Tacticity

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.