the rate of termination and lower the molecular weight. You also end up with lots of unreacted monomer that has to be removed somehow and that costs energy which costs money.

Practically Speaking

Sounds neat, but is it useful? Sure is. In summary, not only are there advantages to doing emulsion polymerizations in terms of the polymer made, it's also inexpensive to do and easy to control. In other words, it's one of the least expensive ways of making useful polymers.Poly(vinyl acetate), polychloroprene, polymethacrylates, poly(vinyl chloride), polyacrylamide, and copolymers of polystyrene, polybutadiene, and polyacrylonitrile are all made commercially by emulsion polymerization.

Congratulations!

You've waded through the muck to get this far, unless, of course, you skipped down to the bottom to see if anything cool was here. So take a look at this handy-dandy, easy to decipher table that lists the good and the bad (and, believe me, it's all ugly!).

Polymerization Process	The Good	The Bad
Bulk	Only monomer is present--no extra stuff, High molecular weights	Thermal hotspots, High viscosity, Unreacted monomer present
Solution	Easy temperature control, Control of molecular weight	Must remove solvent, Chain transfer to solvent
Emulsion	All monomer reacts, Latex is usable "as is", Thermally controllable, Can make low Tg materials, Low viscosity	Presence of surfactant may cause water sensitivity

References and Further Reading

Odian, G., "Principles of Polymerization" 3rd Ed., John Wiley and Sons, Inc., New York, 1991.

Rosen, S.L., "Fundamental Principles of Polymeric Materials" 2nd Ed., John Wiley and Sons, Inc., New York, 1993.

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