Structure and Bonding of Polymers

From small molecules to long chains

A polymer is made of very large molecules. Each molecule has a long chain containing a pattern of atoms that repeats many times. Small molecules called monomers can join together to produce these chains; the recurring section of the finished chain is its repeating unit.

For example, ethene monomers can form poly(ethene). Its repeating unit may be represented as $[\mathrm{-CH_2-CH_2-}]_n$, where $n$ means that the unit is repeated a large number of times. In a complete structural drawing, bonds pass through both sides of the brackets: this shows that each displayed unit is covalently joined to the next part of the chain.

Bonding within a polymer molecule

Atoms along a polymer chain are connected by strong covalent bonds. A covalent bond forms when atoms share a pair of electrons; the shared electrons are attracted to both positively charged nuclei. The covalent bonds therefore hold the atoms together within one long polymer molecule.

The repeating units are not separate particles placed next to one another. They are joined end-to-end by covalent bonds to make a continuous molecular chain.

Separate chains in a polymer sample

A piece of polymer usually contains many long polymer molecules lying alongside and tangled around one another. The chains are separate molecules, so the attractions between them are intermolecular forces, not covalent bonds.

These forces have many opportunities to act along long chains. They can therefore be relatively strong compared with the intermolecular forces between small molecules. This helps explain why polymers are commonly solids at room temperature. When a polymer softens or melts, forces between the chains are overcome; the covalent bonds forming each chain are not normally broken.

Comparing polymer arrangements

A simple molecular substance consists of small, separate molecules. A polymer also consists of separate molecules, but each polymer molecule is an extremely long chain built from repeating units. This larger structure gives many more points of contact between neighbouring molecules.

A polymer is not the same as a giant covalent structure. In a giant covalent structure, covalent bonding continues throughout a continuous network and there are no separate molecules. In a polymer, covalent bonding continues along each molecular chain, while intermolecular forces act between different chains.

Representing the structure

A repeating-unit formula communicates the repeated pattern efficiently, but it does not show the full length, tangling or three-dimensional arrangement of real chains. A ball-and-stick model can make the atoms and covalent connections easier to follow. Several linked copies of a repeating unit can be built to represent part of a chain, but even this model shows only a tiny, simplified section of a polymer molecule. The sticks are representations of bonds; chemical bonds are attractions, not physical rods.