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The Basics of Addition Polymerisation

Think of building a massive, endless chain out of identical snap-together plastic links. In chemistry, a similar process occurs when many small molecules join together to create a giant chain. Addition polymerisation specifically involves the joining of many alkene monomers to form a long-chain polymer. These polymers are generally known as poly(alkenes).

A monomer is a small, unsaturated molecule (usually an alkene) that contains a carbon-carbon double bond ( $C=C$ ). During addition polymerisation, the $C=C$ bond "opens up". Each carbon atom then forms a new single covalent bond to the adjacent monomer molecule.

This transforms many small, unsaturated molecules into a very large polymer chain that is completely saturated (containing only single bonds). Because all the atoms in the original monomers end up in the final polymer chain without any other products being formed, this reaction has an of exactly 100%.

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 ( $CH_2=CHCH_3$ ).

Students often draw the final polymer repeat unit containing a $C=C$ double bond; remember that addition polymers are completely saturated and must only contain $C-C$ single bonds.

When drawing a repeat unit for full marks, make absolutely sure your horizontal extension bonds cross straight through the square brackets; if they stop inside the brackets, you will lose a mark.

If an exam question asks you to draw a "section" of a polymer chain (e.g., drawing 3 repeat units joined together), do not use brackets or the $n$ subscript.

Be careful when drawing side groups like a methyl group ( $-CH_3$ ); the vertical covalent bond must connect directly to the Carbon atom (C), not the Hydrogen (H).

In chemical equations, ensure the $n$ goes before the monomer (to represent many separate molecules) but acts as a subscript outside the bracket for the polymer.

A small, unsaturated molecule containing a carbon-carbon double bond that can combine with many other identical molecules to form a polymer.

A strong covalent bond between two carbon atoms involving four shared electrons, which is present in unsaturated monomers and breaks during polymerisation.

A process where many monomers join together to form a long-chain polymer without the loss of any other atoms or molecules.

A very large molecule made of many repeating units (monomers) joined together by covalent bonds.

The smallest section of a polymer chain that, if repeated over and over again, would form the entire molecule.

Horizontal single covalent bonds extending through the brackets of a repeat unit to indicate that the polymer chain continues.

An addition polymer formed from the joining of many ethene monomers.

An addition polymer formed from the joining of many propene monomers.

An addition polymer formed from the joining of many chloroethene monomers, commonly known as PVC.

An addition polymer formed from the joining of many tetrafluoroethene monomers, commonly known as PTFE or Teflon.

A general term for a long-chain saturated molecule produced from the addition polymerisation of many unsaturated alkene monomers.

An organic compound containing only single covalent bonds between carbon atoms.

An organic compound containing one or more double covalent bonds between carbon atoms.

A measure of the amount of starting materials that end up as useful products; for addition polymerisation, this is always 100%.