Distinguish addition and condensation polymerisation and relate polymer structure to properties

What this dot point is asking

A polymer is a very large molecule, or macromolecule, made by joining many small repeating units called monomers. The process of joining monomers is called polymerisation. Polymers are everywhere, from natural materials such as cellulose, starch and proteins to synthetic materials such as polyethene, nylon and polyester. Understanding polymers means relating the small monomer building blocks to the structure and properties of the final material.

There are two main types of polymerisation, distinguished by what happens to the atoms during the reaction. Addition polymerisation involves unsaturated monomers, usually alkenes. The carbon-carbon double bond opens up and the monomers add to one another to form a long saturated chain. No atoms are lost in the process, so the empirical formula of the polymer matches that of the monomer.

Common addition polymers include polyethene (from ethene), polypropene (from propene), polyvinyl chloride or PVC (from chloroethene), and polystyrene (from styrene). To draw the repeating unit, take the monomer, change the double bond to a single bond, and place the structure inside brackets with a bond extending from each side and a subscript $n$ to show many repeats.

Condensation polymerisation joins monomers with the elimination of a small molecule, most often water. This requires monomers with two reactive functional groups, so that chains can grow at both ends. Two important classes are polyesters and polyamides. A polyester forms when a dicarboxylic acid reacts with a diol, linking through ester bonds and releasing water at each link. A polyamide, such as nylon, forms when a dicarboxylic acid reacts with a diamine, linking through amide bonds.

Proteins are natural condensation polymers (polyamides) made from amino acid monomers joined by peptide bonds, while carbohydrates such as starch and cellulose are condensation polymers of glucose. The same chemistry that builds synthetic plastics also builds the molecules of life.

The properties of a polymer depend on its structure. Chain length affects strength and melting point, with longer chains generally giving stronger, higher-melting materials. The degree of branching matters: highly branched chains pack loosely, giving a low-density, flexible material, while unbranched chains pack closely, giving a high-density, more rigid material. This explains the difference between low-density and high-density polyethene.

Cross-linking, where covalent bonds connect adjacent chains, makes a polymer harder and more rigid because the chains can no longer slide past one another. The strength of intermolecular forces between chains, such as hydrogen bonding in nylon and proteins, also raises melting point and strength. Side groups influence both flexibility and the way chains pack together.

In exams, draw the repeating unit with brackets and the subscript $n$, identify the type of polymerisation with a reason, and connect any property of the material to a specific structural feature.