Synthetic and naturally occurring polymers (chemistry only)

Welcome to the World of Polymers!

In this chapter, we are going to explore polymers—giant molecules that make up everything from the plastic bags in your kitchen to the DNA inside your body. Don’t worry if the names sound long or scary; we will break them down step-by-step. By the end of these notes, you’ll understand how small building blocks join together to create these massive, useful structures.

1. Addition Polymerisation

Imagine you have a box of paperclips. Each single paperclip is a monomer. If you link them all together to form a long chain, you’ve created a polymer.

What is Addition Polymerisation?

In addition polymerisation, many small molecules (monomers) join together to form very large molecules (polymers). For this to work, the monomers must have a double carbon-carbon bond (\( C=C \)). These monomers are usually alkenes, like ethene or propene.

How it works:
1. The double bond in the alkene "opens up."
2. The molecules bond to each other to form a long, single-bonded chain.
3. Important: In addition polymerisation, the polymer is the only product. No other atoms are lost or added. The repeating unit has exactly the same atoms as the monomer.

Representing Polymers

Since polymer chains can be thousands of atoms long, we don't draw the whole thing. Instead, we draw a repeating unit in brackets with a little 'n' outside to show it repeats many times.

Example: Poly(ethene)
Monomer: Ethene \( (C_2H_4) \)
Polymer: Poly(ethene) \( (C_2H_4)_n \)

Common Mistake to Avoid: When drawing the polymer, make sure you change the double bond (\( C=C \)) from the monomer into a single bond (\( C-C \)) in the polymer. Also, ensure the side bonds go through the brackets!

Quick Review: Addition Polymers

• Monomer: Small molecule with a double bond.
• Polymer: Long chain with single bonds.
• Naming: Just put "poly" in front of the monomer name. Ethene becomes poly(ethene).

2. Condensation Polymerisation (Higher Tier Only)

Don't worry if this seems a bit more complex! Just remember the main difference: addition polymerisation makes one product, but condensation polymerisation makes two.

What is Condensation Polymerisation?

This process involves monomers that have two functional groups (special groups of atoms that react). When these monomers join together, they usually lose a small molecule, most commonly water (\( H_2O \)). This is why it is called "condensation"—just like water forming on a cold window!

The Simplest Version

The simplest polymers are made from two different monomers, where each monomer has two of the same functional group. For example:
• A diol (a molecule with two \( -OH \) groups).
• A dicarboxylic acid (a molecule with two \( -COOH \) groups).

When these react, they form a polyester and release water.

Key Takeaway: Unlike addition polymerisation, condensation polymerisation produces the polymer PLUS a small molecule (usually water).

3. Amino Acids (Higher Tier Only)

Amino acids are the biological building blocks of life. They are unique because they have two different functional groups in the same molecule:

1. The basic amine group (\( -NH_2 \))
2. The acidic carboxylic acid group (\( -COOH \))

Making Polypeptides

Amino acids react by condensation polymerisation to produce polypeptides. For example, the simplest amino acid, glycine, polymerises to form a polypeptide and water.

Did you know? When different types of amino acids are combined in a long chain, they form a protein. Your hair, muscles, and enzymes are all made of these proteins!

4. Natural Polymers: DNA and Others

Nature was making polymers long before humans invented plastic! You need to know about a few key ones.

DNA (Deoxyribonucleic Acid)

DNA is a giant molecule that holds the genetic instructions for life. It is made of two polymer chains wrapped around each other in a shape called a double helix.

• The monomers in DNA are called nucleotides.
• There are four different nucleotides (often called A, T, C, and G).

Other Naturally Occurring Polymers

You should be able to name the monomers for these famous natural polymers:

• Proteins: Made from amino acid monomers.
• Starch: Made from glucose monomers.
• Cellulose: Also made from glucose monomers (but they are linked differently, which is why we can eat starch but not wood!).

Quick Review: Monomer Matches

• DNA \(\rightarrow\) Nucleotides
• Protein \(\rightarrow\) Amino acids
• Starch/Cellulose \(\rightarrow\) Glucose

Summary Checklist

1. Addition Polymerisation: Uses monomers with \( C=C \) bonds. One product only.
2. Condensation (HT): Monomers have two functional groups. Makes polymer + water.
3. Amino Acids (HT): Have \( -NH_2 \) and \( -COOH \) groups. They make proteins.
4. Natural Polymers: DNA (nucleotides), Proteins (amino acids), Starch (glucose).

Top Tip: If you are asked to identify a polymer from a diagram, look for the 'n' and the brackets. If you see a double bond in the chain, it's a monomer, not the polymer!