Polymers: WACE Year 12 Chemistry

What this dot point is asking

A polymer is a very large molecule (a macromolecule) built by linking many small repeating units called monomers. The WACE course covers two ways monomers join: addition polymerisation and condensation polymerisation.

Addition polymerisation

Addition polymerisation joins alkene (or substituted alkene) monomers. The C=C double bond opens up and the carbons bond directly to neighbouring units, so no atoms are lost: every atom of the monomer ends up in the polymer. For ethene:

The product is polyethene (polythene). Substituted alkenes give related polymers: chloroethene (vinyl chloride) gives PVC, and propene gives polypropene. The repeating unit is drawn with the double bond now a single bond and an open bond ("trailing valence") at each end.

Condensation polymerisation

Condensation polymerisation joins monomers that each have two reactive functional groups, and a small molecule (usually water) is eliminated at each link. Two important examples:

Polyesters form when a dicarboxylic acid reacts with a diol; the ester linkages ($-\text{COO}-$) join the chain, releasing water. An example is the fibre and bottle plastic PET.

Polyamides form when a dicarboxylic acid reacts with a diamine; amide linkages ($-\text{CONH}-$) join the chain, releasing water. Nylon is a polyamide.

Because a small molecule is lost, the repeating unit does not contain all the atoms of the two monomers.

Structure and properties

A polymer's bulk properties follow from its molecular structure.

• Chain length: longer chains have stronger total dispersion forces, raising melting point, strength and viscosity.
• Branching: straight unbranched chains pack closely (high-density, more crystalline, stronger, e.g. HDPE), while branched chains pack loosely (low-density, more flexible, e.g. LDPE).
• Intermolecular forces and cross-linking: polar groups (as in polyamides) give hydrogen bonding between chains and stronger, higher-melting materials; cross-links between chains make a rigid, hard, often thermosetting material.

Thermoplastics, thermosets and recycling

The way chains are connected also sets how a polymer responds to heat, which matters for recycling. Thermoplastics consist of separate chains held together only by intermolecular forces; on heating these forces weaken, the chains slide past one another, and the material softens so it can be remoulded (polyethene and PET are thermoplastics, which is why they are widely recycled). Thermosetting polymers have covalent cross-links locking the chains into a rigid network; heating cannot make them flow, so they char rather than melt and cannot be remoulded. Addition polymers with only carbon backbones (such as polyethene) are also chemically inert and non-biodegradable, persisting in the environment, whereas condensation polymers containing ester or amide linkages can in principle be hydrolysed back towards their monomers, a point that links polymer chemistry to the green-chemistry theme of designing for degradation.

When answering polymer questions in the WACE examination, decide first whether the monomer has a C=C (addition) or two functional groups (condensation), draw the repeating unit with trailing bonds, and link any property to chain length, branching or intermolecular forces.