Polymers and biomolecules: study guide

Key ideas

A polymer is a long chain built from many repeating units called monomers. Addition polymers join through double bonds with nothing lost, while condensation polymers join with the loss of a small molecule, usually water.

• Addition polymerization uses monomers with C=C: ethene gives polyethylene, propene polypropylene, chloroethene PVC and styrene polystyrene. The repeat unit has the same formula as the monomer.
• Condensation polymerization joins two different functional groups, for example a diol and a dicarboxylic acid for a polyester, or a diamine and a diacid for nylon, and releases water.
• The degree of polymerization is n = M(polymer) ÷ M(repeat unit).
• Linking n monomers into one chain by condensation releases n − 1 molecules of water. Proteins, nucleic acids and polysaccharides are natural condensation polymers.

Key equations

• n = M_polymer ÷ M_repeat unit
• addition: n CH₂=CHX → –(CH₂CHX)ₙ–
• condensation: n monomers → n − 1 H₂O (linear chain)

Worked example

A sample of polyethylene has a molar mass of 28 000 g/mol, and the repeat unit –CH₂CH₂– is 28 g/mol, so n = 1000. Joining 10 amino acids into a peptide releases 9 molecules of water. Nylon-6,6 is made from hexane-1,6-diamine and hexanedioic acid and gives an amide link at each joint.

Common mistakes

• Expecting an addition polymer to release a small molecule. Nothing is lost.
• Counting n water molecules instead of n − 1 when n monomers form a single chain.
• Using the monomer formula for the repeat unit of a condensation polymer, which has lost atoms in the link.

Practice

Test yourself with practice questions on this topic: Polymers and biomolecules