S6 Chemistry | Polymers and plastics prep guide (for tutors)

S6 Chemistry | Polymers and plastics prep guide (for tutors) | addition, condensation, repeat units

2026-11-01

Tutor lesson-prep guides · Secondary Chemistry

Students see “polymer” and start memorising trade names, when the whole chapter reduces to one question: did the monomers lose a small molecule?

Objectives — Students can: (1) classify a polymerisation as addition or condensation and state the criterion; (2) draw repeat units for poly(ethene), nylon-6,6 and PET; (3) convert between Mr and degree of polymerisation, and link structure to disposal.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Functional groups	C=C and COOH	spot the reactive site
Moles and Mr	n = m / Mr	units g/mol
Structural drawing	repeat unit needs brackets and n	open at both ends

02 Lesson flow (50 minutes)

Time	Stage	Tutor does	Student does
0-10 min	The fork	did a small molecule leave?	addition versus condensation
10-22 min	Addition	ethene to poly(ethene), n=5000	Mr = 140000
22-34 min	Condensation	adipic acid 146 plus diamine 116	unit 226
34-44 min	Degree of polymerisation	PET Mr 24000	n = 125
44-48 min	Plastics and disposal	recyclable versus not	structure sets behaviour
48-50 min	Exit check	three questions collected	independent work

03 Teaching script (read it out)

Scene 1: Ask one question: did a small molecule leave?

In addition polymerisation the monomer carries a C=C, the double bond opens and monomers link with nothing expelled, so the repeat unit has the same formula as the monomer. Ethene gives poly(ethene) with the unit CH2-CH2; at n = 5000 the Mr is 28 x 5000 = 140000 g/mol and carbon is 85.7 per cent by mass, identical to the monomer, which is the evidence nothing left.
In condensation polymerisation two functional groups join and each link expels a small molecule such as water or HCl, so the repeat unit is the sum of the monomers minus the small molecules. This single fork replaces memorising ten named reactions.

Student task: classify four monomers (ethene, chloroethene, adipic acid with diamine, terephthalic acid with glycol) with one reason each.

Scene 2: Condensation: subtract the water

Nylon-6,6 comes from adipic acid (Mr 146) and hexamethylenediamine (Mr 116); each link loses one water, so the repeat unit is 146 + 116 – 36 = 226. PET from terephthalic acid (166) and ethylene glycol (62) gives 192; a sample of Mr 24000 therefore has n = 125.
Insist on two waters per repeat unit: each ester or amide link releases one, and the unit contains two links. Answers of 210 or 228 show the wrong number of waters was subtracted.

Student task: work backwards for PVC: unit 62.5, Mr 62500, find n.

Scene 3: Structure decides use and disposal

Unbranched poly(ethene) packs tightly, giving the rigid high-density form used for bottles and pipes; branched chains pack loosely, giving the soft low-density film. Nylon hydrogen-bonds, so it is strong, absorbs water and works as a fibre. PET has ester links that hydrolyse, which is why chemical recycling back to monomers is practical.
Burning chlorine-containing polymers such as PVC (unit 62.5, n up to 1000) releases acidic gases that need scrubbing, while landfill persists because the chains are too long for microbes. This is not an environmental slogan: it is the chain structure to property to disposal argument that longer questions reward.

Student task: in two sentences explain why PET is easier to recycle than PVC, naming a bond or an element.

04 Question types and worked steps

Type	Example	Steps	Answer
Classification	four monomers	check C=C and functional groups	two addition, two condensation
Draw a repeat unit	poly(ethene), nylon-6,6	copy for addition, subtract water for condensation	open ends plus n
Find Mr	n = 5000	unit Mr times n	140000
Find n	PET Mr 24000	Mr divided by unit Mr	n = 125

Sanity check (teach this): 140000 divided by the monomer 28 gives 5000, a whole number, as it should; PET gives 125. An answer of 1250 or 12.5 means a decimal slip. The condensation unit 226 must be smaller than the sum of the two monomers, 262, which is the built-in check that water left.

05 Independent practice (3 questions)

Question	Answer	What to watch
Chloroethene (Mr 62.5), n = 800: find Mr.	50,000	use the monomer, not unit plus water
PET Mr 38400: find n.	200	subtract the two waters first to get 192
Repeat unit Mr of nylon-6,6?	226	are two waters subtracted

06 Common mistakes and what to say

Mistake	Student writes	Say this
Forgetting the water	writes the nylon unit as 262	“Each amide link loses a water; two links, two waters.”
Closed repeat unit	draws a complete molecule	“The ends must stick out of the brackets, or the chain is terminated.”
Mixing the two types	writes poly(ethene) minus H2O	“Nothing leaves in addition polymerisation; the Mr is unchanged.”
Rounding n wrongly	reports 125 from 125.4	“n is a whole number; 125.4 means the data or the waters are wrong.”

07 Homework (paper, section and question numbers)

Secondary chemistry chapter index (moles, bonding, acids, redox, electrolysis, industrial processes) — polymers before organic chemistry
Secondary prep guide hub — all subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
Poly(ethene) or nylon: which is condensation?	nylon	wrong: redo the fork in act one
PET unit Mr (acid 166, glycol 62)?	192	wrong: redo water subtraction
PVC unit 62.5, Mr 62500: find n.	1000	two wrong: back to act two

09 Tutor FAQ

Which year is this for?

S6; it covers the HKDSE polymers topic and the industrial chemistry elective.

How many polymer names must be memorised?

Three: poly(ethene) for addition, nylon-6,6 and PET for condensation; the rest follow the same criterion.

Is there practice attached?

The homework list links to the chemistry chapter index, pairing with organic chemistry and industrial processes.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.