Chain-growth Polymerization

Overview of This Chapter

Ok, if you've read the first chapter, you should have a pretty good idea of what the three main types of polymerizations are and what their characteristics involve. Over on the left of that section (go back one) is a summary of some of those characteristics. Your assignment in that chapter was to make a table or list of these. Be a good idea to do it now if you didn't then; it'll be useful in the future.

Key to this chapter is the mechanistic approach and definition, namely "chain-growth." Sure, most chain-growth polymerizations are also addition polymerizations but that begs the main point: it's the details of the reactions that are important.

"Why?" you might ask. Answer is simple: knowledge, understanding and wisdom. Knowing that a reaction takes place and what the products are is one thing. It's quite another to know how the reactions take place. This knowledge gives you understanding of the process, and that opens up opportunities to use the same mechanism on other monomers to give you new polymers. Combined with wisdom based on ethics and experience, you can then choose monomers and polymers that improve the human condition while making your company profit at the same time. It's the old "two birds and one stone" scenario, although if you make it all the way to the "wisdom" part, it's actually three birds.

So let's explore this area based on the propagating species. This is the type of carbon at the growing end of the polymer chain. This species reacts with a vinyl monomer in such a way that the new two-carbon unit forms the same active species. For example, in radical polymerization, a carbon-centered free radical is the active propagating species on the terminal carbon. Reaction with a vinyl double bond forms a new single bond to one of the carbons, leaving the other with a single unpaired electron that then becomes the new free radical. And this goes on, over and over and over...

I mentioned the main topics in this chapter. They're pretty extensive, actually, so I've broken each one down into subtopics that are covered in separate pages. For example, free radical gets its own page first, followed by one on the unique aspects of radical propagation. Then there are pages for anionic, cationic and transition metal catalyzed vinyl polymerizations. These are all "chain-growth polymerizations," although there are major differences. As long as you keep moving forward in a linear fashion through the topics, no problem, concepts will build on each other. I'll also put in links to the related topics scattered throughout each separate page. So let's get started and and have fun! We'll go ahead and start with free-radical propagation on this page since it has a lot of general characteristics common to all vinyl chain-growth polymerizations.

Radical Chain-growth Details

One of the most common and useful reactions for making polymers is free radical polymerization. It is used to make polymers from vinyl monomers; that is, from small molecules containing carbon-carbon double bonds. Polymers made by free radical polymerization include (but not limited to) polystyrene, poly(methyl methacrylate), poly(vinyl acetate) and branched polyethylene. But enough introduction. What is this reaction, and how does it work?

The discussion above gives you the overall process but let's look at a specific case in detail, low density polyethylene or LDPE. First is the "match," the active species or initiator that gets the ball rolling. There are a number of types of radical initiators as indicated below.

The whole process starts off with a molecule called an initiator. This is a molecule like benzoyl peroxide or 2,2'-azo-bis-isobutyrylnitrile (AIBN). What is special about these molecules is that they have an uncanny ability to fall apart, in a rather unusual way. When they split, the pair of electrons in the bond which is broken, will separate. This is unusual as electrons like to be in pairs whenever possible. When this split happens, we're left with two fragments, called initiator fragments, parts of the original molecule, each of which has one unpaired electron. Molecules like this, with unpaired electrons are called free radicals.

Now remember, these unpaired electrons will be quite discontent with being alone and still want to be paired. If they can find ANY electrons to pair up with, they will do so. The carbon-carbon double bond in a vinyl monomer, like ethylene, has a pair of electrons which is very easily attacked by the free radical. The unpaired electron, when it comes near the pair of electrons, can't help but swipe one of them to pair with itself. This new pair of electrons forms a new chemical bond between the initiator fragment and one of the double bond carbons of the monomer molecule. This electron, having nowhere else to go, associates itself with the carbon atom which is not bonded to the initiator fragment. You can see that this will lead us back where we started, as we now have a new free radical when this unpaired electron comes to roost on that carbon atom. This whole process, the breakdown of the initiator molecule to form radicals, followed by the radical's reaction with a monomer molecule is called the initiation step of the polymerization.

Wouldn't you know it, this new radical reacts with another ethylene molecule in the exact same way as the initiator fragment did. Of course, as we can see, this gets us nowhere as far as pairing electrons goes, because we always form another radical when this reaction takes place over and over again.

This process, the adding of more and more monomer molecules to the growing chains, is called propagation.

Because we keep remaking the radical over and over again, we can keep adding more and more ethylene molecules, and build a long chain of them. Self-perpetuating reactions like this one are called chain reactions. So as long as the chain keeps growing, who really cares if a few electrons remain unpaired?

Fortunately, the electrons care. Radicals are unstable, and eventually they are going to find a way to become paired without generating a new radical. Then our little chain reaction will come grinding to a halt. This happens in several ways. The simplest way is for two growing chain ends to find each other. The two unpaired electrons then join to form a pair, and a new chemical bond joining their respective chains. This is called coupling.

Coupling is one of two main types of termination reaction. Termination is the third and final step of a chain-growth polymerization. Initiation and propagation are the first two steps, of course.

Now here comes the other termination reaction in which our unpaired electrons can shut down the polymerization: it's is called disproportionation. This is a rather complicated way in which two growing polymer chains solve the problem of their unpaired electrons. In disproportionation, when two growing chain ends come close together, the unpaired electron of one chain does something strange. Rather than simply joining with the unpaired electron of the other chain, it looks elsewhere for a mate. It finds one in the carbon-hydrogen bond of the carbon atom next to the other carbon radical. Our unpaired electron grabs not only one of the electrons from this bond, but the hydrogen atom as well. Now our first chain has no unpaired electrons, the end carbon now shares eight electrons with carbons and hydrogens, so everyone is happy.

Everyone, that is, except for the polymer chain which lost its hydrogen atom. It now has not only one carbon atom with an unpaired electron, but two! Now this looks bad but it's really not too difficult a problem, as it turns out. The two carbon radicals, being right next to each other, can easily join their unpaired electrons to form a pair, and thus form a chemical bond between the two carbon atoms. Now the two atoms already shared one pair of electrons, and the second shared pair creates a double bond at the end of the polymer chain.

Enough of the basics. You should now understand initiation, propagation and termination as the three main reactions in any chain-growth polymerization. There are some unique aspects to these for free-radical polyethylene that are different from the next two types, things like back-biting and long-chain branching. Sound interesting? Go to the next section to find out more about free-radical polymerization of ethylene and monomers like styrene and the various acrylates. And don't forget to take the quiz below so you know you're learning and understanding this stuff.

Link to section two of free radical polymerizations where we get into the nitty-gritty of how low density polyethylene is made and what it's unique structure provides, plus see how radical stabilization of the propagating chain end makes some monomers better than others for this type of process.