QCE Chemistry - Unit 4 - Properties and structure of organic materials

Organic Materials: Structure and Function | QCE Chemistry

Understand how structure explains the function of organic materials such as polymers and biomolecules in QCE Chemistry.

Updated 2026-05-18 - 6 min read

Organic materials are useful because their structure produces particular properties. A polymer used in packaging, a fibre used in clothing and a biomolecule used in living systems all work because of bonding, shape and intermolecular forces.

In QCE Chemistry, structure-function questions are really asking: which structural feature causes the useful property?

Polymers

Polymers are large molecules made from repeating units. The small molecules used to make them are called monomers.

There are two major polymerisation patterns you need to recognise.

Addition polymers

Addition polymers form when alkene monomers join together. The carbon-carbon double bond opens, and the monomers link into a long chain.

Example:

$ \mathrm{ethene} \rightarrow \mathrm{poly(ethene)} $

In the polymer, the repeating unit no longer contains the original $\mathrm{C=C}$ double bond. The backbone is mostly carbon-carbon single bonds, which helps explain why many addition polymers are chemically resistant.

Condensation polymers

Condensation polymers form when monomers with two functional groups join together and release a small molecule such as water or hydrogen chloride.

Common examples include:

• polyesters, formed from alcohol and carboxylic acid functional groups
• polyamides, formed from amine and carboxylic acid functional groups

Condensation polymers often contain polar links, such as ester or amide groups, so their intermolecular forces can be stronger than those in non-polar addition polymers.

Repeating units and monomers

The monomer is the starting molecule. The repeating unit is the pattern that repeats inside the polymer chain. These are related, but they are not always written in the same way.

For addition polymers, you can usually recover the monomer by putting the double bond back between the two backbone carbons in the repeating unit.

For condensation polymers, you need to identify the linking group, then split it back into the original functional groups.

Structure-property relationships

Polymer properties depend on several structural features.

| Structural feature | Effect on material properties | | --- | --- | | Longer chains | more entanglement and stronger dispersion forces | | Branching | changes packing and density | | Cross-linking | restricts chain movement, increasing rigidity and heat resistance | | Polar functional groups | increase dipole-dipole forces or hydrogen bonding | | Crystallinity | more ordered packing can increase strength and melting point |

If polymer chains slide past each other easily, the material is more flexible. If chains are held in place by strong forces or cross-links, the material is harder, stronger or more heat resistant.

For example, a cross-linked polymer may be useful for a hard coating because the chains cannot easily move past each other. A lightly branched polymer may be useful for flexible packaging because the chains can move more freely.

Thermoplastics and thermosetting polymers

Thermoplastics soften when heated because their chains are mainly held together by intermolecular forces. Heating gives chains enough energy to move past each other. This makes thermoplastics easier to reshape and recycle.

Thermosetting polymers contain extensive cross-links. When heated, they do not simply soften into a new shape because covalent cross-links hold the network together. They are often more rigid and heat resistant.

Biomolecules

Biomolecules are organic molecules used by living systems. Their function depends on their structure just like synthetic materials do.

Carbohydrates

Carbohydrates contain many hydroxyl groups, so they can form hydrogen bonds with water and with other molecules. This helps explain why many small carbohydrates are water soluble.

Large carbohydrate polymers can have very different properties depending on chain shape and bonding. A structural polysaccharide needs different properties from an energy-storage molecule.

Proteins and amino acids

Amino acids contain both an amine group and a carboxylic acid group. They join by condensation reactions to form peptide links, which are amide functional groups.

Protein function depends on the order of amino acids and the way the chain folds. Hydrogen bonding, ionic interactions, disulfide links and hydrophobic interactions can all help hold a protein in a particular shape.

Lipids

Lipids often contain long non-polar hydrocarbon chains, so they are poorly soluble in water. This makes them useful in membranes and energy storage. Some lipids also contain polar head groups, giving them both hydrophilic and hydrophobic regions.

Material design reasoning

When designing or evaluating an organic material, connect the required use to the structure.

For example:

• A waterproof coating needs non-polar regions that do not interact strongly with water.
• A dissolvable material may need polar or ionic groups that interact with water.
• A heat-resistant polymer may need strong intermolecular forces, aromatic structures or cross-linking.
• A flexible film may need chains that can move past each other without extensive cross-linking.

Quick check

Exam traps worth knowing

• Saying a material is strong without explaining the bonding or structure causing strength.
• Confusing monomers with repeating units.
• Ignoring functional groups when explaining biological function.
• Assuming all polymers are non-polar. Polyesters, polyamides and some biomolecules contain strongly polar functional groups.
• Saying "plastic" as if it describes one property. Different polymers can be flexible, rigid, soluble, insoluble, heat resistant or heat softening depending on structure.

Sources

• QCAA Chemistry subject page
• QCAA Chemistry 2025 syllabus
• OpenStax Chemistry 2e: Alcohols and Ethers
• OpenStax Chemistry 2e: Amines and Amides
• OpenStax Chemistry 2e: Aldehydes, Ketones, Carboxylic Acids, and Esters