Monomer and Polymer Structures

Drawing a Polymer from a Monomer

Chemists do not attempt to draw the entire polymer chain because it is far too long. Instead, they draw a repeat unit, which is the smallest section of the chain that reflects the exact chemical composition of the monomer.

To show that the chain continues indefinitely, we use extension bonds (sometimes called continuation or trailing bonds). These are horizontal bonds extending from the carbon atoms at the ends of the repeat unit.

When writing a balanced chemical equation for addition polymerisation, the letter $n$ represents a very large number. The $n$ is placed before the monomer to show many molecules reacting, and as a subscript after the polymer brackets to show that the unit repeats $n$ times.

$n \text{ Monomer } \rightarrow \text{ [Repeat Unit]}_n$

Draw the repeat unit of poly(tetrafluoroethene) formed from the monomer tetrafluoroethene ($CF_2=CF_2$).

Step 1: Draw the monomer with the C=C bond in the centre.

• Arrange the side groups vertically above and below the two central carbon atoms. For tetrafluoroethene, each carbon atom has two Fluorine (F) atoms attached.

Step 2: Break the double bond.

• Redraw the two carbon atoms with a single bond ($C-C$) between them. Keep the F atoms attached in exactly the same vertical positions.

Step 3: Add the extension bonds.

• Draw horizontal single bonds extending outwards from both the left and right carbon atoms.

Step 4: Add the brackets and subscript.

• Draw large square brackets around the unit. The extension bonds MUST pass exactly through the bracket lines. Place a small subscript $n$ at the bottom right.

Deducing a Monomer from a Polymer

Examiners will frequently ask you to work backwards: taking a section of a polymer chain and deducing the original monomer. The process is the exact reverse of drawing the polymer.

Deduce the structure of the monomer used to make poly(propene), given its repeat unit is $[ -CH_2 - CH(CH_3) - ]_n$.

Step 1: Isolate the simplest repeating section.

• Identify the two central carbon atoms that make up the fundamental repeat unit, along with their vertically attached groups (H atoms and the $-CH_3$ group).

Step 2: Strip away the polymer notation.

• Remove the square brackets, the extension bonds, and the subscript $n$.

Step 3: Restore the double bond.

• Draw a double bond ($C=C$) between the two carbon atoms. Check that each carbon atom now has exactly four bonds in total.

Required Polymers and Naming Rules

The standard naming convention for addition polymers is simply adding the prefix "poly" followed by the monomer name in brackets. The Edexcel specification requires you to deduce structures for four specific polymers:

• Poly(ethene): Formed from ethene ($CH_2=CH_2$).
• Poly(propene): Formed from propene ($CH_2=CHCH_3$).
• Poly(chloroethene): Formed from chloroethene ($CH_2=CHCl$).
• Poly(tetrafluoroethene): Formed from tetrafluoroethene ($CF_2=CF_2$).