Polymers

Polymers are very long molecules made by joining many small, repeated molecules called monomers. Think of a polymer as a long train and each monomer as one carriage. Many everyday plastics are polymers.

Key ideas

• Monomer: a small molecule that can join to others.
• Polymer: a very long molecule made from many monomers linked in a chain.
• Plastics are materials made from polymers (for example, poly(ethene) and PET).

Addition polymerisation (from alkenes)

• Monomers are alkenes with a C=C double bond. The double bonds “open” and link together to form one long chain. No small molecule is produced.

Example with ethene:

$$n\,\mathrm{CH_2{=}CH_2} \ \longrightarrow\ \left[\mathrm{-CH_2-CH_2-}\right]_n$$

Common addition polymers: poly(ethene) for plastic bags, poly(chloroethene) (PVC) for pipes.

Condensation polymerisation

• Monomers have two functional groups each (one at each end). When they join, a small molecule (often water) is removed at each link.

• Polyesters: from a dicarboxylic acid (two –COOH) and a diol (two –OH). $$\text{dicarboxylic acid} + \text{diol} \rightarrow \text{polyester} + \text{water}$$
• Polyamides: from a dicarboxylic acid (–COOH) and a diamine (two –NH₂). $$\text{dicarboxylic acid} + \text{diamine} \rightarrow \text{polyamide} + \text{water}$$

Nylon is a polyamide used in clothing and ropes. Proteins are natural polyamides: amino acid monomers link to form chains using the amide (peptide) link $\mathrm{-CO-NH-}$.

PET and recycling

PET is a polyester used for drink bottles and fabrics. PET can be broken back into its monomers (chemical recycling) and then re-polymerised to make new PET.

Environmental challenges of plastics

• Landfill: takes up space; plastics can take many years to break down.
• Oceans: plastic waste harms wildlife; tiny pieces (microplastics) can spread widely.
• Burning: can release toxic gases (e.g., burning PVC can make hydrogen chloride gas). Controlled incineration with filters is needed.

Common misconceptions

• “All plastics are the same.” Different polymers have different properties and uses.
• “Condensation polymers do not make anything else.” They always produce small molecules like water.