35.1 Condensation Polymerisation

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What is Condensation Polymerisation?

A polymer is a very long molecule made by joining many smaller molecules together. Those smaller molecules are called monomers.

There are two main types of polymerisation. You may already know about addition polymerisation, where monomers simply join together with nothing left over. Condensation polymerisation is different — every time two monomers join, a small molecule is released as a by-product. This small molecule is almost always water (H₂O), but it can also be hydrogen chloride (HCl) in certain reactions.

Objective 1 — Formation of Polyesters

A polyester is a condensation polymer that contains ester linkages (–COO–) along its chain. An ester linkage forms when a carboxylic acid group (–COOH) reacts with an alcohol group (–OH).

(a) Reaction Between a Diol and a Dicarboxylic Acid

• A diol is a molecule with two –OH (hydroxyl) groups, one at each end.
• A dicarboxylic acid is a molecule with two –COOH (carboxylic acid) groups, one at each end.

When these two types of monomer react together, the –OH from the diol and the –COOH from the acid join to form an ester bond (–COO–), and water (H₂O) is released.

Because both monomers have two functional groups, this process keeps repeating along both ends of every monomer, building up a very long polymer chain.

Example — making PET (a very common polyester used in plastic bottles and clothing fibres):

• Diol monomer: ethane-1,2-diol: HO–CH₂CH₂–OH
• Dicarboxylic acid monomer: benzene-1,4-dicarboxylic acid (also called terephthalic acid): HOOC–C₆H₄–COOH

Each time a –OH meets a –COOH, they react to form –COO– and release H₂O.

Repeat unit of PET:

–[–O–CH₂CH₂–O–CO–C₆H₄–CO–]–

The ester linkage is the –COO– part of the chain. Notice it appears twice in each repeat unit because two ester bonds form per pair of monomers.