Metallocene Catalysis Vinyl Polymerization

Keywords:
complex, ligand

Metallocene polymerization is making a big stir in the plastics business. It's making a stir because it's the hottest thing to hit vinyl polymers since the invention of Ziegler-Natta polymerization. So what's all the song and dance surrounding this stuff about? The reason for all the fuss is that metallocene catalysis polymerization allows one to make polyethylene that can stop bullets! This new polyethylene is better than Kevlar for making bullet proof vests. It can do this because it has a much higher molecular weight than polyethylene made by the Ziegler-Natta recipe. How high, you ask? Up to six or seven million, that's how high!

There's more here than high weights. Metallocene polymerization is also good for making polymers of very specific tacticities. It can be tuned to make isotactic and syndiotactic polymers, depending on what you need.

Yes, it's great. What is it?

I knew I couldn't dodge this question forever. I could say simply, metallocene polymerization is polymerization catalyzed by metallocenes.

Big deal. What's a metallocene?

I figured you'd want to know that. Again, I could give simple answer that a metallocene is a positively charged metal ion sandwiched between two negatively charged cyclopentadienyl anions.

Big deal. What's a cyclopentadienyl anion?

My what an inquisitive mind you have! And your inquisitiveness will not go unrewarded! I will tell you that a cyclopentadienyl anion is a nifty little ion that's made from a little molecule called cyclopentadiene. I'm guessing you're just about to ask what that is, so I put a little picture of it right down below:

You may notice that most of the carbon atoms have one hydrogen, but one carbon atom has two hydrogens. One of those two hydrogens are acidic, that is, one will fall off very easily. When this happens, it leaves its bonding electrons (that is, an electron pair) behind. So the carbon it left now has only one hydrogen, just like the others, plus an extra pair of electrons.

Don't you just hate it when you've got extra electrons and nothing to do with them?

But this is not the case with cyclopentadiene, fear not! See those two double bonds in the molecule? Each of those has two electrons, remember, making four in all. Add those two extra electrons on the carbon that lost the hydrogen, and we have six.

This is important. Six electrons in a ring molecule like this will make the ring aromatic. If you've had enough organic chemistry to know what this means, great! If you haven't just know that it means the ring in this anionic form will be very stable.

Got that?

These cyclopentadienide ions have a charge of -1, so when a cation comes along, like Fe with a +2 charge, two of the anions will form an iron sandwich. That iron sandwich is called ferrocene.

Sometimes a metal with a bigger charge is involved, like zirconium with a +4 charge. To balance the charge, the zirconium will bond to two chloride ions, -1 charge on each, to give a neutral compound.

Zirconocenes are a little different from ferrocene. You see, those extra ligands, the chlorines, take up space. It's hard for them to squeeze in-between the cyclopentadienyl rings. So to make room for the chlorines, the rings become tilted with respect to each other, opening like a clam shell. This gives the chlorines space to breathe. Take look at the picture showing this tilt:

As you can see, the cyclopentadienyl rings, shown as the thick dark lines, are parallel to each other in ferrocene, but make an angle in zirconocene. This tilting happens whenever a metallocene has more ligands than just the two cp rings.

We can use some derivatives of bis-chlorozirconocene to make polymers. Take this one for example:

It's different from bis-chlorozirconocene in that each cp ring has a six-carbon aromatic ring fused to it, shown in red. This two-ring system made of a cp ring fused to a phenyl ring is called an indenyl ligand. Plus, there's an ethylene bridge, drawn in blue, that links the top and bottom cp rings. These two features make this compound a great catalyst for making isotactic polymers. You see, the big bulky indenyl ligands, pointed in opposite directions as they are, guide the incoming monomers, so that they can only react when pointed in the right direction to give isotactic polymers. That ethylene bridge holds the two indenyl rings in place. Without the bridge, they could swivel about and might not stay pointed in the right way to direct isotactic polymerization.

The Polymerization

We've talked about what metallocenes are, and a little about why they can make polymers with a specific tacticity. But we haven't talked about how the polymerization actually works. Fear not, for that's what we're going to talk about right now. To make our zirconocene complex catalyze a polymerization, the first thing we have to do is add a pinch of something called MAO. This compound was not discovered by the late Chinese dictator Mao Zedong as some of you might be guessing. Rather, MAO is short for methyl alumoxane. Wouldn't you know it, MAO is itself a polymer, with a structure like this:
It's an unusual polymer because it has metal atoms in the backbone. But we're more interested in what it does than what it is. To get our catalyst to work, we need to use a whole bunch of MAO, almost 1000 times the amount of catalyst. The MAO is going to do something with the chlorines of our zirconocene. You see, those chlorines are what we call labile. That is to say, they like to fall off of the zirconocene. So MAO can replace them with some of its methyl groups. We're left with a catalyst that looks like this:
Wouldn't you know it, the methyl groups can fall off, too. When one of them falls off, we get a complex that looks like this:
You'll notice in the picture that the positively-charged zirconium is stabilized because the electrons from the carbon-hydrogen bond are shared with the zirconium. This is called α-agostic association. But still, the zirconium is lacking in electrons. It needs more than just a wimpy agostic association to satisfy it. That's where our olefin monomer comes in. Imagine an alkene like propylene. Its carbon-carbon double bond is loaded with electrons to share. So it shares a pair with the zirconium, and, at least for now, everyone will be satisfied.
But complexation is a rather complicated process, not nearly as simple as this picture implies. If you already know how this works, you can skip the next section, and go straight to the polymerization. If not, read on, and learn how the complexation works.

Learn about alkene-metal complexation
Skip to the polymerization

Alkene-metal complexes

This is where it starts to get interesting. Suppose at this point that a vinyl monomer showed up, let's say, a molecule of propylene. The zirconium is going to enjoy this. To understand why, let's take a look at vinyl monomer, specifically, its double bond. A carbon-carbon double bond, is made up of a σ bond and a π bond. We're going to take a closer look at that π bond.
Take a look at the picture and you'll see that the π bond consists of two π-orbitals. One is the π-bonding orbital (shown in blue) and the other is the π-antibonding orbital (shown in red). The π-bonding orbital has two lobes sitting between the carbon atoms, and the π-antibonding orbital has four lobes, sticking out away from the two carbon atoms. Normally the pair of electrons stays in the π-bonding orbital. The π-antibonding orbital is too high in energy, so under normal circumstances it's empty.

Let's look again at zirconium for a moment. This picture shows zirconium and two of its d-orbitals. Now to be sure, zirconium has five d orbitals, but we're only going to show two right now for clarity.

One of the d-orbitals which we've shown is that empty orbital. It's made of the green lobes. The pink lobes are one of the filled d-orbitals. That empty d-orbital is going to look for a pair electrons, and it knows just where to look. It knows that the alkene's π-bonding orbital has a pair that it will share. So the alkene's π-bonding orbital and the zirconium's d-orbital come together and share a pair of electrons.
But once they're together, that other d-orbital comes mighty close to that empty π-antibonding orbital. So the d-orbital and the π-antibonding orbital share a pair of electrons, too.
This additional sharing of electrons makes the complex stronger. This complexation between the alkene and the zirconium sets things up for the next step of the polymerization.

The Polymerization

The precise nature of the complex between the zirconium and the propylene is complicated. So to make things simple we're going to just draw it like we did earlier from now on, like this:
This complexation stabilizes the zirconium, but not for long. You see, when this complex forms, it can rearrange itself into a new form. Electrons start to move, as you see in the picture below. The electrons in the zirconium-methyl carbon bond shift, to form a bond between the methyl carbon and one of the propylene carbons. Meanwhile, the electron pair that had been forming the alkene-metal complex shifts to form an outright bond between the zirconium and one of the propylene carbons.
As you can see in the picture, this happens through a four-membered transition state. As you can also see, the zirconium ends up just like it started, lacking a ligand, but with an agostic association with a C-H bond from the propylene monomer.

Being back where we started, another propylene monomer can come along and react just like the first one did.

The propylene coordinates with the zirconium...then the electrons shuffle:
When we're done a second propylene monomer has added to the chain. Notice that we end up with an isotactic polymer; the methyl groups are always on the same side of the polymer chain. As you might predict, the next monomer that comes along will coordinate with zirconium on the same side as the first. The direction of approach switches with each monomer added.

So why do we get an isotactic polymer? Let's look at the catalyst and an incoming propylene monomer for a minute. As you can see, the propylene monomer always approaches the catalyst with its methyl group pointed away from the indenyl ligand.

If the methyl group were pointed toward the indenyl ligand, the two would bump into each other, keeping the propylene from getting close enough to the zirconium to form a complex. So, only when the methyl group is pointed away from the indenyl ligand can the propylene complex with zirconium.

When the second monomer is added, it has to approach from the other side, and it also has to point its methyl group away from indenyl ring:

But notice that this means the methyl group is pointed up rather than down. Because the second propylene is adding from the opposite side as the first, it must be pointed in the opposite direction if the methyl groups are going to end up on the same side of the polymer chain. (Think about this awhile and it should make sense.)

Ok folks, this brings up a question. Knowing why this catalyst gives isotactic polypropylene, what kind of catalyst would give syndiotactic polypropylene?

Have you figured it out yet? It's a catalyst such as this bad boy, which was investigated by Ewen and Asanuma.

I think you can figure out why we get syndiotactic polymerization from this catalyst. Successive monomers approach from opposite sides of the catalyst, but they're always pointing their methyl groups up. This way, the methyl groups end up on alternating sides of the polymer chain.

Identity Crisis

But metallocene catalysts can do even stranger things than that. Let's consider bis(2-phenylindenyl)zirconium dichloride. This metallocene, as you see below, has no bridge between the two indenyl rings.
This means that the two rings can spin around freely. Sometimes the rings will be pointed in opposite directions. We call this the rac form. Other times the rings will be pointing in the same direction. We call this the meso form. The compound spends some time in the rac form, then flips around, and becomes the meso form. After awhile it will flip back again. This happens over and over again.

So what does this mean for our polymerization? It means something really strange will happen. When the zirconocene is in the rac form, poly propylene monomer can only approach in an orientation which will give isotactic polypropylene.

But when the zirconocene flips and becomes the meso form, propylene monomer can approach in any orientation. This will give atactic polypropylene.
Remember that the zirconocene is constantly flipping back and forth between the two forms. It even does this while polymerization is taking place. This means that the same polymer chain will end up having blocks that are atactic and blocks that are isotactic, like this:
This kind of polypropylene is called elastomeric polypropylene because that's what it is, an elastomer. But there's more. It is a special kind of elastomer called a thermoplastic elastomer. To learn more about why this wacky kind of polypropylene works as an elastomer, visit the polypropylene page and the thermoplastic elastomer page!

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