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🏗️ IQ6 · Lesson 21 of 23 · 45 min
Addition Polymers
The plastic in every water bottle, food wrapper, pipe, non-stick pan, and electrical cable is an addition polymer — made from a single small alkene monomer by the same reaction type, with no atoms wasted and no by-product produced.
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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?
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
Monomer formula
Polymer name
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
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
Key Terms — scan these before reading
HydrocarbonAn organic compound containing only carbon and hydrogen atoms.
Functional groupA specific atom arrangement determining characteristic chemical reactions.
Homologous seriesA family of compounds with the same functional group, differing by CH₂.
Addition polymerA polymer formed by monomers adding together without loss of atoms.
Condensation polymerA polymer formed with elimination of a small molecule such as water.
EsterificationA condensation reaction between a carboxylic acid and an alcohol forming an ester.
Core Concepts
Misconceptions to Fix
✗
Wrong: Addition polymers and condensation polymers both release a small molecule during formation.
✗
Right: Addition polymers form by monomers adding together with no by-product (e.g., polyethylene from ethene). Condensation polymers form with the loss of a small molecule like water or HCl. The presence or absence of a by-product is the defining distinction between the two polymerisation types.
01
1 — 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.
Mechanism Overview (conceptual):
Initiation An initiator radical (R•) adds to the C=C, opening it and generating a carbon radical at the chain end.
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.
Termination Two chain radicals combine, ending both chains. Final polymer has n repeat units with 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 TipFor 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.
02
2 — Drawing Polymer from Monomer and Monomer from Polymer
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 (6 steps)
Step 1
Identify the C=C double bond in the monomer.
Step 2
Draw two adjacent monomer units side by side (to show the repeat).
Step 3
Replace the C=C with C-C (single bond). Each carbon now has a bond extending outward to the chain.
Step 4
Enclose ONE complete repeat unit (the two original C=C carbons + substituents) in square brackets.
Step 5
Show open bonds extending from each bracket end.
Step 6
Write subscript n outside the closing bracket.
Polymer → Monomer (6 steps)
Step 1
Identify the repeat unit inside the brackets.
Step 2
Find the two backbone carbons — those with open bonds at the bracket ends.
Step 3
Insert a C=C double bond between those two backbone carbons.
Step 4
Replace the open bonds at the bracket ends with H atoms (to satisfy tetravalency).
Step 5
Check that each C has exactly 4 bonds total.
Step 6
Name the monomer (look up the IUPAC name of the alkene formed).
Worked examples — conversions:
PVC from chloroethene:
Monomer: CH₂=CHCl. C=C between C1 (CH₂=) and C2 (=CHCl).
Open C=C: C1 → -CH₂- (two chain bonds + two H). C2 → -CHCl- (two chain bonds + one H + one Cl). Polymer: [-CH₂-CHCl-]ₙReverse — from [-CH₂-CHCl-]ₙ to monomer:
Two backbone carbons: C1 (CH₂, two chain bonds) and C2 (CHCl, two chain bonds). Insert C=C: CH₂=CHCl. Open bonds → H atoms. Monomer = chloroethene (vinyl chloride) ✓.
Polystyrene from styrene:
Monomer: CH₂=CH-C₆H₅. C=C between C1 and C2. The phenyl ring (C₆H₅) stays as a side group on C2. Polymer: [-CH₂-CH(C₆H₅)-]ₙ
The phenyl ring is intact in the repeat unit — it does not react, just hangs off every second backbone carbon.
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.
03
3 — Properties and Uses of Common Addition Polymers
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
Stiffer than LDPE, MP ~165°C, good chemical resistance
Food containers, rope, carpet, car parts, living hinges
PVC
Chloroethene (CH₂=CHCl)
C-Cl bonds give rigidity + flame resistance
Rigid & strong (natural); flexible with plasticisers; flame-resistant
Pipes, window frames (rigid); cable insulation, hoses, vinyl records (plasticised)
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, medical implants
PS
Styrene (CH₂=CHC₆H₅)
Bulky phenyl groups prevent crystallisation → amorphous → clear
Clear, rigid, brittle; expanded PS (Styrofoam) is very low density
Disposable cups, CD cases, insulation foam, lab containers
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 — instead of gas, he found a white, waxy, incredibly slippery solid. PTFE's C-F bond energy (~485 kJ/mol, vs C-H ~413 kJ/mol) means no common chemical — not even aqua regia (which dissolves gold) — can attack the polymer backbone. Large-scale industrial acid reactors are lined with PTFE for exactly this reason.
04
4 — Thermoplastics, Thermosets, and Environmental Context
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. Thermoplastics are recyclable — they can be melted and reshaped.
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.
Addition polymerisation definition: Alkene monomers (C=C) undergo chain reaction — pi bond opens, new C-C sigma bonds form between monomers, no by-product produced. Repeat unit notation: [-repeat unit-]ₙ — square brackets + open bonds at each end + subscript n outside closing bracket. No C=C inside the repeat unit. LDPE vs HDPE: Same monomer (ethene). LDPE = branched chains → poor packing → flexible, low density, low MP. HDPE = linear chains → close packing → rigid, high density, high MP. Both thermoplastics (recyclable). PVC: Cl substituent → rigid + flame-resistant. Plasticisers → flexible PVC. PTFE: C-F bond (~485 kJ/mol) = strongest to carbon. F sheath shields backbone. Only weak dispersion forces at surface → non-stick. Stable to ~260°C. Environmental issues: C-C backbone resists biodegradation (hundreds to thousands of years). UV + mechanical → microplastics (<5 mm) → accumulate in food chains.
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.
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: CH₂= already has no spare bond; =CHCN is 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 (no polar bonds, no H-bonding in polyethylene). 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. Compare with nylon (-NH- groups allow H-bonding at the surface → stronger adhesion → higher 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).
Interactive
Check Your Understanding
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?
Short Answer Questions
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.
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 (rearranges to ethanal) 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. Option A: same repeat unit = same C-C bonds per unit. Option C: same molecular mass per repeat unit. Option D: covalent cross-links = thermoset property; HDPE is a thermoplastic.
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. Option B confuses covalent bonds (within the polymer) with surface adhesion forces (between the polymer and other surfaces). Option D: C-F bonds are polar but the shielding is steric/energetic, not electrostatic repulsion.
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. This is why PE, PP, PVC, and PS persist for hundreds to thousands of years while biological polymers (proteins, polyesters like PLA under industrial conditions) degrade.
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 (including saline, amino acids, lipids, enzymes). No common biological molecule can attack PTFE. HDPE and PVC are less chemically inert — HDPE can swell in organic solvents; PVC can leach plasticisers. [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) and does not soften at body temperature (well below its MP). LDPE (MP ~110°C) is also stable, but PTFE's chemical inertness advantage makes it superior. [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]
Revisit Your Think First Response
At the start of this lesson you predicted what happens to the C=C double bond in addition polymerisation. Now you know: the pi bond opens and is consumed to form two new C-C sigma bonds linking monomers. The sigma bond framework stays — only the pi bond disappears. This is why: (1) there is no by-product; (2) the repeat unit has no C=C; (3) the empirical formula of the polymer is the same as the monomer.