Ziegler-Natta Polymerization - Online Tutor, Practice Problems & Exam Prep

The Ziegler-Natta polymerization reaction is a highly stereoselective process that produces isotactic and syndiotactic polymers using a catalyst, typically an organometallic complex of titanium and aluminum. The mechanism involves three key steps: activation of the catalyst, coordination of the alkene monomer with the titanium, and chain growth through the insertion of monomers. This method avoids radical formation, resulting in linear polymers without branching. Notably, the catalyst is deactivated by polar groups in monomers, emphasizing the importance of monomer selection in polymer synthesis.

Ziegler-Natta Polymerization Concept 1

Hey, everyone. Let's take a look at the Ziegler-Natta polymerization reaction. Here, we're going to say that it's highly stereoselective, and it's stereoselective for isotactic and syndiotactic polymers. Here, we're going to say it uses its catalyst, and we can think of this as an organometallic complex, most commonly containing titanium and aluminum. Now here, we're going to say the polymer stereochemistry is going to be catalyst specific.

And there are no radicals formed which results in our linear polymers. So we're not going to have to worry about branching. Alright. So if we take a look here, we have titanium 4 chloride. This represents our inactive catalyst.

And here we have our cocatalyst. Here we have aluminum connected to 3 ethyl groups. Remember, aluminum's in group 3A. Typically, we would see it connecting to 3 things because it has 3 valence electrons. So here, we have one of these ethyls connecting to our titanium, thus activating it.

So we're going to say this is our active catalyst. Here, we're going to introduce our R group which is our monomer. And what it does is it interjects itself, separating, we're going to say it's going to separate out the ethyl from the titanium. It's going to step in there in between. And then the ethyl is going to connect to it.

So this is the broad overview of how this polymerization reaction works. Alright? So keep that in mind when we're talking about this particular type of polymerization.

Ziegler-Natta Polymerization Concept 2

We now take a look at the Ziegler-Natta polymerization reaction in terms of its mechanism. Step 1 is the activation of its catalyst. The Ziegler-Natta catalyst is activated. Here, we have our titanium(IV) chloride and our aluminum with its three ethyls, one of which is attached to the titanium, thus activating it.

Next, we're going to say coordination is what occurs next. With coordination, electrons from the π bond of our alkene monomer share with the titanium. In step 2, we have ourselves sharing this π bond with the titanium. And that leads us to step 3, which is our chain growth.

Electrons from the titanium ethyl group shift, and a monomer is inserted between our titanium and our ethyl group. Here is our alkene, now it's a CH₂CH₂. It has moved the ethyl out of the way. So there goes our group.

It is said here that steps 2 and 3 are repeated as needed. If desired, we can just keep adding a higher number of these groups here, which we will denote as n. Now here, we emphasize that the catalyst cannot be used with monomers that contain polar groups as it deactivates the catalyst. This provides a more specific, deeper dive into the actual mechanism that occurs. Remember, it's composed of three steps: activation, coordination, and then chain growth.

Ziegler-Natta Polymerization Example 1

Here we're told that polyproline is polymerized using a Ziegler, not a catalyst, selected for isotactic stereochemistry. Draw a segment of the resulting polymer. So if we're talking about isotactic, that means all the R groups are on the same side. So if we wanted to draw this out, we'd have our chain. And if we're talking about propylene, it looks like this.

So, propyl is a 3-carbon chain, and ene is an alkene. It looks like this. Alright. Then here, the R group would be this methyl group right here. And since it's isotactic, they're all on the same side, so I'm going to make them all wedged.

Okay. So this would be the segment of the resulting polymer that forms if it has an isotactic stereochemistry.