What is the fundamental requirement for a molecule to act as a monomer in addition polymerisation?

The molecule must be unsaturated, containing at least one carbon-carbon double bond ($C=C$). This double bond breaks its $\pi$-bond to allow the monomers to link together into a saturated chain.

How does the saturation of the carbon chain change during addition polymerisation?

The chain changes from unsaturated to saturated. The $C=C$ double bonds in the monomers are converted into $C-C$ single bonds in the polymer backbone as new covalent bonds are formed between units.

What is the difference between a monomer and a repeat unit in a chemical equation?

The monomer is the starting alkene with a double bond, while the repeat unit is the section of the polymer shown in square brackets with single bonds and continuation bonds. They have the same atoms but different bonding structures.

Why is the atom economy of addition polymerisation considered to be 100%?

Because addition polymerisation involves the joining of monomers to form a single product with no by-products. Every atom in the reactants ends up in the final polymer product.

What is a common error when drawing the repeat unit of poly(ethene)?

A common error is leaving the $C=C$ double bond inside the brackets. In the polymer, the double bond must be replaced by a single bond to show that the molecule has reacted and linked to others.

What happens if the continuation bonds do not pass through the square brackets in a repeat unit diagram?

This is a structural error that implies the chain ends at those atoms. The bonds must extend through the brackets to indicate that the polymer chain continues indefinitely in both directions.

How do you name a polymer formed from the monomer 'phenylethene'?

The polymer is named by adding the prefix 'poly' to the monomer name in brackets, resulting in 'poly(phenylethene)'. This convention clearly identifies the chemical origin of the polymer.

When identifying a monomer from a polymer chain, how many carbon atoms should you look for in the repeating segment?

You should look for a two-carbon segment in the main backbone. Since addition polymerisation involves the $C=C$ bond of a single monomer, the repeat unit will always be based on those two specific carbons.

Why do polymers have different physical properties than their monomers?

Polymers have much higher molecular masses and significantly stronger intermolecular forces (like London forces) due to their long chains. This makes them solids at room temperature, whereas many monomers are gases or liquids.

What is the significance of the 'n' in the general formula for polymerisation?

The 'n' represents a large number of repeating units. On the reactant side, it indicates the number of monomer molecules; on the product side, it indicates how many times the unit repeats within the polymer chain.

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Addition Polymerisation

Summary

Addition polymerisation is a chemical process where numerous small, unsaturated molecules called monomers join together to form a single, long-chain molecule known as a polymer. This reaction is fundamental to the plastics industry, relying on the reactivity of the carbon-carbon double bond in alkenes to create diverse synthetic materials.

1. Definition & Core Concepts

Monomers are the small, reactive starting molecules that contain at least one carbon-carbon double bond ( $C=C$ ). These are typically alkenes or substituted alkenes.
A Polymer is the resulting long-chain molecule composed of thousands of repeating structural units linked by strong covalent bonds.
Addition Polymerisation is characterized by the fact that the polymer is the only product of the reaction, meaning the atom economy is 100% as no small molecules are lost during the process.

Diagram showing the transition from individual alkene monomers with double bonds to a continuous polymer chain with single bonds.

2. Underlying Principles

3. Methods: Deducing Repeat Units

4. Key Distinctions

5. Exam Strategy & Tips

The 'n' Rule: Always ensure the 'n' is present on the left side of the equation (before the monomer) and as a subscript on the right side (after the brackets of the repeat unit).
Continuation Bonds: Ensure the bonds on the repeat unit extend through the square brackets. If they stop inside the brackets, it implies a terminal group rather than a repeating chain.
Naming Convention: The name of the polymer is always 'poly' followed by the name of the monomer in brackets, such as poly(propene) or poly(chloroethene).
Check the Backbone: In addition polymers, the main chain backbone consists exclusively of carbon atoms. If you see oxygen or nitrogen in the main chain, it is likely a condensation polymer, not an addition polymer.