Polymerisation: condensation and degradable polymers (A Level only)

A-Level Chemistry Study Notes: Polymerisation (Topic 35)

Welcome to the fascinating world of polymers! This chapter connects everything you've learned about organic functional groups (alcohols, carboxylic acids, alkenes) and shows you how chemists build massive molecules. Understanding polymers is crucial because they form the basis of all plastics, synthetic fabrics, and even the proteins that make up your body!
Don't worry if the reaction names seem intimidating; at its core, polymerisation is just linking up small units (monomers) into long, repeating chains.

1. Introduction to Polymers and Monomers

A polymer is a very large molecule (a macromolecule) made up of many small, identical or similar units called monomers, linked together in a chain.

Analogy: Think of a polymer as a long LEGO train, and the monomers are the individual LEGO bricks.

Key Terminology Review

• Monomer: The small reactive molecule that is capable of bonding with other monomers.
• Polymer: The giant molecule formed when many monomers join together.
• Polymerisation: The chemical reaction process that forms the polymer.
• Repeat Unit: The smallest portion of the polymer chain that, when repeated, forms the entire structure.

Key Takeaway: Polymers are chains, and monomers are the building blocks. We focus on two main ways to link these blocks: Addition and Condensation.

2. Addition Polymerisation (The Join-Up Reaction)

Addition polymerisation happens when monomers simply add together to form a polymer chain. Critically, no atoms are lost during the process.

2.1 Monomer Requirement

For addition polymerisation to occur, the monomer must contain a carbon-carbon double bond (\(C=C\)), i.e., it must be an alkene or a substituted alkene. The double bond breaks, allowing the monomer to link up on both sides.

Memory Aid: Think of a zip fastener. The double bond unzips, allowing new links to be added sequentially.

2.2 Examples and Repeat Unit Deduction (Syllabus 20.1, 20.2)

The most important skill here is identifying and drawing the repeat unit.

Example 1: Poly(ethene) (Polythene)

Monomer: Ethene (\(CH_2=CH_2\)).

1. Break the \(\pi\) bond in the \(C=C\). 2. The two carbons that were double-bonded now link up with neighboring molecules. 3. The repeat unit is simply the two carbon atoms from the original monomer, enclosed in brackets, with extension bonds drawn outside.

\(n \left( \begin{array}{c} H \\ | \\ C=C \\ | \\ H \end{array} \begin{array}{c} H \\ | \\ H \end{array} \right) \longrightarrow \left[ \begin{array}{c} H \quad H \\ | \quad | \\ -C-C- \\ | \quad | \\ H \quad H \end{array} \right]_n\)
(The monomer is ethene, the polymer is poly(ethene).)

Example 2: Poly(chloroethene) (PVC)

Monomer: Chloroethene (or vinyl chloride, \(CH_2=CHCl\)).

If the monomer is asymmetric, ensure the side groups (like Cl) remain attached to the correct carbon atoms in the chain.

\(n \left( \begin{array}{c} H \\ | \\ C=C \\ | \\ H \end{array} \begin{array}{c} Cl \\ | \\ H \end{array} \right) \longrightarrow \left[ \begin{array}{c} H \quad Cl \\ | \quad | \\ -C-C- \\ | \quad | \\ H \quad H \end{array} \right]_n\)
Deducing the Monomer from the Polymer: To reverse this, simply identify the repeat unit (the shortest section of the main chain that repeats) and insert a double bond between the two carbons in that unit.

3. Condensation Polymerisation (The Linking Reaction)

Condensation polymerisation occurs when monomers link together by removing a small molecule, typically water (\(H_2O\)) or hydrogen chloride (\(HCl\)).

For this to happen, the monomers need to have two functional groups that react with each other (one on each end of the molecule, or two different functional groups on the same molecule).

3.1 Formation of Polyesters (Syllabus 35.1)

Polyesters are formed via the condensation reaction that produces an ester linkage (\(-COO-\)).

Reagents for Polyesters

There are two main ways to make polyesters:

1. Diol + Dicarboxylic Acid:
   • A diol has two alcohol (\(-OH\)) groups.
   • A dicarboxylic acid has two carboxylic acid (\(-COOH\)) groups.
   • The \(-OH\) from the alcohol reacts with the \(-COOH\) from the acid, eliminating \(H_2O\).
   Example: Ethanediol and Benzene-1,4-dicarboxylic acid form PET (polyethylene terephthalate).
2. Diol + Dioyl Chloride:
   • Using a dioyl chloride (containing two \(-COCl\) groups) instead of a dicarboxylic acid makes the reaction much faster and easier, often eliminating \(HCl\).
3. Hydroxycarboxylic Acid:
   • A single monomer contains both an \(-OH\) group and a \(-COOH\) group (e.g., Lactic acid). The molecules link head-to-tail.

Repeat Unit Deduction: Look for the ester link: \(-O-C(=O)-\)

3.2 Formation of Polyamides (Syllabus 35.1)

Polyamides are formed via the condensation reaction that produces an amide linkage (\(-CONH-\)). (This is also called a peptide bond when dealing with amino acids).

Reagents for Polyamides

There are three main ways to make polyamides:

1. Diamine + Dicarboxylic Acid:
   • A diamine has two amine (\(-NH_2\)) groups.
   • A dicarboxylic acid has two carboxylic acid (\(-COOH\)) groups.
   • The amine reacts with the acid, eliminating \(H_2O\). (e.g., forming Nylon 6,6).
2. Diamine + Dioyl Chloride:
   • Using a dioyl chloride speeds up the reaction, eliminating \(HCl\).
3. Aminocarboxylic Acid (or Amino Acids):
   • A single monomer contains both an \(-NH_2\) group and a \(-COOH\) group. These monomers link head-to-tail to form proteins or synthetic polyamides (e.g., Nylon 6).

Repeat Unit Deduction: Look for the amide link: \(-N(H)-C(=O)-\)

4. Predicting the Type of Polymerisation (Syllabus 35.2)

To predict whether a monomer will undergo addition or condensation polymerisation, look closely at its functional groups:

4.1 Prediction Table

If the Monomer(s) contain...	Type of Polymerisation	Key Feature
Only a \(C=C\) double bond.	Addition	Monomers just "open up" and join.
Two reactive groups (e.g., \( -OH\), \(-COOH\), \(-NH_2\)) at opposite ends.	Condensation	Small molecule (\(H_2O\) or \(HCl\)) is eliminated.

Deducing from the Polymer Structure:

• If the main chain contains only carbon atoms, it is an Addition Polymer. (e.g., Poly(ethene)).
• If the main chain contains carbon atoms plus heteroatoms (N or O) as part of a functional linkage (ester or amide), it is a Condensation Polymer.

5. Environmental Concerns and Degradability (Syllabus 20.4, 35.3)

Polymers are incredibly useful, but their durability causes major environmental problems related to disposal.

5.1 Disposal Difficulty of Poly(alkenes)

Polymers formed by addition polymerisation, such as poly(ethene) and poly(chloroethene) (PVC), are classified as poly(alkenes).

• Chemical Inertness: The backbone of a poly(alkene) consists entirely of strong, non-polar C-C single bonds. These bonds are highly stable and resistant to chemical attack (like hydrolysis) and biological decomposition by microbes (biodegradation).
• Non-biodegradability: Because they are chemically inert, poly(alkenes) take hundreds of years to break down naturally, leading to massive landfill issues.
• Harmful Combustion Products: When poly(alkenes) are burned (incinerated), they can release harmful substances. For example, burning PVC (poly(chloroethene)) releases toxic hydrogen chloride gas (\(HCl\)).

5.2 Degradation of Polymers

Chemists are trying to develop more degradable polymers. Degradation is the process of breaking the polymer chain back into smaller molecules.

1. Degradation by Light (Photodegradation)

Some polymers are designed to contain groups that are sensitive to ultraviolet (UV) light. When exposed to sunlight, these polymers break down into fragments. However, this often only solves the visible litter problem, as the fragmented pieces remain inert, micro-plastic pollutants.

2. Biodegradation of Condensation Polymers

Polyamides and Polyesters are typically considered more degradable than poly(alkenes). Why?

• They contain reactive ester (\(-COO-\)) or amide (\(-CONH-\)) linkages in their backbone.
• These links can be broken via hydrolysis (reaction with water).
• Acidic and Alkaline Hydrolysis: Polyesters and polyamides are readily broken down by heating with dilute aqueous acid or alkali. This is a crucial concept when considering recycling or decomposition.

Example of Polyester Hydrolysis (using acid catalyst):
\(\text{Polymer} + n\text{H}_2\text{O} \rightleftharpoons n(\text{Diol} + \text{Dicarboxylic Acid})\)

Key Takeaway: Addition polymers (poly(alkenes)) are hard to dispose of due to their strong C-C backbone. Condensation polymers are generally more easily hydrolysed (broken down) because of the presence of ester or amide linkages.

Summary Checklist

You should now be able to:

• Identify monomers for addition polymers (must have \(C=C\)).
• Deduce and draw the repeat unit of polymers like poly(ethene) and PVC.
• Identify the reagents needed for polyesters (diols + dicarboxylic/dioyl chlorides) and polyamides (diamines + dicarboxylic/dioyl chlorides).
• Recognise that condensation involves the loss of a small molecule (\(H_2O\) or \(HCl\)).
• Explain why poly(alkenes) are environmentally problematic (inert C-C bonds, toxic combustion products like \(HCl\)).
• Explain why polyesters and polyamides are biodegradable via hydrolysis.

Keep practising those repeat unit drawings! You got this!