Revise Predicting the type of polymerisation for Cambridge 9701 (35.2)

Addition polymerisation is characterised by the joining of monomers without the loss of any atoms. The only requirement for a monomer to undergo this process is the presence of a carbon-carbon double bond ($C=C$). During the reaction, the pi ($pi$) bond within the double bond breaks, and the electrons are used to form new single bonds with adjacent monomer molecules. This creates a long, saturated carbon chain which forms the backbone of the polymer.

Condensation polymerisation involves a reaction between two functional groups. In this process, a covalent bond is formed between monomer units, and a small, stable molecule (like water, ammonia, or hydrogen chloride) is eliminated. For a chain to form, each monomer must be bifunctional, meaning it has two reactive functional groups. These can be two identical groups on one monomer that reacts with another bifunctional monomer (e.g., a diol with a dicarboxylic acid), or two different functional groups on a single monomer type (e.g., an amino acid).

To determine the polymerisation route, follow a simple two-step check of the monomer(s). First, look for a $C=C$ double bond. If present, addition polymerisation is the most likely pathway. If there is no $C=C$ bond, or if the reaction conditions favour it, look for two reactive functional groups. If a monomer has two such groups (or you have two different monomers that are both bifunctional), condensation polymerisation will occur. A special case is a diene (a monomer with two $C=C$ bonds, e.g. buta-1,3-diene). This undergoes addition polymerisation, but the presence of a second double bond in each monomer unit allows for potential cross-linking between chains, as seen in the vulcanisation of rubber.