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.
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
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?
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.
Mechanism Overview (conceptual)
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.
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.
Worked Examples — Conversions
Convert chloroethene (CH₂=CHCl) to its polymer, then reverse the process.
Identify the monomer from [-CH₂-CH(CH₃)-]ₙ and name it.
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.
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.
| Polymer | Monomer | Key structural feature | Key property | Common uses |
|---|---|---|---|---|
| LDPE | Ethene (CH₂=CH₂) | Branched chains — poor packing | Flexible, transparent, low density (0.91–0.93 g/cm³), MP ~110°C | Plastic bags, cling film, squeeze bottles |
| HDPE | Ethene (CH₂=CH₂) | Linear chains — close packing, semi-crystalline | Rigid, opaque, higher density (0.94–0.97 g/cm³), MP ~130°C | Milk crates, piping, detergent bottles |
| PP | Propene (CH₂=CHCH₃) | CH₃ side groups every second C | Stiffer than LDPE, MP ~165°C, good chemical resistance | Food containers, rope, carpet, car parts |
| PVC | Chloroethene (CH₂=CHCl) | C-Cl bonds give rigidity + flame resistance | Rigid & strong (natural); flexible with plasticisers; flame-resistant | Pipes, window frames; cable insulation, vinyl records |
| PTFE (Teflon) | Tetrafluoroethene (CF₂=CF₂) | C-F sheath shields backbone; F only on surface | Extremely chemically inert (C-F ~485 kJ/mol); non-stick; stable to ~260°C | Non-stick cookware, plumber's tape, chemical vessel linings |
| PS | Styrene (CH₂=CHC₆H₅) | Bulky phenyl groups prevent crystallisation → amorphous → clear | Clear, rigid, brittle; expanded PS very low density | Disposable cups, CD cases, insulation foam |
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.
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.
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. Thermoplastics are recyclable — they can be melted and reshaped.
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).
(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.
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.
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).
[-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).
n CF₂=CF₂ → [-CF₂-CF₂-]ₙ (addition polymerisation; no by-product)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
| Monomer | Formula | Polymer | Abbrev. | Repeat unit |
|---|---|---|---|---|
| Ethene | CH₂=CH₂ | Polyethylene | PE (LDPE/HDPE) | [-CH₂-CH₂-]ₙ |
| Propene | CH₃CH=CH₂ | Polypropylene | PP | [-CH₂-CH(CH₃)-]ₙ |
| Chloroethene | CH₂=CHCl | Poly(vinyl chloride) | PVC | [-CH₂-CHCl-]ₙ |
| Tetrafluoroethene | CF₂=CF₂ | Polytetrafluoroethylene | PTFE (Teflon) | [-CF₂-CF₂-]ₙ |
| Styrene (phenylethene) | CH₂=CHC₆H₅ | Polystyrene | PS | [-CH₂-CH(C₆H₅)-]ₙ |
| Propenenitrile | CH₂=CHCN | Polyacrylonitrile | PAN | [-CH₂-CH(CN)-]ₙ |
(a) Repeat unit is –[CH₂–CH₂]ₙ– (polyethylene). (b) No small molecule is lost because both pi-bond electrons are used to form new C–C sigma bonds between monomers — addition polymerisation uses ALL atoms of the monomer. This distinguishes it from condensation polymerisation, which eliminates H₂O or HCl. (c) The C=C double bond contains a reactive pi bond — pi electrons are more exposed and higher in energy than sigma electrons, making them available for electrophilic attack. Breaking the pi bond releases energy, driving the reaction forward.
Complete the Learn phase to unlock Practice.
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
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.
Q1. Which correctly identifies the monomer and polymerisation type used to make PVC?
Q2. LDPE and HDPE are both polyethylene made from ethene. What structural difference gives HDPE a higher melting point?
Q3. A student draws the repeat unit of a polymer as -CH₂-CH₂- without square brackets and without subscript n. What is wrong?
Q4. PTFE (Teflon) is used as a non-stick coating and to line chemical storage tanks. Which statement best explains its non-stick property using intermolecular force reasoning?
Q5. Why are most addition polymers (PE, PP, PVC, PS) not biodegradable, while proteins and polyesters can be broken down by microorganisms?
(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.
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.
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.
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.
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.
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.
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.
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.
(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]
(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]
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]
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?
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?
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