Welcome to Synthetic Polymers! Your Chemistry Survival Guide

Hello future chemists! Get ready to dive into the amazing world of polymers. Don't worry if the name sounds complicated; you interact with polymers every single day—we’re talking about plastics, nylon, Styrofoam, and rubber.

This chapter is essential because it connects the small organic molecules we’ve studied (like alkenes) to the huge, useful materials that make up modern society. Understanding how these massive molecules are built is a key part of organic chemistry.

What You Will Learn:

  • What monomers and polymers are.
  • How addition polymerisation works using ethene.
  • Why plastics are so durable (and why that causes problems).

1. Monomers, Polymers, and Polymerisation: The Building Blocks

Key Definitions: Think Lego!

Imagine you have a box full of identical small Lego bricks. Now imagine you snap them all together to form a very long, long chain.

1. Monomer:
A monomer (mono- means ‘one’) is the small, simple organic molecule that acts as the basic building block.
Analogy: A single Lego brick.

2. Polymer:
A polymer (poly- means ‘many’) is the large molecule formed when thousands of monomers link together in a chain. Polymers are sometimes called macromolecules (mega-molecules!).
Analogy: The long chain you built from all the Lego bricks.

3. Polymerisation:
This is the chemical reaction where the small monomers join together to create the long polymer chain.

Quick Review Box

Key Takeaway: Small units (monomers) join up to make one very large chain (polymer) through the process of polymerisation.

2. Addition Polymerisation: Making Plastics from Alkenes

In IGCSE Chemistry, the main way we study polymers being made is through addition polymerisation. This type of reaction only works with monomers that have a carbon-carbon double bond (i.e., alkenes).

The Monomer: Ethene

The simplest and most important monomer you need to know is ethene (C2H4).

  • Ethene is an unsaturated hydrocarbon (it has a C=C double bond).
  • This double bond is the secret weapon! It allows the molecule to open up and connect to other ethene molecules.

The Polymer: Poly(ethene) (Polythene)

When thousands of ethene molecules link together, they form poly(ethene), commonly known as polythene or polyethylene. This is the plastic used to make shopping bags, plastic films, and many types of containers.

Step-by-Step: How Addition Polymerisation Works

The entire process is driven by the opening up of the double bond:

  1. Start with Monomers: You have many ethene molecules (the small units), all with a C=C double bond.
  2. Break the Double Bond: Under high pressure and high temperature (and often using a catalyst), the double bond in the ethene molecule breaks (opens up).
  3. Form Single Bonds: When the double bond breaks, it leaves two free bonding sites (one on each carbon atom).
  4. Chain Formation: These free bonding sites immediately link up with the free bonding sites of the next monomer, creating a very long, continuous chain of single C-C bonds.

Important Rule: In addition polymerisation, the polymer formed is the only product. All the atoms in the monomers are added directly into the chain.

Analogy: Think of the C=C bond as a clasp on a belt. When you polymerise, you unhook the clasp so the belt can link directly onto the next belt.

The General Equation (The Key Structure)

You must be able to represent the polymerisation of ethene using the standard structure. We use the letter \(n\) to show that a very large, indefinite number of monomers are involved.

n Monomers (Ethene) \(\rightarrow\) 1 Polymer Chain (Poly(ethene))

The chemical equation is represented as:

\( n \left( \begin{matrix} H \\ | \\ C=C \\ | \\ H \end{matrix} \begin{matrix} H \\ | \\ \\ | \\ H \end{matrix} \right) \rightarrow \left[ \begin{matrix} H \\ | \\ -C-C- \\ | \\ H \end{matrix} \begin{matrix} H \\ | \\ \\ | \\ H \end{matrix} \right]_n \)

Crucial Tip for Exams:
To draw the polymer structure, take the monomer, remove the double bond, put a single bond between the carbons, and add brackets and the small subscript \(n\) on the outside. This small unit inside the bracket is called the repeating unit.

Did You Know?

The name "poly(ethene)" is IUPAC standard, but the brackets show that the polymer is derived from the ethene monomer. You will often see the common name "polythene" or the abbreviation "PE."

3. Common Synthetic Addition Polymers and Their Uses

By changing the monomer (the starting material), we can make different types of polymers, each with unique properties. The name of the polymer is simply "poly" followed by the name of the monomer.

Poly(ethene) (PE) – The Most Common Plastic

  • Monomer: Ethene
  • Properties: Flexible, chemically inert (doesn't react easily), good electrical insulator.
  • Uses: Plastic bags, wash bottles, milk cartons, electrical wire insulation.

Poly(propene) (PP)

  • Monomer: Propene (Propylene)
  • Properties: Stronger and tougher than poly(ethene), high melting point.
  • Uses: Crates, ropes, carpets, microwave containers (because it resists heat better).

Poly(chloroethene) (PVC)

This is often called PVC. The monomer is chloroethene (or vinyl chloride), where one hydrogen atom on the ethene molecule is replaced by a chlorine atom.

  • Monomer: Chloroethene
  • Properties: Rigid, tough, flame-resistant.
  • Uses: Drain pipes, window frames, protective clothing.
Memory Aid for Naming

The polymer's name always tells you the monomer's name! If it's Poly(propene), the monomer was Propene. Easy peasy!

4. Environmental Impact of Synthetic Polymers

Synthetic polymers are incredibly useful because they are durable, strong, and inert (chemically unreactive). However, these exact properties lead to major environmental issues.

Why Plastics Last Forever

The reason most plastics are problematic when thrown away is their non-biodegradability.

What is Biodegradable?
A substance is biodegradable if it can be naturally broken down by bacteria, fungi, or other living organisms (like food waste or paper).

Why Plastics Resist Decomposition:
Polymers are giant molecules made of long chains of carbon atoms held together by very strong carbon-carbon single bonds.

  • These strong bonds are extremely difficult for natural microorganisms (like soil bacteria) to break.
  • Since the bacteria cannot break the chains, the polymer remains intact for hundreds or thousands of years, filling up landfills and polluting oceans.

The Solution: Dealing with Plastic Waste

Since we cannot easily break down polymers naturally, managing plastic waste focuses on two main strategies:

1. Recycling

Recycling involves collecting used plastic, sorting it by type (e.g., separating PE from PVC), melting it, and remolding it into new products.

  • Advantage: Saves finite raw materials (crude oil) and reduces the amount of waste going to landfill.
  • Disadvantage: Sorting plastics is expensive and energy-intensive.
2. Burning (Incineration)

Plastics can be burned to release energy, which can be used to generate electricity.

  • Advantage: Reduces the volume of waste rapidly.
  • Disadvantage: Burning polymers (especially those containing chlorine, like PVC) can release toxic gases into the atmosphere.

Common Mistake Alert!

Don't confuse burning with biodegrading! Burning uses high heat to destroy the material and causes pollution. Biodegradation is a natural, clean process involving microorganisms.

Chapter Summary Review

Key Terms to Remember:
  • Monomer: Small molecule (e.g., Ethene).
  • Polymer: Long chain formed from monomers (e.g., Poly(ethene)).
  • Addition Polymerisation: Reaction where C=C bonds open up to join chains.
  • Non-biodegradable: Cannot be broken down by natural microbes due to strong C-C bonds.

Great job making it through! You now know how to turn small hydrocarbon molecules into the building blocks of the modern world!