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