Addition Polymerisation

• The fundamental mechanism involves the breaking of the $\pi$-bond in the $C=C$ double bond of the monomer.

• Each carbon atom in the former double bond now has an available electron to form a new $\sigma$-bond with a carbon atom from an adjacent monomer molecule.

• This process continues in a chain reaction, effectively 'opening up' the double bonds to create a continuous backbone of single-bonded carbon atoms.

• Because no atoms are gained or lost, the empirical formula of the polymer is identical to that of the monomer, though its physical properties (like melting point and density) differ significantly.

Drawing the Polymer from a Monomer

• Step 1: Draw the monomer structure but replace the $C=C$ double bond with a $C-C$ single bond.
• Step 2: Extend horizontal bonds (trailing bonds) outwards from the two carbon atoms that were originally double-bonded.
• Step 3: Place large square brackets around the repeating unit, 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 the Monomer from a Polymer

• Locate the repeating unit within the chain (the simplest section that repeats).
• Remove the trailing bonds and the brackets.
• Re-insert the $C=C$ double bond between the two central carbon atoms to restore the monomer structure.

• It is vital to distinguish between the repeating unit (the specific arrangement of atoms inside the brackets) and the monomer (the independent molecule with a double bond).

• The 'n' Rule: Always include the '$n$' in the equation. On the left side, it is a coefficient ($n$ Monomer); on the right side, it is a subscript (Polymer)$_n$.

• Bond Alignment: Ensure that the groups attached to the carbons in the monomer remain attached to the same carbons in the polymer. Do not rearrange side groups during the process.

• Trailing Bonds: In diagrams, trailing bonds must clearly extend through the brackets to show that the chain continues; stopping them inside the brackets is a common error that loses marks.

• Verification: Check that the number of atoms of each element is the same on both sides of the reaction equation to confirm mass conservation.