Addition Polymerisation

The chemical mechanism relies on the breaking of the $\pi$-bond within the $C=C$ double bond of the monomer. The $\pi$-bond is weaker than the $\sigma$-bond and can be broken under specific conditions of temperature, pressure, or in the presence of a catalyst.

When the $\pi$-bond breaks, the two electrons that formed it become available to form new $\sigma$-bonds with adjacent monomer units. This creates a continuous carbon backbone of single bonds, effectively 'opening up' the molecule to link with its neighbors.

The resulting polymer is saturated, meaning it contains only single carbon-carbon bonds. This change in saturation significantly alters the physical and chemical properties of the material compared to the original monomer.

Drawing a Repeat Unit from a Monomer

• Step 1: Draw the monomer molecule but replace the $C=C$ double bond with a $C-C$ single bond.
• Step 2: Extend horizontal 'trailing bonds' from each of the two carbon atoms out through the sides.
• Step 3: Place large square brackets around the structure, ensuring the trailing bonds pass through the brackets.
• Step 4: Add a subscript '$n$' at the bottom right of the bracket to indicate that the unit repeats many times.

Identifying a Monomer from a Polymer Chain

• Step 1: Identify the repeating pattern in the polymer backbone, which will always consist of a two-carbon segment.
• Step 2: Isolate this two-carbon section and remove the trailing bonds.
• Step 3: Replace the $C-C$ single bond between those two carbons with a $C=C$ double bond to restore the monomer's structure.

It is vital to distinguish between the monomer, the repeat unit, and the polymer chain to avoid structural errors in chemical notation.

• Double Bond Retention: A common mistake is leaving the double bond inside the square brackets of the polymer. Remember that the polymerisation process consumes the double bond to create the links between units.

• Atom Loss: Students often confuse addition polymerisation with condensation polymerisation. In addition polymerisation, no atoms are lost; the mass of the polymer is exactly equal to the total mass of the monomers used.

• Incorrect Monomer Identification: When identifying a monomer from a polymer like poly(propene), students sometimes try to include three carbons in the double bond. The double bond only ever forms between the two carbons that make up the main chain backbone.