Chemistry (9701) | Predicting the type of polymerisation

Welcome to Polymer Prediction!

Hello future chemists! This chapter is all about becoming a detective: looking at a molecule (the monomer) and predicting how it will link up with its neighbours to form a giant chain (the polymer).

Don't worry if 'polymerisation' sounds complicated. We are essentially just learning two main rules to classify these reactions. Mastering these rules helps you understand the structures of all the plastics and biological molecules around us, like Nylon, PVC, and DNA!

Section 1: The Two Fundamental Types of Polymerisation

In A-Level Chemistry (9701), we focus on two fundamental ways monomers join together:

1. Addition Polymerisation
2. Condensation Polymerisation

1.1 Addition Polymerisation

Think of this process like joining identical LEGO bricks end-to-end. Nothing is lost, the final polymer is simply the sum of all the monomers.

How to Recognise the Monomer for Addition Polymerisation

• Key Requirement: The monomer must contain a carbon-carbon double bond (\(C=C\)).
• The Reaction: The \(\pi\) (pi) bond in the double bond breaks, and the electrons are used to form two new single bonds (\(\sigma\) bonds) to the next monomer units in the chain.
• Mass Balance: The polymer formed is the only product. The repeat unit has the exact same empirical formula as the monomer. No atoms are lost!

Example: Making Poly(ethene) (Polythene)
The monomer is ethene (\(CH_2=CH_2\)). It has a C=C double bond, so it can undergo addition polymerisation.

1.2 Condensation Polymerisation

This is like joining complex LEGO bricks where, to make the connection, a tiny side-piece (like water) must pop off first.

How to Recognise the Monomer(s) for Condensation Polymerisation

Condensation polymerisation occurs when two monomers react together and lose a small molecule, such as water (\(H_2O\)) or hydrogen chloride (\(HCl\)).

• Key Requirement: The monomer(s) must have two functional groups capable of reacting, usually one at each end.
• The Monomers can be:
  1. One type of monomer that has two different reactive functional groups (e.g., an amino acid has \(-NH_2\) and \(-COOH\)).
  2. Two different monomers, each having two of the same functional group (e.g., a diol \((HO-R-OH)\) and a dicarboxylic acid \((HOOC-R'-COOH)\)).
• Mass Balance: The final polymer unit will have a different empirical formula from the sum of the monomers, because a small molecule (usually \(H_2O\)) is lost.

Common Condensation Polymers in the Syllabus

You need to be able to predict the formation of two main polymer types via condensation:

1. Polyesters (The Ester Link)
These are formed by the reaction between alcohol groups (\(-OH\)) and carboxylic acid groups (\(-COOH\)) or acyl chloride groups (\(-COCl\)).

• Monomer 1: A Diol (two \(-OH\) groups).
• Monomer 2: A Dicarboxylic Acid (two \(-COOH\) groups) or a Dioyl Chloride (two \(-COCl\) groups).
• Alternatively: A Hydroxycarboxylic Acid (one \(-OH\) and one \(-COOH\) group in the same molecule).
• Lost Molecule: \(H_2O\) (if using acid) or \(HCl\) (if using acyl chloride).

2. Polyamides (The Amide/Peptide Link)
These are formed by the reaction between amine groups (\(-NH_2\)) and carboxylic acid groups (\(-COOH\)) or acyl chloride groups (\(-COCl\)).

• Monomer 1: A Diamine (two \(-NH_2\) groups).
• Monomer 2: A Dicarboxylic Acid (two \(-COOH\) groups) or a Dioyl Chloride (two \(-COCl\) groups).
• Alternatively: An Aminocarboxylic Acid (one \(-NH_2\) and one \(-COOH\) group in the same molecule, like amino acids).
• Lost Molecule: \(H_2O\) (if using acid) or \(HCl\) (if using acyl chloride).

Section 2: Predicting the Type from the Monomer Structure (Syllabus 35.2.1)

This is the core skill. To predict the type of polymerisation, you must examine the monomer(s) structure.

Step-by-Step Guide to Prediction

1. Check for the C=C Double Bond

Look closely at the monomer structure. Does it contain a \(\mathbf{C=C}\) double bond?

• If YES, and no other highly reactive groups are present (like \(-OH\), \(-COOH\)), the reaction is Addition Polymerisation.
  (e.g., chloroethene, propene, but-2-ene).
• If NO, move to Step 2.

2. Check for Bi-Functional or Di-Functional Groups

Look for functional groups that can easily react with each other (like an acid reacting with an alcohol or amine).

• If the monomer is one molecule containing two different reactive groups (e.g., \(-OH\) and \(-COOH\)), the reaction is Condensation Polymerisation.
• If the monomers are two separate molecules, and each molecule has two identical reactive groups (e.g., \(HO-R-OH\) and \(HOOC-R'-COOH\)), the reaction is Condensation Polymerisation.

Analogy: The Velcro Test

Imagine you need to link up monomers to make a chain:

• Addition Monomer: The C=C double bond is like a strip of Velcro that is forced open to stick to the next piece. It's a simple sticky bond.
• Condensation Monomer: The two functional groups are like two different connecting parts, say a hook on one end and an eyelet on the other. When the hook and eyelet connect, they release a small molecule (like a little bead of sweat) to form the permanent, strong bond.

Common Mistakes to Avoid

1. Mixing the Rules: Do not look for \(\mathbf{C=C}\) bonds in condensation reactions, or functional groups that form amide links in addition reactions. They are mutually exclusive in terms of reaction mechanism.
2. Forgetting the "Di" or "Bi": For condensation to form a polymer chain, the molecule(s) MUST be bi-functional (two different groups) or di-functional (two of the same group). If a molecule only has one \(-OH\) group, it can't form a long chain; it would just stop the growth.

Section 3: Deducing the Type from a Polymer Structure (Syllabus 35.2.2)

Sometimes, you are given a section of the polymer chain and asked to deduce how it was formed (Addition or Condensation). You'll be a polymer detective now!

Step-by-Step Guide to Deduction

1. Identify the Repeat Unit

Find the repeating section of the molecule. This is the unit that links end-to-end.

2. The 'Lost Molecule' Check (The Key Distinction)

Look at the bonds connecting the repeat units together.

• If the chain links are ONLY Carbon-Carbon single bonds (\(C-C\)):
  This is a straightforward chain of carbon atoms. All atoms from the original monomer are present in the polymer.
  Deduction: The polymer was formed by Addition Polymerisation.
• If the chain links include atoms other than carbon, specifically Oxygen or Nitrogen, forming an Ester Link (\(-COO-\)) or an Amide Link (\(-CONH-\)):
  This means a small molecule must have been ejected during the joining process.
  Deduction: The polymer was formed by Condensation Polymerisation.

Examples of Polymer Backbones

Addition Polymer Backbone:
\(-CH_2-CH(X)-CH_2-CH(X)-\)
(The main chain is purely carbon atoms)

Condensation Polymer Backbone (Polyester):
\(-R-COO-R'-COO-R-COO-\)
(Contains the ester link \(-COO-\), indicating loss of \(H_2O\)).

Condensation Polymer Backbone (Polyamide):
\(-R-CONH-R'-CONH-R-CONH-\)
(Contains the amide/peptide link \(-CONH-\), indicating loss of \(H_2O\) or \(HCl\)).

Quick Review Table: Addition vs. Condensation

To ensure you can make the correct prediction quickly in the exam, use this contrast table:

Feature	Addition Polymerisation	Condensation Polymerisation
Required Monomer Feature	Carbon-Carbon Double Bond (\(C=C\))	Two functional groups (e.g., \(-OH\), \(-COOH\), \(-NH_2\)) at opposite ends.
By-products	None (the polymer is the only product)	A small molecule is lost (e.g., \(H_2O\) or \(HCl\))
Polymer Backbone	Purely Carbon chain (\(C-C-C-C\))	Contains linkage groups (Ester \(-COO-\) or Amide \(-CONH-\))
Formula Check	Repeat unit formula = Monomer formula	Repeat unit formula ≠ Monomer formula (lighter due to lost molecule)

Remember, predicting the type of polymerisation simply requires a careful examination of the monomer's structure. Look for the \(\mathbf{C=C}\) bond for addition, or two complementary functional groups for condensation. You've got this!