Q.What are polymers ?

Polymers (Greek poly = many, mer = part/unit) are high molecular mass substances consisting of a very large number of repeating structural units, derived from simple molecules called monomers, joined together by covalent bonds. The process by which monomers combine to form a polymer is called polymerisation.

For example, polythene is a polymer built from thousands of ethene ($CH_2=CH_2$) monomer units joined end to end:

The small repeating unit inside the brackets is called the repeat unit, and $n$ (the degree of polymerisation) is usually a very large number, giving the polymer its characteristic high molecular mass (often ranging from $10^4$ to $10^6$ g/mol). Polymers may be natural (cellulose, proteins, natural rubber) or synthetic (polythene, nylon, PVC), and may form by addition or condensation mechanisms.

Define the term using its structural meaning (poly + mer), state that monomers combine via covalent bonds through polymerisation, and give one illustrative example with the repeat-unit notation.

• Confusing 'polymer' with 'monomer' — the polymer is the large final molecule, the monomer is the small starting unit.
• Forgetting to mention that the linkage between monomer units is covalent (not just physical mixing/aggregation).
• Omitting an example, which the answer usually expects for full credit.