Free radical polymerisation
One of the most common and useful reaction 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 polystyrene, poly(methyl methacrylate), poly(vinyl acetate) and branched polyethylene. But enough introduction. What is this reaction, and how does it work?
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, 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, 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?
Sadly, 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:
Another way in which our unpaired electrons can shut down the polymerization
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, and everyone is happy.
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 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.
Sometimes, the unpaired electron at the end of a growing chain is so unhappy
that it will pair itself with an electron from a carbon-hydrogen bond along
the backbone of another polymer chain. This leaves an unpaired electron which
is nowhere near the propagating chain end. This electron can't form a double
bond the way the electron from the last example did, but it can and will react
with a monomer molecule, just the way the initiator fragment did. This starts
a new chain growing out of the middle of first chain! This is called chain
transfer to polymer, and the result is a branched polymer. It is
especially a problem with polyethylene,
so much that linear non-branched polyethylene can't be made by free radical
This branching has a big effect on how polyethylene behaves. To find out how, and how getting rid of branching helped make lowly polyethylene better than Kevlar in bulletproof vests, visit the fun-filled, fact-filled polyethylene page.