30-1 Polymer synthesis: introduction to polymers and polymer synthesis


Uploaded by JUSTANEMONERD on 17.12.2012

Transcript:
PROFESSOR CIMA: Polymers. And, of course, this follows the chemistry
of carbon, because a lot of polymers we use today are based on carbon.
Not all, and I'm going to use some examples today.
But it is from the Greek poly, obviously meaning many, and mers,
which means part or unit. Many units, many parts.
And we usually start talking about these from the standpoint of how
they're made, at least the synthetic ones, how they're made.
And there are really three main methods that we will talk about today.
First is addition, condensation, and anionic. These are the most common ones that you find
around. Actually, I'd say condensation and anionic
are the ones that you most often experience in your every day life.
Addition, you typically won't run across that except in a big reactor.
It's an important synthesis method for making the kind of polymers
we use all the time. So let's do addition reactions.
So addition reactions require an initiation to start a chain reaction.
The chain reaction is something that just keeps on repeating itself until
it runs out of reactants. And the most common initiators are a family
of compounds called peroxides. Often those are organic groups.
They're not typically hydrogen peroxide, but there are some reactions
that can use hydrogen peroxide as an initiator. You put a small amount of this into the materials
that you're going to have the chain reaction with.
And if you heat it up, or put light on it, delta sometimes means heat, this
bond has a tendency to break into two what we call radicals.
They're long enough lived that it can exist in a very small amount.
But this can exist inside my soup that will eventually be a reaction.
Now, that's just the first part of the initiation. The second part requires a double bond around.
So let's just do ethylene. Hence, you need a reactor for this.
Ethylene, it's a gas, but in order to condense it into a liquid you have to
be at high pressures. So a lot of polyethylene will be made at high
pressures. This guy sits around and reacts with it to
initiate what will now be the species that carries out the chain reaction.
So this is the initiation, is to create this new ethylene radical.
So basically what happened is this bond is formed.
It breaks this double bond to form a carbon oxygen bond,
right here at the end. And this carbon is now starved for bonding,
so it's now the new radical. And, of course, we have a lot of other ethylene
around, so we undergo what's known as propagation, where this guy can now react with more ethylene to
form the R-O-CH2-CH2-CH2-CH2. You get the idea; still another radical.
And it just keeps on going. So this, now, loops back, and that's my propagation
scheme. This just keeps on going until we run into--
All polymerization reactions have a stopping point.
Obviously, if this ever runs into another radical, it'll stop.
If it runs out of ethylene, this reaction becomes highly probable.
It's the only thing it can do. And then there's other things.
There are poisons, I'll call them P star, which will stop the
reaction, or P dot. Anything else that's not supposed to be there,
that scavenges up radicals, will stop; things like oxygen are a typical
thing that will stop these reactions.
And so this can repeat itself. The number of times that it's repeated--
so you start with something at the end, and then it's CH2-CH2, then
somehow terminates at the end. This repeat unit is repeated so many times.
n is the degree
of polymerization. And obviously, the larger n is, the greater
the molecular weight of this polymer will be.
So some typical examples in polyethylene; n can be 10,000.
And with molecular weight of the ethylene at 28, that means this
particular polymer will have a molecular weight of around 300,000
grams per mole. So this can get pretty monstrously long molecules.
So an absolute length, that distance there is about 3 angstroms.
So to get its length, if you were to stretch it out--
Of course, it coils back on itself, and we'll talk about that.
But if you were to grab one end and stretch it out, it would be, in this
case, 10,000 times the 3 angstrom size. So it's straight length.
If you could, as I said, stretch it, it would be around 30,000 angstroms,
which is about 3 x 10^ -4 centimeters, or 3 microns.
So you could almost see that under a microscope, if you stretched it out.
Some of these, there's a kind of polyethylene that's called ultra high
molecular weight polyethylene you can buy. And this one has a molecular weight up to
6 million. So that ends up being about n is 214,000.
Huge. It's length, if you do it through this calculation,
is 60 microns. So you can see that, that distance, with your
eye. That's a good fraction of a human hair dimension.
We'll talk about others that are even longer. OK, so that's addition reactions.
This radical polymerization is very common for making things like
polyethylene.