Step 1: Identify the Monomer: Draw the monomer molecule with the double bond in the center and its four substituents clearly branching off. Place a lowercase in front of the molecule to represent a large, unspecified number of units.
Step 2: Draw the Repeat Unit: Redraw the molecule but replace the double bond with a single bond (). Ensure all substituents remain attached to the same carbon atoms as they were in the monomer.
Step 3: Add Continuation Bonds: Draw single bonds extending horizontally from the two central carbon atoms. These 'trailing bonds' must pass through the brackets to show that the chain continues indefinitely.
Step 4: Finalize with Brackets and n: Enclose the repeat unit in square brackets and place the lowercase as a subscript at the bottom right to indicate the repeating nature of the structure.
| Feature | Monomer | Polymer (Repeat Unit) |
|---|---|---|
| Bonding | Unsaturated () | Saturated () |
| Representation | precedes the formula | is a subscript after brackets |
| Physical State | Often gas or liquid | Usually solid (plastic) |
| Reactivity | Highly reactive at double bond | Generally chemically inert |
Check the Bonds: A common mistake is leaving the double bond inside the polymer brackets. Always ensure the polymer has only single bonds between the main chain carbons.
Continuation Bonds: Ensure the bonds extending from the carbons actually cross the bracket lines. If they stop inside the brackets, the structure represents a molecule, not a repeating polymer unit.
Naming Convention: To name any addition polymer, simply place the monomer name in parentheses and add the prefix 'poly-'. For example, the polymer made from chloroethene is poly(chloroethene).
Conservation of Atoms: Verify that the number and type of atoms attached to the carbons in your repeat unit exactly match those in your monomer. Do not add or remove hydrogens or functional groups.
The 'n' Placement: Students often forget to put the in front of the monomer or after the polymer. Without , the equation is not balanced and does not represent a polymerisation reaction.
Substituent Alignment: When drawing monomers like propene, students often draw the group in a straight line. It is easier to visualize the polymerisation if you draw the horizontally and the group pointing vertically up or down.
Biodegradability: A common misconception is that all plastics are biodegradable. In reality, most addition polymers are non-biodegradable because their carbon-carbon backbones are extremely strong and chemically inert, meaning microbes cannot break them down.
