Session 5:

A Closer Look at the Polymerization of Propylene


Question:

How can propylene molecules be joined together to form polypropylene chains?


Text:

When ethylene is polymerized into linear chains, only one arrangement of atoms is possible. However, the incorporation of substituents into the olefin monomer introduces the opportunity for some structural variability. For example, when propylene is polymerized, the monomers can arrange themselves along the chain in three different ways. If we call the CH2 end of the propylene the "head" and the CH(CH3) end the "tail", then a head-to-tail polymerization would lead to a polymer chain with a methyl group (CH3) located on every other carbon (see Figure 3). On the other hand, if the polymerization occurred in a head-to-head, tail-to-tail fashion, methyl groups would be located on adjacent carbons in pairs. A third possibility involves random orientation of monomer units along the polymer chain. These three different structural forms of polypropylene would be expected to have different physical properties. Generally, the head-to-tail polymer is produced using heterogeneous Ziegler-Natta or homogeneous cyclopentadienyl-zirconium catalysts. The reason for this result should become apparent in Session 6.

head-to-tail

head-to-head, tail-to-tail

random

Figure 3:

Two-dimensional representations of portions of linear polypropylene that result from different arrangements of monomer units along the polymer chain.

Corresponding 3D stick representations are shown above. The hydrogens are shown in white and the carbons are in green. The polymer is terminated by hydrogens.
Note: To download pdb files for the polymers shown above, please click on the 2D representation, the stick representation, or the words for the different arrangements.

There is another kind of structural variability in polypropylene that is perhaps a bit more subtle. Recall from Session 2 that the groups around each carbon atom in a polyolefin chain are tetrahedrally oriented. Hence, the chain has three-dimensional structure. Even if we assume a regular head-to-tail polymerization pattern for polypropylene, the methyl groups (which will be situated on alternating carbon atoms) can adopt different relative orientations with respect to the polymer backbone (see Figure 4).

isotactic

syndiotactic

atactic

Figure 4:

Portions of linear polypropylene with different orientations of pendant methyl groups along the polymer backbone. Carbon labels are omitted for clarity.

Corresponding 3D stick representations are shown above. The hydrogens are shown in white and the carbons are in green. The polymer is terminated by hydrogens.
Note: To download pdb files for the polymers shown above, please click on the 2D representation, the stick representation, or the words for the different orientations.

If all the methyl groups lie on the same side of the chain, the polymer is called isotactic. If the methyl groups alternate in a regular fashion from one side of the chain to the other, the polymer is syndiotactic. Finally, if the orientation of the methyl groups is random, the polymer is given the name atactic. The tacticity of a polymer can have a dramatic effect on its physical properties. For example, the melting points of isotactic, syndiotactic, and atactic polypropylene are 160-170 oC, 125-131 oC, and <0 oC, respectively. Atactic polypropylene is a soft, rubbery polymer, while isotactic polypropylene is strong and hard with excellent resistance to stress, cracking, and chemical reaction. Syndiotactic polypropylene has only recently been made on a large scale. It is somewhat softer than the isotactic polymer, but also tough and clear. It is stable to gamma radiation and will therefore find applications in medicine.


Hands-On Activity:

Using Cochrane's Molecular Models, build models of head-to-tail and head-to-head polypropylene. Build models of isotactic and syndiotactic polypropylene.


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