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.
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.