Polymer and monomer — the two-part definition
A polymer is a large molecule built up from many smaller molecules, called monomers, joined together in a long repeating chain. Both halves score: it is a large molecule, and it is built from many repeating monomer units. The repeat unit is the smallest group of atoms that, repeated many times, builds the whole chain — written inside brackets with a subscript outside. Once joined into the chain, a repeat unit is no longer "a monomer".
Addition polymerisation — poly(ethene) from ethene
Many ethene () monomers join together: . Each monomer's double bond opens to a single bond inside the chain. No atoms are gained or lost, so the repeat unit has exactly the same molecular formula as the monomer (). The polymer is the only product — no small molecule is released.
(Extended) Condensation polymerisation — nylon, a polyamide
Here each monomer carries two reactive functional groups instead of a , so the chain can grow at both ends, and a small molecule is released at every new link. Nylon is a polyamide: an group on one monomer reacts with a (or acyl chloride) group on the other to form an amide link (), losing one water molecule each time. The syllabus repeat unit shows two amide links, so two waters per repeat unit.
Drawn from real examiner reports.
Polymer ≠ long-chain hydrocarbon
"A long-chain hydrocarbon" is not credited as a definition. It omits the idea that a polymer is built from many repeating monomer units, and it is factually wrong for nylon, whose amide links contain nitrogen and oxygen — a hydrocarbon contains carbon and hydrogen only. Poly(ethene) happens to be a hydrocarbon, but that is a coincidence, not the definition.
Flagged Jun 2022 P41 Q2ci; Jun 2023 P32 Q8ci
(Extended) Generic nylon answers score nothing
Nylon questions supply the two monomer structures, because their IUPAC names are not required recall at this level. "Monomers join together and water is released" is too generic to earn full marks. The answer must reference the specific reactive groups shown: which end of each supplied monomer reacts, where the amide link forms, and which atoms leave as water.
Flagged Jun 2023 P41 Q11b
(Extended) One water released, not two
Each supplied monomer carries two reactive end-groups, so two amide links form per repeat unit and two water molecules are released, not one. Check by atom conservation on : hydrogen ; oxygen .
Expecting the repeat unit to gain atoms
In addition polymerisation nothing is added or removed — only the double bond opens — so the repeat unit has exactly the same molecular formula as its monomer. Candidates who expect the formula to change are importing the rule for condensation polymers, where the repeat unit is the monomers' formulae minus the atoms lost as the small molecule.
Miscounting atoms in a formula
The formula contains four hydrogen atoms, two on each carbon — miscount here and every mass that follows is wrong. In a four-element formula such as , the nitrogen or the oxygen is easily dropped. Use values C 12, H 1, N 14, O 16, Cl 35.5 and sum element by element.
Leaving a C=C inside the repeat unit
When addition polymerisation happens, one of the two bonds making up the breaks, and those electrons form new single bonds to the monomers on each side. Every carbon-carbon bond inside the finished chain is therefore a single bond. A repeat unit drawn with a double bond still inside the brackets loses the structure mark.
Flagged Jun 2022 P41 Q2ci; Jun 2023 P32 Q8ci
(Extended) Deduce the repeat unit in four steps
Draw the alkene's displayed formula with its shown. Change that double bond to a single bond — the only structural change. Extend a bond line outward from each of those two carbons, through a pair of brackets. Write a subscript outside, bottom right.
(Extended) Work back from repeat unit to monomer
For the "vice versa" version: find the two carbon atoms whose bonds pass out through the brackets, change the single bond between them back into a double bond, then delete the extending lines, the brackets and the subscript . What is left is the original alkene monomer.
Use to check a repeat unit
Addition polymerisation gains and loses no atoms, so a monomer and its repeat unit share a molecular formula and the same relative formula mass. If the repeat unit you deduced gives a different from the monomer, the structure is wrong.
Match the points to the mark tariff
"Describe two differences" over 2 marks wants two contrasted pairs — both sides of each difference — not one difference worded twice, and not a one-sided description of addition polymerisation alone. Count your distinct points against the tariff before moving on.
Cambridge 0654 spec reference: Section C11 "Organic chemistry", sub-topic C11.7 "Polymers". Core: (1) define polymers as large molecules built up from many smaller monomer molecules; (2) describe the formation of poly(ethene) as an example of addition polymerisation using ethene monomers. Supplement (Extended): (3) identify the repeat units in addition and condensation polymers; (4) deduce the structure/repeat unit of an addition polymer from a given alkene and vice versa; (5) describe the differences between addition and condensation polymerisation; (6) describe and draw the structure of nylon, a polyamide.
| Term | Mark-scheme-precise meaning |
|---|---|
| Polymer | A large molecule built up from many smaller molecules called monomers, joined together in a repeating chain |
| Monomer | A small molecule that joins together, with many others of the same or a paired kind, in a repeating pattern, to form a polymer |
| Repeat unit | The smallest group of atoms in a polymer chain that, when repeated many times, builds up the whole polymer molecule -- shown inside brackets with a subscript outside |
| Addition polymerisation | Many monomers, each containing a carbon-carbon double bond (), join together, the double bond opening to a single bond, forming a polymer as the only product (no other substance is released) |
| Condensation polymerisation | Monomers (usually carrying two reactive functional groups each) join together to form a polymer, with a small molecule (for nylon, water) released at every new link formed |
| Polyamide | A condensation polymer whose monomer units are joined by amide links (); nylon is the polyamide named in this specification |
Define "polymer".
Poly(ethene) is formed by the addition polymerisation of ethene, .
Calculate the relative formula mass, , of the ethene monomer. (2 marks)
