Mechanism of Condensation Polymerization

You know that monomers that are joined by condensation polymerization have two functional groups. You also know (from Part 6) that a carboxylic acid and an amine can form an amide linkage, jand a carboxylic acid and an alcohol can form an ester linkage. Since each monomer has two reactive sites, they can form long-chain polymers by making many amide or ester links. Let’s look at two examples of common polymers made from the monomers we have studied.

Example 1:
A carboxylic acid monomer and an amine monomer can join in an amide linkage.

As before, a water molecule is removed, and an amide linkage is formed. Notice that an acid group remains on one end of the chain, which can react with another amine monomer. Similarly, an amine group remains on the other end of the chain, which can react with another acid monomer.

Thus, monomers can continue to join by amide linkages to form a long chain. Because of the type of bond that links the monomers, this polymer is called a polyamide. The polymer made from these two six-carbon monomers is known as nylon-6,6. (Nylon products include hosiery, parachutes, and ropes.)

Example 2:
A carboxylic acid monomer and an alcohol monomer can join in an ester linkage.

A water molecule is removed as the ester linkage is formed. Notice the acid and the alcohol groups that are still available for bonding. ( )

Because the monomers above are all joined by ester linkages, the polymer chain is a polyester. This one is called PET, which stands for poly(ethylene terephthalate). (PET is used to make soft-drink bottles, magnetic tape, and many other plastic products.)

Let’s summarize:
As difunctional monomers join with amide and ester linkages, polyamides and polyesters are formed, respectively. We have seen the formation of the polyamide nylon-6,6 and the polyester PET. There are numerous other examples.

The above process is called condensation polymerization because a molecule is removed during the joining of the monomers. This molecule is frequently water.