Addition Polymerisation: Building the Giant Chains
Welcome to the world of polymers! Chances are, you are surrounded by them right now—your phone casing, your water bottle, your clothes. This chapter is vital because it explains how small, simple molecules link up to create the massive, useful materials we call plastics. We will focus on addition polymerisation, the process that makes molecules like poly(ethene) (that's polythene!).
Let's dive into how these chemical chains are built!
1. Understanding the Vocabulary
In the world of polymer chemistry, you need to know three key terms:
a) Monomers
A monomer is a small, simple molecule that acts as the basic building block.
Analogy: If you are building a LEGO wall, one LEGO brick is the monomer.
b) Polymers
A polymer is a very long chain molecule (a macromolecule) formed by linking thousands of monomers together.
Analogy: The entire, finished LEGO wall is the polymer.
c) Polymerisation
Polymerisation is the chemical reaction where many monomers join together to form a polymer.
Key Takeaway: Polymers are giant molecules made from repeating units of small monomers.
2. What is Addition Polymerisation?
Addition polymerisation is a specific type of reaction where monomers join up without the loss of any atoms. The total mass of the polymer is the sum of the masses of all the monomers that reacted.
a) The Essential Ingredient: Alkenes
Addition polymerisation almost always involves monomers that are unsaturated, meaning they contain a carbon-carbon double bond (\(\text{C=C}\)). These molecules are typically alkenes (or substituted alkenes).
Why alkenes? The \(\text{C=C}\) bond consists of one strong sigma (\(\sigma\)) bond and one weaker pi (\(\pi\)) bond. The weaker \(\pi\) bond can easily break, allowing the carbon atoms to form two new single bonds, connecting them to the next monomer unit.
Process in simple steps:
1. The \(\pi\) bond in the \(\text{C=C}\) of the monomer breaks (unstable).
2. This forms two "free ends" (or sites for bonding) on each carbon atom.
3. These free ends link up with the free ends of thousands of adjacent monomers.
4. A long, continuous carbon backbone is created.
Memory Aid: Think of the C=C bond as a handshake you don't really want to do. It breaks easily, and then the hands reach out (forming new bonds) to grab the next molecule in line.
3. Key Examples of Addition Polymers
The syllabus requires you to be familiar with poly(ethene) and poly(chloroethene).
a) Poly(ethene) (Polythene/PE)
This is the most common plastic, used in shopping bags and bottles.
- Monomer: Ethene (\(\text{C}_2\text{H}_4\)).
- Structure: \(\text{H}_2\text{C=CH}_2\)
During polymerisation, the double bond breaks to form a long chain:
$$n \left( \text{H}_2\text{C=CH}_2 \right) \rightarrow \left[ - \text{CH}_2 - \text{CH}_2 - \right]_n$$
The resulting polymer has a simple, inert carbon backbone.
b) Poly(chloroethene) (PVC)
Commonly known as PVC (Polyvinyl Chloride), used in window frames, pipes, and electrical cable insulation.
- Monomer: Chloroethene (or vinyl chloride).
- Structure: \(\text{H}_2\text{C=CH}(\text{Cl})\) (an ethene molecule where one H is replaced by a Cl).
The polymer repeat unit is shown below:
$$n \left( \text{H}_2\text{C=CH}(\text{Cl}) \right) \rightarrow \left[ - \text{CH}_2 - \text{CH}(\text{Cl}) - \right]_n$$
Quick Review Box: The Basics
Type of Monomer: Must contain \(\text{C=C}\) (unsaturated).
Reaction Type: Addition (no small molecules lost).
Polymer Backbone: Continuous chain of C-C single bonds.
4. Deducing Monomers and Repeat Units (The Crucial Skill)
You must be able to move confidently between the monomer and the repeat unit structure.
a) Deducing the Repeat Unit from the Monomer
The repeat unit is the smallest section of the polymer chain that, if repeated, would reproduce the entire structure.
Step 1: Identify the Double Bond. Focus only on the two carbon atoms involved in the \(\text{C=C}\) double bond.
Step 2: Draw the Box. Draw the two carbon atoms connected by a single bond, and keep all the substituents (H, Cl, \(\text{CH}_3\), etc.) exactly where they were on those two carbons.
Step 3: Add Brackets and \(n\). Add square brackets and the subscript \(n\) to show that this unit is repeated many times, drawing single bonds extending outwards through the brackets.
Example: Propene Monomer (\(\text{CH}_2\text{=CH}(\text{CH}_3)\))
The repeat unit is: \(\left[ - \text{CH}_2 - \text{CH}(\text{CH}_3) - \right]_n\)
b) Deducing the Monomer from the Polymer
This is the reverse process.
Step 1: Identify the Repeat Unit. Look for the two carbon atoms in the main chain that have the exact same pattern of substituents.
Step 2: Reintroduce the Double Bond. Remove the extending single bonds and replace the single bond between the two carbons of the repeat unit with a double bond.
Common Mistake to Avoid: When deducing the repeat unit, make sure the extending bonds start and end outside the two carbons that used to have the double bond. Don't include other atoms (like side groups) in the backbone of the repeat unit.
Key Takeaway: The repeat unit and the monomer look structurally identical, apart from the double bond turning into two single bonds extending the chain.
5. Environmental Consequences of Poly(alkene) Disposal (Syllabus 20.4)
While addition polymers are incredibly useful, their disposal presents significant environmental challenges.
a) Non-Biodegradability
Non-biodegradability means the material cannot be naturally decomposed by microorganisms (like bacteria or fungi).
The primary reason poly(alkene)s are non-biodegradable is their structure:
1. The polymer chains consist solely of very strong, non-polar carbon-carbon (\(\text{C-C}\)) single bonds.
2. These chains are chemically inert (unreactive) and lack the specific chemical groups (like C-O or C-N bonds found in natural materials) that microorganisms produce enzymes to break down.
3. The dense structure of the polymer chains prevents water and enzymes from accessing the bonds easily.
This chemical inertness leads to huge amounts of plastic waste accumulating in landfills, oceans, and environments for hundreds of years.
b) Harmful Combustion Products
Burning waste plastics (incineration) seems like an alternative to landfill, but it releases harmful gases.
1. Simple poly(alkene)s (like poly(ethene)): Complete combustion yields \(\text{CO}_2\) and \(\text{H}_2\text{O}\). However, incomplete combustion (which often happens in incinerators) produces carbon monoxide (\(\text{CO}\)), a poisonous gas, and soot (carbon particles).
2. Halogenated Polymers (like PVC - poly(chloroethene)): When these polymers are burned, the halogen atoms are released, forming highly acidic and corrosive gases.
Example: Combustion of PVC releases hydrogen chloride gas (\(\text{HCl}\)).
$$\left[ - \text{CH}_2 - \text{CH}(\text{Cl}) - \right]_n \quad \xrightarrow{\text{Heat/Combustion}} \quad \text{HCl}(\text{g}) \quad + \quad \text{other products}$$
This \(\text{HCl}\) gas is toxic and contributes significantly to acid rain.
Did you know? Modern incinerators often have acid scrubbers installed to neutralise these acidic gases (\(\text{HCl}\), etc.) before they are released into the atmosphere, mitigating some of the environmental harm.
Key Takeaway: Poly(alkene)s are difficult to dispose of because their strong, non-polar bonds resist biodegradation, and burning them (especially PVC) releases harmful acidic or poisonous gases.
6. Chapter Summary Checklist
You should now be able to:
- Define monomer, polymer, and addition polymerisation.
- Explain that addition polymerisation involves monomers with \(\text{C=C}\) bonds.
- Identify the monomers and draw the repeat units for poly(ethene) and poly(chloroethene).
- Deduce the repeat unit from any given alkene monomer.
- Explain the environmental problems of poly(alkene)s, referencing non-biodegradability and toxic combustion products (\(\text{HCl}\) from PVC).
Keep practising those monomer-to-polymer conversions—it’s a key exam skill!