Year 12 Chemistry Module 7 · Organic Chemistry ⏱ ~45 min 5 MC · 3 Short Answer Lesson 21 of 23 Core

Addition Polymers

Polyethylene, PVC, PTFE, every plastic wrapping, pipe, and non-stick pan comes from one key idea: the C=C pi bond opens under the right conditions, linking thousands of monomers into a single chain with nothing added and nothing wasted.

Today's hook: LDPE and HDPE are both polyethylene, same monomer, same chemical repeat unit. Yet one makes cling film and the other makes milk crates. How can the same molecule produce two such different materials?
0/5TASKS
1

Connect: trace an object back to its monomer

Warm up first

Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.

Worksheets

Practise this lesson

Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application practice Master Mastery challenge Exam Exam-style questions
Before You Read

A single molecule of ethene (CH₂=CH₂) is a colourless gas that diffuses away instantly. Link thousands of those molecules together end-to-end with no atoms added or removed, and you get polyethylene, a white solid that can be hard enough to make milk crates or soft enough to make cling film, depending only on how the chains are arranged.

Before reading: what do you think physically happens to the C=C double bond when ethene molecules link together? Does the double bond stay, or does it disappear? Where do the new connections between monomers come from if nothing is added?

Learning Intentions
goals

Know

  • What addition polymerisation is and that no by-product is produced
  • The 5–6 common addition polymers, their monomers and repeat units
  • LDPE vs HDPE, same monomer, different chain architecture

Understand

  • Why the C=C double bond opens and forms new C-C bonds
  • How chain branching (LDPE) vs linearity (HDPE) affects density and melting point via IMF
  • Why PTFE is chemically inert and non-stick (C-F bond energy, surface IMF)

Can Do

  • Draw polymer repeat unit from monomer and monomer from polymer
  • Correctly use square bracket + subscript n + open bond notation
  • Explain polymer properties using structural and IMF reasoning
Scan these before reading
vocab
Addition polymerisationA chain reaction where alkene monomers add together without loss of any atoms; forms a long-chain polymer with no by-product.
MonomerA small molecule with a C=C double bond that undergoes repeated addition to form a polymer.
Repeat unitThe basic structural unit that repeats in a polymer chain; shown in square brackets with subscript n and open bonds.
Poly(ethene) / polyethyleneAddition polymer of ethene; two types: LDPE (branched chains, flexible) and HDPE (linear chains, rigid).
Poly(propene) / polypropyleneAddition polymer of propene; harder and higher melting point than polyethylene; used in packaging and fibres.
Free radical initiationAddition polymerisation begins when an initiator (e.g., peroxide) generates a free radical that attacks the C=C of the first monomer.
📚 Core Content
2

Explain: account for every atom in addition

01
How Addition Polymerisation Works: The C=C Opens

Addition polymerisation is conceptually the simplest polymer-forming reaction, the pi bond of a C=C opens, each freed electron forms a new sigma bond to the next monomer, and the chain grows one link at a time with nothing added, nothing removed, and nothing wasted.

What happens at the molecular level: In addition polymerisation, the pi bond of each monomer's C=C opens. The two carbons each form a new single bond to adjacent monomer carbons. The sigma bond framework of the original C=C remains intact, only the pi bond is consumed to form new C-C single bonds linking monomers into a chain.

Critically: no by-product is produced. Every atom in every monomer ends up in the polymer chain. This distinguishes addition polymerisation from condensation polymerisation (Lesson 22), which releases water or HCl.

The π bond of ethene opens so many monomers add together into polyethylene with no by-product.

Mechanism Overview (conceptual)

1 · Initiation
An initiator radical (R•) adds to the C=C, opening it and generating a carbon radical at the chain end.
2 · Propagation
The chain radical attacks the next monomer's C=C, extending the chain by one unit and creating a new radical. Repeats thousands of times.
3 · Termination
Two chain radicals combine, ending both chains. Final polymer has n repeat units, degree of polymerisation n = hundreds to hundreds of thousands.
Notation checklist, every repeat unit needs all three:
(1) Square brackets: [ ] around the repeat unit.
(2) Subscript n outside the closing bracket: ]ₙ
(3) Open bonds: lines extending from both bracket ends showing chain continuation.
Missing any one of these is an incomplete answer and loses marks.
Common error, C=C inside the polymer: Students draw the repeat unit with the C=C still present inside the bracket. The C=C does not exist in the polymer, the pi bond is consumed to form the new C-C single bonds linking monomers. If you see C=C inside a polymer repeat unit, something is wrong. The repeat unit has only single bonds.
Exam Tip: For organic chemistry questions, draw full structural formulas showing all atoms and bonds, condensed or skeletal formulas alone may lose marks in HSC extended-response questions.

Addition polymerisation: alkene monomers (C=C) undergo chain reaction, pi bond opens, new C-C sigma bonds form, no by-product produced. General equation: n(CH₂=CHR) → [-CH₂-CHR-]ₙ. Notation: (1) square brackets, (2) subscript n, (3) open bonds, all three required for full marks.

Pause, copy the highlighted points into your book before the check below.

Quick check: Which statement correctly describes addition polymerisation?
3

Represent: convert monomers and repeat units

02
Drawing Polymer from Monomer and Monomer from Polymer

You've seen that the C=C pi bond opens to link monomers. The next question: how exactly do you draw a polymer repeat unit from a monomer, or work backwards from a polymer to find its monomer? This card gives you a systematic 6-step process in both directions, every HSC polymer question uses these same rules.

Converting between monomer and polymer structures is a two-way skill that appears in almost every HSC polymer question, and it is systematic: the same rules apply every time.

→Monomer → Polymer
  1. Identify the C=C double bond in the monomer.
  2. Draw two adjacent monomer units side by side.
  3. Replace C=C with C-C; each carbon gains an outward chain bond.
  4. Enclose ONE repeat unit (two backbone carbons + substituents) in square brackets.
  5. Show open bonds extending from each bracket end.
  6. Write subscript n outside the closing bracket.
↔
reverse
←Polymer → Monomer
  1. Identify the repeat unit inside the brackets.
  2. Find the two backbone carbons, those with open bonds at bracket ends.
  3. Insert a C=C double bond between those backbone carbons.
  4. Replace open bonds at bracket ends with H atoms (tetravalency).
  5. Check that each C has exactly 4 bonds total.
  6. Name the monomer, the IUPAC name of the alkene formed.

Worked Examples, Conversions

EXAMPLE 1, Monomer → Polymer and back

Convert chloroethene (CH₂=CHCl) to its polymer, then reverse the process.

→
CH₂=CHCl → [-CH₂-CHCl-]ₙ
C=C opens: C1 → -CH₂- · C2 → -CHCl-. Enclose in [ ], add open bonds, subscript n.
Polymer: PVC (poly(vinyl chloride)).
←
[-CH₂-CHCl-]ₙ → CH₂=CHCl
Backbone carbons: C1 (CH₂) and C2 (CHCl). Insert C=C; open bonds → H atoms.
Monomer = chloroethene (vinyl chloride) ✓
EXAMPLE 2, Polymer → Monomer

Identify the monomer from [-CH₂-CH(CH₃)-]ₙ and name it.

1
Backbone: C1 = -CH₂- (two chain bonds + 2H); C2 = -CH(CH₃)- (two chain bonds + 1H + CH₃).
Identify the two carbons that had the C=C.
2
Insert C=C → CH₂=CHCH₃. Replace open bonds with H atoms. Check tetravalency ✓.
Monomer = propene (CH₂=CHCH₃) → polymer is polypropylene (PP).
Must-do: When drawing a polymer from a monomer, draw 2–3 adjacent repeat units first to see the pattern clearly, THEN enclose ONE unit in square brackets. The bracket must enclose the minimum repeating unit, for vinyl monomers (CH₂=CHR), this is always two backbone carbons plus their substituents.
Common error: Students draw the "monomer" from a polymer by copying the repeat unit without inserting the C=C. The repeat unit [-CH₂-CHCl-]ₙ contains no double bond, you MUST insert C=C between the two backbone carbons to regenerate the alkene monomer. A polymer monomer without C=C cannot undergo addition polymerisation, writing CH₂-CHCl (single bond) as the monomer is wrong.

Monomer → polymer (6 steps): identify C=C; draw 2 adjacent monomers; replace C=C with C-C; enclose ONE repeat unit in [ ]; add open bonds; write subscript n. Polymer → monomer (6 steps): find backbone carbons; insert C=C; replace open bonds with H; check tetravalency. Key rule: the repeat unit of an addition polymer has NO C=C.

Pause, copy the 6-step processes and the key rule into your book before the check below.

True or False: The repeat unit of polystyrene [-CH₂-CH(C₆H₅)-]ₙ contains a C=C double bond between the two backbone carbons.
4

Respond: connect side groups to properties

03
Properties and Uses of Common Addition Polymers

You can now draw any addition polymer structure. The next question: does the monomer's substituent or chain architecture predict the polymer's physical properties? This card connects structure to property for the 6 HSC addition polymers you need to know, LDPE, HDPE, PP, PVC, PTFE, and PS.

The properties of an addition polymer, stiffness, flexibility, chemical resistance, melting point, follow directly from its monomer structure, chain length, and degree of chain branching or cross-linking.

Branching controls how closely chains pack, which sets density, rigidity and melting point.
PolymerMonomerKey structural featureKey propertyCommon uses
LDPEEthene (CH₂=CH₂)Branched chains, poor packingFlexible, transparent, low density (0.91–0.93 g/cm³), MP ~110°CPlastic bags, cling film, squeeze bottles
HDPEEthene (CH₂=CH₂)Linear chains, close packing, semi-crystallineRigid, opaque, higher density (0.94–0.97 g/cm³), MP ~130°CMilk crates, piping, detergent bottles
PPPropene (CH₂=CHCH₃)CH₃ side groups every second CStiffer than LDPE, MP ~165°C, good chemical resistanceFood containers, rope, carpet, car parts
PVCChloroethene (CH₂=CHCl)C-Cl bonds give rigidity + flame resistanceRigid & strong (natural); flexible with plasticisers; flame-resistantPipes, window frames; cable insulation, vinyl records
PTFE (Teflon)Tetrafluoroethene (CF₂=CF₂)C-F sheath shields backbone; F only on surfaceExtremely chemically inert (C-F ~485 kJ/mol); non-stick; stable to ~260°CNon-stick cookware, plumber's tape, chemical vessel linings
PSStyrene (CH₂=CHC₆H₅)Bulky phenyl groups prevent crystallisation → amorphous → clearClear, rigid, brittle; expanded PS very low densityDisposable cups, CD cases, insulation foam
HSC requires four things for each polymer: (1) monomer name and formula; (2) repeat unit in correct bracket notation; (3) one key physical property linked to structure; (4) one specific use. Writing only a use without structural reasoning earns one mark. All four earns all marks.
Common error, LDPE vs HDPE confusion: Students write "LDPE is more rigid than HDPE." This is the reverse, HDPE (linear chains, close packing, strong dispersion forces) is STIFFER and has a HIGHER melting point. "High density" = denser = closer packing = more rigid = higher melting point. LDPE has branched chains, lower packing efficiency, weaker dispersion forces, lower density, lower melting point.
Insight, PTFE discovered by accident: In 1938, Roy Plunkett at DuPont found that a cylinder of tetrafluoroethene gas had polymerised overnight in storage. PTFE's C-F bond energy (~485 kJ/mol, vs C-H ~413 kJ/mol) means no common chemical, not even aqua regia, can attack the polymer backbone.

HDPE = linear chains → close packing → stronger dispersion forces → higher density and MP (~130°C). LDPE = branched chains → poor packing → weaker dispersion forces → lower density and MP (~110°C). PTFE: C-F sheath (~485 kJ/mol) → chemically inert + non-stick. HSC formula: monomer + repeat unit + structural property + specific use = full marks.

Pause, record the LDPE vs HDPE comparison and the PTFE reasoning in your book before the check below.

HDPE has a higher melting point than LDPE. Which structural explanation is correct?
5

Feedback: compare structures and environmental claims

Beyond the syllabus. The named addition polymers (polyethylene, PVC, polystyrene, PTFE), their monomers, repeat units and structure–property–use links are core. Thermosets, polymerisation mechanisms and resin codes are extension — exam questions draw on the named polymers.
04
Thermoplastics, Thermosets, and Environmental Context

You know the structure and properties of the 6 key addition polymers. The next question: what happens to these polymers when they're heated or discarded in the environment? This card covers thermoplastics vs thermosets and why the C-C backbone leads to the microplastics problem.

Whether a plastic can be melted and remoulded (thermoplastic) or permanently hardens when heated (thermoset) is a direct consequence of molecular structure, whether the polymer chains are free to slide past each other or are chemically locked in place.

Thermoplastics

Most addition polymers (LDPE, HDPE, PP, PVC, PS) are thermoplastics, they soften and can be remoulded when heated, re-harden on cooling. This is reversible and repeatable.

Why: chains are held by intermolecular forces (dispersion, dipole-dipole in PVC). Heating weakens these reversibly → chains flow. Cooling re-establishes them → hard again. This makes mechanical recycling possible, but does not guarantee practical recyclability: contamination, additives, degradation, sorting and economics can prevent recovery.

Thermosets (for context)

Thermoset polymers (epoxy resins, bakelite, vulcanised rubber) are permanently set by covalent cross-links between chains during curing. Heating does not melt them, the covalent cross-links are permanent.

Thermosets cannot be recycled by melting. Primarily relevant in condensation polymer context (Lesson 22).

Environmental Context:

(1) Persistence: Most addition polymers are very resistant to biodegradation. PE, PP, PVC, and PS can persist in the environment for hundreds to thousands of years. The C-C backbone of addition polymers is not readily attacked by microbial enzymes, unlike ester linkages in polyesters or amide linkages in proteins, which bacteria can hydrolyse.

(2) Microplastics: Physical breakdown by UV light and mechanical action produces microplastics (<5 mm fragments) that enter food chains and are found in organisms at all trophic levels, including in human blood and tissue.

(3) Recycling identification: The Resin Identification Code (RIC) allows sorting, 1 = PET, 2 = HDPE, 3 = PVC, 4 = LDPE, 5 = PP, 6 = PS. Different polymers cannot be mixed in recycling streams. Actual recycling rates remain low because sorting and reprocessing is economically challenging.

(4) Bioplastics: Polylactic acid (PLA) is made from fermented plant sugars and is biodegradable under industrial composting conditions (55–70°C, humidity). It does not degrade meaningfully in home compost or ocean environments.

Environmental impact, be specific: "Plastic is bad for the environment" earns no marks. Address two specific issues: (1) persistence, C-C backbone is not biodegradable, persists hundreds to thousands of years; (2) microplastics, physical breakdown produces fragments <5 mm that accumulate in food chains. Specific chemistry + specific consequences = marks.
Common error, LDPE and HDPE have "different monomers": Both LDPE and HDPE have the SAME monomer (ethene) and SAME chemical repeat unit (-CH₂-CH₂-). The difference is chain branching, a structural difference from different manufacturing conditions, not different chemistry. Same polymer, different process, different physical properties.

Thermoplastic = chains held by IMF → reversible on heating → recyclable (LDPE, HDPE, PP, PVC, PS). Thermoset = covalent cross-links → permanent → not recyclable. Environmental: C-C backbone resists microbial enzymes → persists hundreds–thousands of years; UV + mechanical → microplastics (<5 mm) → accumulate in food chains.

Pause, record the thermoplastic vs thermoset distinction and both environmental issues in your book before the check below.

Fill in: Addition polymers resist biodegradation because microbial enzymes cannot cleave ___ bonds. Physical breakdown produces ___ (<5 mm) that accumulate in food chains.
6

Apply: justify a polymer choice

🔬 Worked Examples
05
Worked Example 1, Drawing Polymer and Identifying Monomer
Problem: (a) Draw the repeat unit of the polymer formed from propene (CH₂=CHCH₃). Name the polymer. (b) Repeat unit given: [-CH₂-CH(CN)-]ₙ. Identify and name the monomer and the polymer.
a

Propene → polypropylene:
Propene: CH₂=CHCH₃. C=C between C1 (CH₂=) and C2 (=CHCH₃).
Open the C=C: C1 → -CH₂- (two chain bonds + 2H). C2 → -CH(CH₃)- (two chain bonds + 1H + CH₃).
Repeat unit: [-CH₂-CH(CH₃)-]ₙ (square brackets, subscript n, open bonds at each end)
Polymer name: polypropylene (PP) or poly(propene).

b

[-CH₂-CH(CN)-]ₙ → monomer:
Backbone: C1 = CH₂ (two chain bonds + 2H). C2 = CH(CN) (two chain bonds + 1H + CN group).
Insert C=C between C1 and C2: CH₂=CH(CN). Replace open bonds with H: complete.
Monomer: CH₂=CHCN (propenenitrile, also called acrylonitrile)
Polymer: polyacrylonitrile (PAN) or poly(propenenitrile).

Worked Example 2, Explaining LDPE vs HDPE Properties

Problem: LDPE (density 0.91–0.93 g/cm³, MP ~110°C) and HDPE (density 0.94–0.97 g/cm³, MP ~130°C) both have repeat unit [-CH₂-CH₂-]ₙ. Explain the differences in density and melting point using structural and IMF reasoning.
1
Structural difference: Same monomer (ethene), same repeat unit, same chemistry. The difference is chain architecture: LDPE chains are branched (~every 50 backbone carbons); HDPE chains are linear (unbranched).
2
Density link: HDPE linear chains align parallel and pack closely → semi-crystalline regions → more mass per unit volume → higher density (0.94–0.97). LDPE branches force adjacent chains apart → looser packing → less mass per unit volume → lower density (0.91–0.93).
3
Melting point via IMF: Both polymers have only London (dispersion) forces between chains. Dispersion force strength depends on surface area of contact. HDPE (straight chains, close packing) → maximised contact area → strong cumulative dispersion forces → more energy to pull chains apart → higher MP (~130°C). LDPE (branched chains, reduced contact) → weaker dispersion forces → lower MP (~110°C).
4
Conclusion: Branching in LDPE prevents close chain packing, reducing both density and intermolecular force strength. Both properties (lower density AND lower melting point) arise from the same structural cause. Same polymer backbone chemistry, completely different properties from chain architecture alone.

Worked Example 3, Extended Response on PTFE (7 marks)

Problem: Polymer X has repeat unit [-CF₂-CF₂-]ₙ. (a) Name X, draw its monomer, write the polymerisation equation. (b) Explain why X is extremely resistant to chemical attack using bond chemistry. (c) Explain why X has an exceptionally low coefficient of friction using IMF reasoning. (d) Evaluate whether X would be an ideal material for constructing acid storage tanks.
a
Repeat unit [-CF₂-CF₂-]ₙ → insert C=C → monomer: CF₂=CF₂ (tetrafluoroethene). Polymer name: polytetrafluoroethylene (PTFE, Teflon). n CF₂=CF₂ → [-CF₂-CF₂-]ₙ (addition polymerisation; no by-product)
b
The C-F bond energy is ~485 kJ/mol, significantly stronger than C-H (413), C-Cl (339), or C-O (360). The small, electronegative F atoms pack tightly around the carbon backbone, creating a fluorine "sheath" that physically shields the carbon atoms from approaching reagents. No common acid, base, oxidiser, or reducer can break C-F bonds under normal conditions, not even aqua regia.
c
The PTFE surface consists entirely of F atoms, no polar groups, no H-bond donors/acceptors, no ionic charges. The only surface interactions are very weak London (dispersion) forces. Other materials in contact with PTFE can form only these weak forces → extremely low adhesion → low friction.
d
Advantage: PTFE is essentially non-reactive with all common acids (HF, H₂SO₄, HNO₃, HCl), excellent for acid storage, used industrially as a tank liner.
Limitation: PTFE degrades above ~260°C, releasing toxic fluorinated decomposition products. For high-temperature acid processes, PTFE may not be suitable. Also relatively expensive and mechanically soft (requires thick walls for large tanks).
→
Key Formulas & Reference

General addition polymerisation equation:

n(CH₂=CHR) → [-CH₂-CHR-]ₙ

No by-product: addition polymer has the SAME empirical formula as its monomer

Notation rule: repeat unit MUST be in square brackets with subscript n and open bonds at each end

MonomerFormulaPolymerAbbrev.Repeat unit
EtheneCH₂=CH₂PolyethylenePE (LDPE/HDPE)[-CH₂-CH₂-]ₙ
PropeneCH₃CH=CH₂PolypropylenePP[-CH₂-CH(CH₃)-]ₙ
ChloroetheneCH₂=CHClPoly(vinyl chloride)PVC[-CH₂-CHCl-]ₙ
TetrafluoroetheneCF₂=CF₂PolytetrafluoroethylenePTFE (Teflon)[-CF₂-CF₂-]ₙ
Styrene (phenylethene)CH₂=CHC₆H₅PolystyrenePS[-CH₂-CH(C₆H₅)-]ₙ
PropenenitrileCH₂=CHCNPolyacrylonitrilePAN[-CH₂-CH(CN)-]ₙ
Interactive Tool, Polymers Builder Open fullscreen ↗
True or False: In the polymers builder above, the repeat unit for polyethylene [-CH₂-CH₂-]ₙ shows only C-C single bonds, there is no C=C double bond in the polymer backbone.
1

Practice: construct and justify addition polymers

🔬Predict, Then Reveal+8 XP
Predict ethene's repeat unit, explain the atom accounting and identify the role of the C=C bond.
Your predictionExpert answerCompare
✏️ Activities

Activity 1, Monomer↔Polymer Conversions

For each monomer, write the correct polymer repeat unit in square bracket notation. For each polymer, identify the monomer. Show your working.

Set A, Monomer → Polymer: (i) CH₂=CHCN   (ii) CF₂=CF₂   (iii) CH₂=CH-C₆H₅
Set B, Polymer → Monomer: (iv) [-CH₂-CHCl-]ₙ   (v) [-CF₂-CF₂-]ₙ   (vi) [-CH₂-CH(CH₃)-]ₙ

Activity 2, IMF Reasoning for Polymer Properties

Explain, using intermolecular force reasoning, why: (a) HDPE has a higher melting point than LDPE; (b) PTFE has a lower coefficient of friction than nylon; (c) PTFE has a higher melting point than LDPE.

📝 Check Your Understanding

Q6. (3 marks)

(a) Write the structural formula of the monomer that produces poly(tetrafluoroethylene). (b) Write the correct repeat unit notation for PTFE. (c) State one property of PTFE and link it directly to the C-F bond.

Q7. (4 marks)

A polymer has the repeat unit [-CH₂-CHCl-]ₙ. (a) Name the polymer and draw the monomer. (b) Explain why this polymer is classified as a thermoplastic rather than a thermoset. (c) State one environmental concern about the disposal of this polymer, using specific chemical reasoning.

Q8. (5 marks)

A pharmaceutical company requires a polymer for a medical implant that must: (i) be chemically inert in body fluids; (ii) have low surface friction to minimise tissue damage; (iii) be stable at body temperature (37°C). Evaluate which of the following polymers, PTFE, HDPE, or PVC, is most suitable, using structural and property reasoning. Identify one limitation of your chosen polymer for this application.

Show All Answers
Q1, Answer: B

PVC is made by addition polymerisation of chloroethene (vinyl chloride, CH₂=CHCl). The C=C opens; each monomer joins the next via new C-C single bonds; no by-product is produced. Option A: vinyl alcohol (CH₂=CHOH) is not stable and PVC contains Cl, not OH. Option C: 1,2-dichloroethane is an alkane (no C=C), cannot undergo addition polymerisation. Option D: addition polymerisation never produces HCl as a by-product, that would be condensation polymerisation.

Q2, Answer: B

Both LDPE and HDPE have the same repeat unit (-CH₂-CH₂-) and the same chemistry. The structural difference is chain architecture: HDPE = linear chains → close packing → maximised dispersion force contact → higher MP. LDPE = branched chains → poor packing → reduced contact → weaker dispersion forces → lower MP.

Q3, Answer: C

Correct notation requires square brackets AND subscript n: [-CH₂-CH₂-]ₙ. Without them, the drawing is indistinguishable from butane (C₄H₁₀). Square brackets define the repeat unit boundary; subscript n indicates n repetitions. Both are required and both are specifically marked in HSC marking guidelines.

Q4, Answer: A

PTFE's non-stick property arises from its surface chemistry: the surface consists entirely of fluorine atoms with no H-bond donors, no polar -OH or -NH groups, and no ionic charges. Only very weak London (dispersion) forces can act between PTFE and any contacting surface, this produces extremely weak adhesion and therefore extremely low friction.

Q5, Answer: D

Biodegradation by microorganisms requires hydrolase enzymes that can cleave specific bond types, specifically ester bonds (in polyesters and lipids), amide bonds (in proteins), and glycosidic bonds (in cellulose). Addition polymer backbones consist entirely of C-C single bonds, which no common microbial hydrolase can cleave. Without an enzyme-accessible bond, microorganisms cannot break the chain into smaller fragments they can metabolise.

Q6, Sample Answer (3 marks)

(a) Monomer: CF₂=CF₂ (tetrafluoroethene). [1 mark]
(b) Repeat unit: [-CF₂-CF₂-]ₙ, square brackets, subscript n, open bonds at both ends. [1 mark]
(c) Property: exceptional chemical resistance (or: very low coefficient of friction / stable to ~260°C). Link: C-F bond energy ~485 kJ/mol, stronger than C-H, C-Cl, or C-O bonds. F atoms form a tight sheath around the carbon backbone, shielding it from chemical attack by acids, bases, and oxidisers. [1 mark]

Q7, Sample Answer (4 marks)

(a) Polymer: poly(vinyl chloride), PVC. Monomer: CH₂=CHCl (chloroethene / vinyl chloride). [1 mark]
(b) PVC is a thermoplastic because its chains are held together by intermolecular forces (dispersion forces and dipole-dipole interactions from the polar C-Cl bonds). These forces weaken reversibly on heating → chains can flow → can be remoulded. On cooling, the forces re-establish and the plastic re-hardens. There are no permanent covalent cross-links between chains, unlike thermosets (e.g. bakelite). [2 marks]
(c) PVC's C-C backbone is resistant to biodegradation by microbial enzymes (no hydrolysable linkages) → persists hundreds to thousands of years in landfill or ocean environments. Physical breakdown produces microplastics (<5 mm) that accumulate in food chains. Additionally, PVC incineration can release toxic chlorinated by-products (e.g. HCl, dioxins). [1 mark]

Q8, Sample Answer (5 marks)

Most suitable: PTFE. [1 mark]
(i) Chemical inertness in body fluids: PTFE's C-F bonds (~485 kJ/mol) are extremely strong; the F sheath shields the carbon backbone from attack by aqueous biological fluids. No common biological molecule can attack PTFE. HDPE and PVC are less chemically inert. [1.5 marks]
(ii) Low surface friction: PTFE surface consists entirely of F atoms with only weak dispersion forces → extremely low adhesion → minimal friction → less tissue damage and inflammation. [1 mark]
(iii) Thermal stability at 37°C: All three polymers are stable at 37°C, but PTFE is the most thermally stable (to ~260°C). [0.5 marks]
Limitation of PTFE: PTFE is mechanically soft (low stiffness) and expensive, for load-bearing implants requiring structural rigidity, PTFE may deform under sustained mechanical stress; HDPE or titanium may be needed for structural components. [1 mark]

How did your thinking change?

Back at the start you predicted what happens to the C=C double bond during addition polymerisation, and why cling film is flexible while a milk crate is rigid. Now you know: when ethene polymerises, the pi bond in each C=C opens and the carbons form new single bonds to neighbouring monomers, creating long, flexible polyethylene chains. LDPE (low-density PE) has branched chains that prevent close packing → amorphous regions → flexible. HDPE (high-density PE) has linear chains that pack closely and crystallise → rigid, strong. Same monomer, different reaction conditions, completely different macroscopic properties.

Look back at what you wrote before reading this lesson. How has your understanding changed?

1

Review: retrieve structure-property links

Check what actually stuck
🏆 Review

What is the general equation for addition polymerisation of a vinyl monomer CH₂=CHR?

Explain why HDPE is denser and has a higher melting point than LDPE, even though they have the same repeat unit.

State three notation requirements for a correctly drawn polymer repeat unit.

Why is PTFE chemically inert and non-stick? Give a bond energy and an IMF argument.

Why are addition polymers like polyethylene not biodegradable?