Overview

In this Lesson, we consider some intricacies of chain polymerization which we have disregarded to this point. For example, you all have learned about the concept of chirality in organic chemistry - is there such a thing for polymers, and how is it controlled? There is, and we call it tacticity - and the tacticity of the polymer has very important ramifications for the polymer properties. What about monomers that are conjugated and can have resonance? We will look at what happens when we polymerize dienes, which have conjugated double bonds, both of which can participate in polymerization. Lastly, we consider a special type of chain growth polymerization where are monomers are actually cyclic, called ring opening polymerization.

Learning Outcomes

By the end of this lesson, you should be able to:

• Identify the tacticity of a polymer.
• Draw the mechanisms for polymerization of dienes leading to 1,2 addition, 3,4 addition, and cis/trans 1,4 addition products.
• Describe the benefits of coordination polymerization and why Ziegler Natta polymerization is used.
• Draw mechanisms and describe the tacticity for cationic and anionic ring opening polymerization.

Lesson Checklist

Please refer to the Canvas Calendar for specific timeframes.

Questions?

If you have questions, please feel free to post them to the General Questions and Discussion forum. While you are there, feel free to post your own responses if you, too, are able to help a classmate.

Many commercially important polymers that you may use in your daily lives, such as epoxy glue, rely on ring opening polymerization. Ring opening polymerization is characterized by using a monomer that is cyclic (a ring) and the mechanism can proceed by radical, anionic, or cationic pathways depending on the specific monomer and initiator. During polymerization, the ring opens and creates a linear polymer. In many ways, ROP is familiar to our other mechanisms, and we can use it to make many of the same polymers (and some new ones). It still requires initiation, it still goes through propagation and termination, it still follows the same rules for electron donating and electron withdrawing groups as we learned for cationic and anionic polymerization.

FIgure 7.1: Examples of ring opening polymerizations

Source: Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

So why use a cyclic monomer instead of a non-cyclic one? Well, one important thing going for cyclic monomers (with small rings) is that they have ring strain, which makes them somewhat less stable (and hence more reactive) than non-cyclic monomers. For larger rings, they have less ring strain, but more steric repulsion between the ring substituents, which also makes them unstable.

Let's first look at a cationic ring opening polymerization of epoxides, which are very common cyclic monomers, and are used to produce polyethers. We can use the same initiator molecules. Note the use of double-headed arrows for moving pairs of electrons. We see some equilibrium structures here as well that are new - the cationic intermediates are in equilibrium between the ring open and ring closed state. Termination can occur by rearrangement, just as we saw before in cationic polymerization.

Figure 7.2: Cationic ring opening polymerization of epoxides

Source: Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

We can also consider ring opening polymerization of epoxides by anionic polymerization, as shown below.

Figure 7.3: Anionic ring opening polymerization of epoxides

Source: Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

Notice that again we see the same kinds of initiators as was typical for anionic polymerization. We have added an "R" group onto the epoxide to help stabilize the anion, and because of this substitution, the nucleophilic attack will occur on the least hindered carbon. We also see no inherent termination process, unlike cationic polymerization.

In addition to epoxides, another common class of cyclic monomers are lactones:

Figure 7.4: Anionic ring opening polymerization of lactones

Source: Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

PROBLEM 1

Of the monomers shown below, which would yield a polyether when polymerized by ring opening polymerization?

1. Isotactic polybutadiene
2. Syndiotactic polybutadiene
3. Atactic polybutadiene

Click here for the answer to Problem 1.

ANSWER 1

A. Isotactic polybutadiene

Isotactic polybutadiene would yield a polyether

PROBLEM 2

Of the monomers shown below, which one would you choose to make this polymer?

Click here for the answer to Problem 2.

ANSWER 2

D.

Because we usually draw polymers as flat 2D structures on a piece of paper, it's easy to forget that actually polymers are three-dimensional and the structure often has very important ramifications on the polymer properties and chemistry. For example, in organic chemistry, you learned about chirality or the "handedness" of molecules, where the position of the atoms in 3D space differ by mirror images of one another. Molecules that are chiral have the same connectivity of the atoms, but are not the same molecules - the structures would not be able to be overlaid with each other when the 3D structure is considered. We use a similar concept for polymers, tacticity.

Figure 7.5: Using 3D notation to show polymer tacticity

Source: Dr. Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

Look at the two repeat units shown in Figure 7.5. Recall from organic chemistry that we used the dashed lines to indicate a bond that goes "into" the page, and a bold line to indicate a bond "coming out of" the page, to give a 3D representation of the molecule. We see in Figure 7.5 that the connectivity of all the atoms is identical between the two structures, but the X and Y substituents are flipped in their 3D orientation. (The carbon that has the X and Y substituents is called an asymmetric carbon.) Therefore, these repeat units are actually not exactly the same; they cannot be overlaid with each other, no matter what orientation you choose. So even though the same monomer could be used to make both of these repeat units, these repeat units are different! In order to describe this difference in 3D structure, we use the concept of tacticity.

There are three terms we use to describe polymer tacticity. Isotactic means that all asymmetric carbons in the polymer have the same configuration. Syndiotactic means that the asymmetric carbons have alternating configuration. Atactic means that the asymmetric carbons have random orientation. Drawings of each of these is shown below. Notice that in order to have tacticity, you must have asymmetric carbons. If X=Y in Figure 7.5, then there is no asymmetric carbon, and no relevant tacticity. Also realize that the entire polymer does not have to be just one of these categories, there may be short stretches along the polymer which can take on any tacticity. Tacticity is important because it can affect the chemistry, reactivity, and mechanical properties of polymers. You can imagine that if the polymer structure is more "regular", such as an isotactic polymer versus an atactic polymer, that the isotactic polymer may be able to pack better, and thus crystallize more easily, which would significantly affect its material properties.

Figure 7.6: Three variations of polymer tacticity

Source: Dr. Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

So how does this tacticity arise? To understand this, we have to remember what the 3D structure of our active center looks like. Recall that the radical on our active center is in an unhybridized p orbital that is perpendicular to the plane with the sp² orbitals bonded to the substituents, as shown in Figure 7.7 below. In a slight oversimplification, we can imagine that the orientation of the next repeat unit will depend on whether the next monomer approaches the active center from above or below. If you are interested, for a more detailed figure depicting how this orientation gets translated to the tacticity, please see Figure 6.2 in the text.

Figure 7.7: Tacticity varies depending on the approach of the monomer

Source: Dr. Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

PROBLEM

The polymer below was formed from ring opening polymeriztion of the below monomer. What is the tacticity of this polymer?

1. Isotactic
2. Syndiotactic
3. Atactic

Click here for the answer.

ANSWER

A. Isotactic

Notice that isotactic and syndiotactic refer to the stereoisomerism of the asymmetric carbons, not necessarily the direction those bonds are "pointing". Here, we see the R groups alternate between going into and out of the page, so our first instinct is likely to say that this is a syndiotactic polymer - but also notice that if you were to rotate all the carbons so that they are pointing in the same direction, that in fact, all the carbons with the R group have the same stereochemistry, making it isotactic.

There are multiple variables that can affect the resulting stereochemistry of the polymer, such as counter ion, solvent, or temperature. However, it is still very difficult to make highly regular structures by any of the polymerization mechanisms we have discussed thus far. If we need to make highly stereoregular structures, we need a different approach: coordination polymerization. Coordination polymerization involves the addition of monomers to an organometallic active enter.

One of the most important coordination polymerization catalysts is the Ziegler-Natta catalyst. The Ziegler-Natta catalyst allows for the synthesis of highly linear, stereoregular polymers. For example, at the beginning of the course, we introduced both high and low density polyethylene; well, to make the high density polyethylene (HDPE), we noted that you had to have very highly linear polyethylene that packed together well. Such HDPE is often made using coordination polymerization, and cannot be accomplished by other polymerization routes. Similarly, polypropylene can also be produced with a highly steroregular structure (isotactic or syndiotactic) through coordination polymerization. A simplified mechanism of how this works is shown using a Zr catalysts is shown in Figure 7.8.

Figure 7.8: Simplified mechanism for a simplified catalyst for ethylene polymerization.

Source: Wikipedia - Ziegler–Natta catalyst

Since you have not likely taken inorganic chemistry, and for coordination polymerization we are dealing with organometallic molecules, I am not going to expect you to learn or understand exactly how the polymerization mechanism works. But there are a couple important differences to note. First, we see that the organometallic catalyst is located at our active center and "holds on" to the monomer. It holds on to the monomer is a very specific orientation, and so every monomer reacts in the same orientation. Recall how in Figure 7.7, we showed that the stereochemistry is controlled by whether the monomer approaches the active center from above or below? Well, we are basically using this organometallic catalyst to help us control that. But we also see that since the Zr has to coordinate to the monomer that the monomer has to be able to approach the catalyst and that bulky groups are going to prevent this coordination from happening. Therefore, we can't use this technique for every polymer, and those with bulky substituents are going to be less active towards coordination polymerization.

Thus far, we have only considered chain polymerization of monomers that have isolated reactive groups. But we also recall from general chemistry and organic chemistry that double bonds are conjugated and can participate in resonance (depending on the molecular structure). So what happens if we have a conjugated monomer? For example, 1,3-butadiene and isoprene are common monomers (shown below) which have conjugated double bonds. How do these monomers react?

Figure 7.9: 1,3-isobutadiene and isoprene

Source: Dr. Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

There are going to be multiple ways in which these unsaturated bonds can react. To keep track of which carbon is where, we need to number our carbons. Following tradition from the naming of organic molecules, we number our carbons along the carbon chain with the diene and start counting with the carbon nearest the substituent. Take this generic diene as an example, where "R" represents some substituent. We number the carbons as follows:

Figure 7.10: Generic diene with numbered carbons

Source: Dr. Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

It is very important that you number your carbons correctly, or you will end up drawing the wrong polymerization products. Please note that you do not necessarily number from left to right, it just depends on where that substituent "R" is located. We see we have two unsaturated bonds that we could polymerize: the 1,2 and 3,4 bonds. Figure 7.11 shows the products we get if we polymerize through those bonds:

Figure 7.11: Products of polymerizing through the 1,2 and 3,4 bonds

Source: Dr. Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

We see we get two very different polymer depending on whether we react the 1,2 or 3,4 bond. (Also, don't forget that each of these polymers can have tacticity!) But we aren't quite done, because these dienes have conjugated double bonds, which we know can have resonance. This means, if we generate an active center, that it can move through the molecule via resonance and this will also affect our products. To make things even more complicated, single bonds can rotate, and the bond rotation in the diene will also affect our polymer! As shown in Figure 7.12, because of these resonances and bond rotation we can also get cis 1,4 addition and trans 1,4 addition. (Remember the "cis" and "trans" geometric isomers from general chemistry and organic chemistry?).

Figure 7.12: Polymerization of dienes showing the formation of the cis and trans 1,4 addition products. The * signifies the active center.

Source: Dr. Lauren Zarzar based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

Just as tacticity was important to the polymer structure and properties, so is the mode of addition for dienes. Depending on whether you have primarily cis or trans addition, you can have polymers with very different mechanical properties.

PROBLEM

Shown below are (A) trans, (B) cis, and (C) vinyl polybutadiene (from left to right). Which one is most crystalline?

1. Trans polybutadiene
2. Cis polybutadiene
3. Vinyl polybutadiene

Click here for the answer.

ANSWER

A. Trans polybutadiene

The regular zig zag structure of the backbone will allow the polymer to pack better giving it more crystallinity.

Summary

After this lesson, you should now feel comfortable identifying the tacticity of a polymer, describing how the tacticity impacts the polymer properties, and suggesting methods by which to control tacticity. Additionally, you should be able to draw the products of polymerization of dienes, and describe the mechanism of the various products are formed. Lastly, you now can suggest cyclic monomers to use to create polymer via ring opening polymerization, and notice how many of those polymers you can also make by step growth! Next, we move onto copolymers – where we have not just one type of monomer, but two or more. We will find that when we mix monomers, it’s likely that one monomer is more reactive (and incorporated more quickly into the polymer). How do we measure that, and determine which monomers will actually end up in the polymer? All coming soon, in Lesson 8!

Reminder - Complete all of the Lesson 7 tasks!

You have reached the end of Lesson 7! Review the checklist on the Lesson 7 Overview / Checklist page to make sure you have completed all of the activities listed there before you begin Lesson 8.