Welcome to the World of Giant Molecules!
In your previous lessons, you might have looked at Addition Polymerisation (how plastics like polyethene are made). Today, we are exploring its sophisticated cousin: Condensation Polymerisation. While addition polymers just "add" together, condensation polymers "click" together and release a tiny molecule as a "thank you" gift in the process. This is how nature builds proteins and how we build high-tech fabrics like Nylon!
1. What is Condensation Polymerisation?
In simple terms, condensation polymerisation is a process where many small molecules (monomers) join together to form a long chain (polymer), and every time a new link is made, a small molecule—usually water (\(H_2O\)) or hydrogen chloride (\(HCl\))—is "spat out" or eliminated.
Prerequisite Check:
Remember from your study of Organic Chemistry (Section 13.2) that a condensation reaction is just two molecules joining while losing a small one. Polymerisation is simply doing that reaction thousands of times in a row!
An Everyday Analogy
Imagine two people holding hands. To hold hands, they both have to let go of the suitcases they are carrying. The people are the monomers, the hand-hold is the chemical bond, and the suitcases they dropped are the water molecules being released.
Key Takeaway: Polymer = Many monomers joined + many small molecules lost.
2. The Two Main Types You Need to Know
For your Cambridge 9701 syllabus, you need to focus on two main families of condensation polymers: Polyesters and Polyamides.
A. Polyesters
As the name suggests, these are polymers held together by ester links. You have already learned about making a simple ester from a carboxylic acid and an alcohol (Section 18.2). To make a polymer, we just need monomers that have "functional groups" on both ends.
The Recipe:
• A dicarboxylic acid (a molecule with a \(-COOH\) group at both ends)
• A diol (a molecule with an \(-OH\) group at both ends)
What happens?
The \(-OH\) from the acid and the \(-H\) from the alcohol join to form water (\(H_2O\)). The remaining pieces join to form the ester link: \(-COO-\).
Example: Terylene (PET)
This is used in plastic bottles and clothing. It is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol.
B. Polyamides
These are held together by amide links (also called peptide links in biology). These are the bonds that hold your muscles (proteins) together!
The Recipe:
• A dicarboxylic acid (or a dioyl chloride)
• A diamine (a molecule with an \(-NH_2\) group at both ends)
What happens?
The \(-OH\) from the acid and a \(-H\) from the amine join to form water. The remaining pieces join to form the amide link: \(-CONH-\).
Example: Nylon 6,6
Nylon is used in everything from guitar strings to parachutes. The "6,6" tells you that both monomers have 6 carbon atoms each.
Quick Review Box:
• Polyester = Diol + Dicarboxylic Acid (releases \(H_2O\))
• Polyamide = Diamine + Dicarboxylic Acid (releases \(H_2O\))
3. How to Draw a "Repeat Unit"
Don't worry if drawing these looks intimidating! There is a simple "box trick" you can use. Exam questions often represent the middle of the monomer chain as a simple shaded box.
Step-by-Step for a Polyester:
1. Draw the two monomers side by side: \(HO-[\text{Box 1}]-OH\) and \(HOOC-[\text{Box 2}]-COOH\).
2. Circle the \(H\) from the alcohol and the \(OH\) from the acid. Cross them out (that's your water!).
3. Join the remaining oxygen from the alcohol directly to the carbon of the acid's carbonyl group (\(C=O\)).
4. To show it's a repeat unit, leave "extension bonds" sticking out of the ends of the boxes to show the chain continues.
Memory Aid:
In a Polyester, you see "C-O-C=O".
In a Polyamide, you see "N-H-C=O".
4. Addition vs. Condensation: The Big Differences
Students often mix these up. Here is a simple way to keep them straight:
Addition Polymerisation:
• Monomer must have a C=C double bond.
• The polymer is the only product.
• The backbone is just a chain of Carbon atoms.
Condensation Polymerisation:
• Monomers have functional groups (\(-OH, -NH_2, -COOH\)) at the ends.
• Two products are formed: the polymer + a small molecule (like water).
• The backbone contains atoms like Oxygen or Nitrogen.
Did you know?
Because condensation polymers have these "links" (esters or amides) in their backbone, they can be broken back down by reacting with water. This process is called hydrolysis. This makes some condensation polymers biodegradable, unlike addition polymers (like polyethene) which can sit in a landfill for hundreds of years!
5. Common Mistakes to Avoid
• Forgetting the small molecule: In an exam, if they ask for an equation, always remember to write \(+ (2n-1)H_2O\) or simply \(+ H_2O\).
• Incorrect Links: Make sure you don't flip the ester link. It should always be \(C=O\) connected to an \(O\).
• Missing Extension Bonds: Always draw the bonds going "through" the brackets to show the chain is infinite.
Summary Checklist
Can you:
• Define condensation polymerisation?
• Identify the monomers needed for a polyester and a polyamide?
• Draw the amide and ester linkages accurately?
• Explain why condensation polymers are often more environmentally friendly than addition polymers?
Don't worry if this seems tricky at first! The more you practice drawing the "click" between the monomers, the more natural it will feel. You've got this!