Monomers and polymers
A polymer is a very large molecule (a macromolecule) built from many small molecules called monomers joined together. The repeating section of the chain is the repeat unit. There are two main ways monomers join.
Key idea / 要点
Addition polymerisation: many unsaturated monomers (with a C=C double bond) add together with no other product. e.g. n(CH2=CH2) → -(CH2-CH2)n- , poly(ethene).
Addition and condensation
Addition polymers such as poly(ethene) and poly(chloroethene) (PVC, from the monomer chloroethene) form only from monomers with a C=C bond, and nothing else is made. Condensation polymerisation joins monomers with two functional groups and releases a small molecule (usually water) each time a link forms. Nylon is a polyamide (amide linkages) and Terylene is a polyester (ester linkages); both are condensation polymers. Natural polymers such as proteins also contain amide linkages, like nylon.
Polymers and the environment
Many addition polymers are non-biodegradable: microorganisms cannot break them down, so waste plastics build up in landfill and pollute the sea, harming wildlife. Burning them can release toxic or acidic gases such as hydrogen chloride from PVC. Recycling, reusing and reducing plastic use all help to limit this problem.
Worked example / 例题
If 2500 ethene monomers (Mr of ethene = 28) join to make one poly(ethene) molecule, the polymer's relative molecular mass is 2500 × 28 = 70 000.
Remember / 记住
- Addition: C=C monomers, one product only.
- Condensation: two functional groups, releases water.
- Nylon = polyamide; Terylene = polyester.