What are polymers and macromolecules, and what do monomer and degree of polymerisation mean?

In one line

A polymer is a macromolecule (very large molecule, molecular weight typically 10³ to 10⁷) built by joining many small repeating units called monomers through covalent bonds.

Explained simply

Think of a long chain of paper clips. Each clip is a monomer; the whole chain is a polymer. "Poly" means many and "mer" means part. Plastic bags, nylon clothes, rubber tyres, and even DNA, proteins and cellulose in cotton are polymers. Macromolecular science is the study of how such giant molecules are made, how they are arranged and how their size and shape decide their properties.

How it works

1. Monomer: a small molecule with at least two reactive sites (a double bond or two functional groups), e.g., ethylene CH₂=CH₂, vinyl chloride CH₂=CHCl.
2. Functionality: number of bonding sites in a monomer. Functionality 2 gives linear chains; 3 or more can give branched or cross-linked (network) polymers.
3. Polymerisation: the reaction joining monomers. In addition polymerisation monomers add without losing any atoms (n CH₂=CH₂ → –(CH₂–CH₂)_n–). In condensation polymerisation a small molecule such as H₂O or HCl is eliminated (e.g., nylon 6,6 from hexamethylenediamine and adipic acid).
4. Repeat unit: the structural unit that repeats along the chain, e.g., –CH₂–CH₂– in polyethylene.
5. Degree of polymerisation (DP): the number of repeat units in a chain.

Key formula

\( \text{Molecular weight of polymer} = DP \times \text{molecular weight of repeat unit} \)

• DP = degree of polymerisation (dimensionless count)
• Molecular weight in g/mol

Quick example

A polyethylene sample has molecular weight 28,000 g/mol. Repeat unit –CH₂–CH₂– = 28 g/mol. DP = 28000/28 = 1000. Low DP products are called oligomers (waxy or oily), while high DP gives tough plastics; properties such as strength improve as DP increases.

Polymers versus small molecules: polymers have very high molecular weights, show a range of chain lengths (polydispersity), soften over a range of temperatures, have high viscosity in solution and show strength, elasticity and film-forming ability that small molecules do not.

Common mistakes

• Calling every large molecule a polymer. A polymer must have repeating units.
• Saying a monomer needs one reactive site; it needs at least two to form a chain.
• Mixing up repeat unit and monomer in condensation polymers (the repeat unit of nylon 6,6 is not either monomer alone).

How MU asks it

"Define polymer, monomer, degree of polymerisation and functionality with examples" (3–5 marks), or short numericals on DP (2 marks). For full marks give each definition with one example and the DP formula; for numericals show repeat unit mass calculation.