62. Condensation Polymers (Polyesters)

1. What are Condensation Polymers?

Condensation polymerization involves monomers joining together while losing a small molecule as a byproduct, usually water (H2O). This is why it is called "condensation" polymerization.

2. Making Polyesters

To form a polyester, you typically need two different types of monomers, each with two functional groups:

  • A Dicarboxylic Acid: Contains two carboxylic acid groups (-COOH).

  • A Diol: Contains two alcohol groups (-OH).

3. The Reaction Process

  • The dicarboxylic acid loses its -OH group.

  • The diol loses a hydrogen atom (H) from its -OH group.

  • These three atoms combine to form a water molecule (H2O).

  • A new bond forms between the remaining carbon and oxygen, creating an ester link.

  • This process happens at both ends of the monomers, allowing them to join into long chains.

4. Representing the Equation

Because the reaction involves many monomers, we use the letter n to represent the number of molecules:

  • n (Dicarboxylic Acid) + n (Diol) —→ [Repeating Unit]n + 2n H2O

  • Note: We write 2n for water because two water molecules are produced for every repeating unit formed (one from each side of the monomer).

5. Key Requirements for Condensation Polymerization

  1. Each monomer must have at least two functional groups.

  2. There must be at least two different functional groups overall (e.g., carboxyl and alcohol).

  3. A small molecule (usually water) is given off during the process.

6. Biodegradability

A major difference between condensation polymers and addition polymers is their impact on the environment:

  • Polyesters: Generally biodegradable. Bacteria and microorganisms can break down the ester links naturally.

  • Addition Polymers (e.g., Plastics): Generally not biodegradable, meaning they persist in the environment for a very long time.

Example: Ethanedioic acid + Ethanediol —→ Poly(ethyl ethanoate) + Water.