Welcome to Polyesters and Polyamides!
Hi there! Welcome to one of the most practical chapters in your A Level Chemistry journey. In this section, we are going to look at condensation polymers. If you’ve ever worn a "polyester" shirt or heard of "nylon" stockings, you’ve already encountered these molecules in the real world!
Don't worry if the structures look big and scary at first. We are going to break them down into simple shapes so you can master them in no time. Let’s dive in!
1. What is Condensation Polymerisation?
Before we look at the specific polymers, we need to understand the process. You might remember addition polymerisation (from alkenes) where the double bond simply opens up. Condensation polymerisation is different.
In this process, two different functional groups react together to form a long chain. Every time a new link is made, a small molecule is "spit out" as a byproduct. This is usually water (\(H_2O\)) or hydrogen chloride (\(HCl\)).
An Everyday Analogy
Imagine a line of people holding hands. To hold hands, each person must drop whatever they are holding in their left and right hands (like a bag of crisps or a water bottle) to make the connection. Those dropped items are the "small molecules" released during the reaction!
Quick Review:
- Addition Polymerisation: One monomer (alkene), no byproduct.
- Condensation Polymerisation: Two functional groups, releases a small molecule (like \(H_2O\)).
2. Polyesters
A polyester is formed when an ester link is created repeatedly. There are two ways this usually happens in your syllabus:
Method A: Dicarboxylic Acid + Diol
You react a molecule with two carboxylic acid groups (\(-COOH\)) and a molecule with two alcohol groups (\(-OH\)).
- 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: \( -C(=O)-O- \).
Method B: Diacyl Chloride + Diol
This is very similar, but instead of a carboxylic acid, we use a diacyl chloride (\(-COCl\)).
- The \(-Cl\) from the acyl chloride and the \(-H\) from the alcohol join to form hydrogen chloride (\(HCl\)).
- This reaction is much faster and more reactive than using a carboxylic acid!
Key Takeaway: Polyesters always contain the \( -COO - \) linkage repeating throughout the chain.
3. Polyamides
Polyamides are held together by amide links (also known as peptide bonds in biology). These are the polymers used to make things like Nylon and Kevlar.
How they form: Dicarboxylic Acid + Diamine
You react a molecule with two carboxylic acid groups (\(-COOH\)) and a molecule with two amine groups (\(-NH_2\)).
- The \(-OH\) from the acid and a \(-H\) from the amine join to form water (\(H_2O\)).
- The remaining pieces join to form the amide link: \( -C(=O)-NH- \).
Note: Just like polyesters, you can also use diacyl chlorides to make polyamides even faster, releasing \(HCl\) instead of water.
Did you know?
Proteins in your body are actually natural polyamides! They are made from amino acids, which contain both an amine and a carboxylic acid group on the same molecule.
Key Takeaway: Polyamides always contain the \( -CONH - \) linkage repeating throughout the chain.
4. Predicting Structures (Exam Skills)
The OCR examiners love to ask you to draw repeat units or identify monomers. Here is a step-by-step guide:
How to draw a Repeat Unit from Monomers:
- Line up the two monomers side-by-side.
- Remove the molecules: Take the \(-OH\) off the acid and the \(-H\) off the alcohol/amine.
- Join the "naked" ends together.
- Draw square brackets around the unit and add the letter \(n\) outside.
- Crucial Tip: Make sure the bonds extend through the square brackets to show the chain continues!
How to find Monomers from a Polymer chain:
Don't worry if this seems tricky at first—just look for the "cut" points!
- Find the ester link (\(C-O\)) or amide link (\(C-N\)).
- Imagine "cutting" that bond right down the middle.
- Add back the components of water: Add \(-OH\) back to the \(C=O\) side to make it an acid, and add \(-H\) back to the \(O\) or \(N\) side to make it an alcohol or amine.
Common Mistake to Avoid: When drawing a repeat unit, students often forget to remove the \(-H\) or \(-OH\) from the far ends of the monomers. Remember, the ends have to be "open" to connect to the next unit!
5. Hydrolysis: Breaking them down
Because polyesters and polyamides are made by "spitting out" water, they can be broken back down by "adding" water back in. This is called hydrolysis. However, plain water is very slow, so we use hot aqueous acid or hot aqueous alkali.
Acid Hydrolysis (\(H^+ / H_2O\))
- Polyesters: Break into the original dicarboxylic acid and diol.
- Polyamides: Break into the dicarboxylic acid and the ammonium salt of the diamine (because the amine reacts with the acid catalyst).
Base Hydrolysis (\(NaOH / H_2O\))
- Polyesters: Break into the diol and the sodium salt of the dicarboxylic acid (\(-COO^-Na^+\)).
- Polyamides: Break into the diamine and the sodium salt of the dicarboxylic acid.
Memory Aid: "Base loves Acids"
In base hydrolysis, the carboxylic acid part of the polymer won't end up as an acid; it will end up as a salt because the base reacts with it!
Key Takeaway Summary:
- Hydrolysis is the opposite of polymerisation.
- Acid hydrolysis gives you the acid (or its protonated form).
- Base hydrolysis gives you the salt of the acid.
Quick Review Quiz
Check your understanding with these three questions:
- What functional group is found in a polyester? (Answer: Ester link, \( -COO- \))
- What small molecule is lost when a diacyl chloride reacts with a diamine? (Answer: Hydrogen Chloride, \(HCl\))
- Which type of hydrolysis (acid or base) produces a carboxylate salt? (Answer: Base hydrolysis)
Well done for making it through these notes! Keep practicing drawing those structures, and you'll be an expert in no time.