Polymers — the complete exam guide

1.1 Introduction to Polymers

A polymer is a long-chain molecule made of thousands or millions of repeating units called monomers. The word comes from Greek — poly = many, mer = unit. So a poly-mer is literally "many units."

Natural vs Synthetic Polymers

Polymers occur naturally and can be synthesised industrially.

• Natural polymers: cellulose (plant cell walls), silk and hair (proteins), DNA, natural rubber.
• Synthetic polymers: often called "plastics" — polyethylene shopping bags, PVC pipes, polyester clothing, nylon stockings, PTFE non-stick cookware.

Synthetic Polymers vs "Plastics" — a chemist's distinction

In everyday speech, "plastic" means a synthetic material. For chemists, "plastic" describes a property — a substance is plastic if it is malleable, pliable, and mouldable under heat and pressure.

Therefore: all plastics are synthetic polymers, but not all synthetic polymers are plastics. Synthetic polymers split into 4 categories based on their intermolecular forces.

The 4 Categories of Synthetic Polymers

✏️ Mapping to this guide: Category 1 → Part 1 (Addition polymers, PE/PVC/PS/PTFE), Category 4 → Part 2 (Condensation polymers, Nylon/PET). Categories 2 and 3 appear only as structural contrasts.

Thermoplastics vs Thermosetting — quick contrast

Thermoplastics — including every syllabus polymer in this guide (PE, PVC, PS, PTFE, Nylon-6,6, PET) — have weak intermolecular forces between chains. Heat overcomes these → chains slide → polymer melts and re-moulds. By contrast, thermosetting plastics (e.g. Bakelite) have covalent cross-links tying chains together — heat cannot break these without decomposing the polymer. This contrast is included only for context; NESA does not test thermosetting plastics directly.

1.2 Addition Polymerisation — Mechanism

Addition polymerisation is the process of forming a polymer through a series of addition reactions between unsaturated monomers in the presence of an initiator or catalyst.

Step 1 · Addition Reaction Revision (from IQ1)

An addition reaction is a reaction in which an unsaturated molecule (with a C=C or C≡C bond) gains atoms by breaking the π-bond. The double bond opens up; no atoms are eliminated. (Compare with substitution, which exchanges one group for another, and condensation, which eliminates a small molecule.)

Step 2 · The Polymerisation Mechanism

In addition polymerisation, instead of ethene + Cl₂, we have ethene + ethene + ethene + …. Each π-bond breaks under the influence of a catalyst at high temperature and pressure, freeing one electron per carbon to form a new covalent bond with the next monomer.

1. One bond in each C=C breaks (π-bond breaks).
2. Each carbon now has an unpaired electron.
3. New C–C bonds form between adjacent ethene molecules → long chain of repeating −CH₂−CH₂− units.

General equation:

n CH₂=CH₂   →   −(CH₂−CH₂)_n−

where n = the number of monomer units in the polymer chain (typically 10³–10⁵).

Addition polymerisation needs a catalyst or initiator together with high temperature and pressure to break the C=C π-bond. The exact catalyst type determines whether chains end up branched or linear — this is what causes the LDPE vs HDPE difference. When explaining LDPE/HDPE in an exam, mention "different catalyst conditions" — you do not need to name the catalysts.

Naming Addition Polymers

Place the prefix poly- in front of the monomer name. Use brackets when the monomer name is two words or starts with a number.

⚠️ Note: "phenylbenzene" is not a synonym for styrene — that name refers to biphenyl. Acceptable IUPAC names for styrene's monomer are ethenylbenzene (preferred IUPAC name, treating benzene as the parent) or phenylethene (treating ethene as the parent). The older common name "vinylbenzene" is also valid.

Distinguishing Test — Bromine Water

A common exam question asks: "How would you distinguish the monomer from the polymer using a chemical test?"

This is a direct application of the bromine test for unsaturation from IQ1 Hydrocarbons. Tested explicitly in PEM 2019 Q33 and Loreto 2019 Q26 (both 3-4 marks).

1-Mark Gateway Question — why is the polymer a solid but the monomer a gas?

Frequently asked as a 1-mark "warm-up" before harder polymer Qs (Independent 2020 Q35(a), Hurlstone 2020 Q31(a)).

Worked Example — Drawing the polymer from a monomer

Given the monomer vinyl chloride (chloroethene), CH₂=CHCl, draw the addition polymer formed.

1.3 Comparing PE / PVC / PS / PTFE — Structure → Properties → Uses

The 4 syllabus addition polymers are all derivatives of ethene. Replacing one or more H atoms of ethene with another substituent gives the monomer that builds each polymer:

Ethene (ethylene, C₂H₄) is the master starting material. Replace 1 H → vinyl chloride / ethenylbenzene. Replace all 4 H with F → tetrafluoroethene.

Structure: Molecular Comparison

The Polymer Comparison Checklist core

Properties of any addition polymer depend on 4 structural factors. Memorise these 4 — they're the comparison checklist for every Part 1 polymer.

Properties of Polyethylene — LDPE vs HDPE

Polyethylene exists in two industrial forms that differ only in chain branching — yet their properties are dramatically different. This is the canonical "branching changes everything" case study.

The Causal Chain — How to Answer "Compare" Questions

Whenever NESA asks "compare the uses of X and Y" or "explain why X is used for Z", the marker wants this 3-link chain:

Example for LDPE = plastic bag:

Use: plastic shopping bag → requires properties: flexible (so it folds), lightweight, low cost, chemically inert (food contact) → justified by structure: high branching → chains can't pack → amorphous → flexible + low density; pure hydrocarbon → non-polar → chemically inert.

1.4 The 4-Step Model Answer Scaffold — Addition Polymers

Almost every "compare-and-contrast" or "discuss the structure / properties / uses" question on addition polymers can be answered with this 4-step scaffold. NESA markers reward this structure explicitly — use it on every multi-mark addition polymer question.

Worked Example — Compare LDPE and HDPE

Per-Polymer 5-Step Summary Cards — for memorisation

Each of the 5 addition polymers below has its own 5-step summary card following the scaffold above. Use these as flashcards before the exam — one card per polymer, with the monomer + abbreviated polymer structure included for easy reference.

LDPE — Low-Density Polyethylene

PE

1. Equation: n CH₂=CH₂ → −(CH₂−CH₂)_n−   (free-radical conditions; branched chains)
2. Use: plastic shopping bags, cling film, squeeze bottles — needs flexibility + low cost + chemical inertness.
3. Properties: flexible (phys) · low density ~0.92 g/cm³ (phys) · translucent (phys) · chemically inert (chem).
4. Structure → Properties chain:
   • High degree of branching → chains physically cannot pack closely → amorphous (low crystallinity) → low density (lots of empty space) + flexible (chains slide past each other easily) + translucent (no large crystal domains to scatter light).
   • Side group: none (just –H) → pure non-polar C–C/C–H backbone.
   • Chain length: long (~10³–10⁵ units) → enough dispersion forces to be solid at RT, but no polar attractions → low m.p. (~110 °C).
5. Polar bonds: none — pure non-polar C–H/C–C backbone → only weak dispersion (London) forces between chains → chemically inert (no functional groups for acids/bases/water to attack) + low m.p.

📝 Sample exam answer (3 marks) — "Why is LDPE used for plastic shopping bags? Refer to its structure."

LDPE has highly branched polyethylene chains, so the chains cannot pack closely together. This produces an amorphous structure with low density (~0.92 g/cm³), making LDPE flexible — the chains slide past each other easily when stretched, so the material folds without breaking. The pure non-polar C–C/C–H backbone gives LDPE chemical inertness, which is essential for food contact. The combination of flexibility + low cost + inertness makes LDPE ideal for plastic shopping bags.

⚡ 30-sec recall: Highly branched chains → amorphous → low density (~0.92 g/cm³) + flexible. Non-polar C–C/C–H backbone → dispersion forces only → chemically inert.
⚠️ Top 2 mistakes: (1) Saying LDPE has "low molecular weight" — wrong; LDPE chains are still long. Branching, not chain length, is what lowers density. (2) Confusing LDPE and HDPE — they're the same polymer chemistry; only the chain geometry (branched vs linear) differs.

HDPE — High-Density Polyethylene

HDPE

1. Equation: n CH₂=CH₂ → −(CH₂−CH₂)_n−   (coordination catalyst; linear chains)
2. Use: milk jugs, water pipes, buckets, toys — needs rigidity + durability + opacity (food contact).
3. Properties: rigid (phys) · high density ~0.95 g/cm³ (phys) · opaque (phys) · chemically inert (chem).
4. Structure → Properties chain:
   • Low branching (linear chains) → chains pack tightly together → highly crystalline → high density (~0.95) + rigid (chains can't slide past each other) + opaque (crystal domains scatter light).
   • Side group: none → pure non-polar C–C/C–H backbone, same as LDPE.
   • Tighter packing → stronger total dispersion forces → higher m.p. (~135 °C) than LDPE despite identical chemistry.
5. Polar bonds: none — same backbone as LDPE; the difference (rigidity, density, opacity) comes entirely from packing geometry, not from polarity or chemistry.

📝 Sample exam answer (3 marks) — "Why is HDPE used for milk jugs? Refer to its structure."

HDPE has linear (unbranched) polyethylene chains, which pack tightly together in a highly crystalline structure. This high crystallinity gives HDPE rigidity (the milk jug holds its shape under the weight of the liquid) and opacity (the crystal domains scatter light, blocking UV that would degrade the milk inside). The pure non-polar C–C/C–H backbone provides chemical inertness, essential for food contact. Rigidity + opacity + inertness make HDPE the ideal material for milk jugs.

⚡ 30-sec recall: Linear unbranched chains → high crystallinity → high density (~0.95) + rigid + opaque. Same chemistry as LDPE; only the chain geometry differs.
⚠️ Top 2 mistakes: (1) Treating HDPE as "different chemistry" from LDPE — they share the same ethene monomer and C–C backbone; only the chain arrangement differs. (2) Forgetting that HDPE's stronger inter-chain attraction comes from closer packing → more dispersion contact, NOT from any polar group.

PVC — Poly(vinyl chloride)

PVC

1. Equation: n CH₂=CHCl → −(CH₂−CHCl)_n−   (monomer: vinyl chloride / chloroethene)
2. Use: PVC pipes (rigid form), electrical wire insulation (flexible form), window frames, vinyl flooring.
3. Properties: rigid (phys) · slightly transparent (phys) · higher m.p. than PE (phys) · chemically inert (chem).
4. Structure → Properties chain:
   • Polar C–Cl side group on every second carbon → permanent dipole → dipole-dipole forces between chains (stronger than PE's dispersion-only) → more rigid + higher m.p. than PE.
   • Low branching + intermediate crystallinity → chains pack moderately well → slightly transparent (smaller crystal domains than HDPE).
   • Stable C–Cl and C–C bonds in the backbone → chemical inertness in everyday conditions (no –OH/–COOH for acids/bases to attack).
5. Polar bonds: C–Cl is polar → permanent dipole on every other C → dipole-dipole IMFs between chains → key difference from PE.

📝 Sample exam answer (4 marks) — "Compare PVC and PE. Explain why PVC is more rigid than PE."

Both PVC and PE are addition polymers of ethene derivatives. The key structural difference is the side group: PE has only hydrogen atoms, while PVC has a chlorine atom on every second carbon. The C–Cl bond is polar (Cl is much more electronegative than C), creating a permanent dipole. This means PVC chains attract each other through dipole-dipole forces in addition to dispersion forces. PE chains, having only non-polar C–C/C–H bonds, attract each other through dispersion forces alone. The stronger inter-chain forces in PVC restrict chain movement, making PVC more rigid with a higher melting point than PE.

⚡ 30-sec recall: One H replaced by Cl per repeat unit → polar C–Cl → dipole-dipole forces → more rigid + higher m.p. than PE. Plasticisers can convert rigid PVC into flexible PVC.
⚠️ Top 2 mistakes: (1) Saying PVC forms H-bonds — Cl is electronegative but has no H attached to it, so PVC has dipole-dipole only, NOT H-bonds. (2) Treating "flexible PVC" and "rigid PVC" as different polymers — both are PVC; the only difference is plasticiser additive.

PS — Polystyrene

PS

1. Equation: n CH₂=CH(C₆H₅) → −(CH₂−CH(C₆H₅))_n−   (monomer: styrene / ethenylbenzene)
2. Use: CD cases, plastic cutlery (Type 1 commercial) · foam packaging, disposable cups (Type 2 Styrofoam — gas-expanded).
3. Properties: brittle (phys) · transparent — Type 1 (phys) · lightweight (phys) · chemically inert in everyday conditions (chem).
4. Structure → Properties chain:
   • Bulky phenyl (C₆H₅) side group on every second C → chains cannot pack regularly → atactic (amorphous) → transparent (no crystal domains) + brittle (no crystal reinforcement to absorb stress).
   • No polar groups, no N–H/O–H → the only inter-chain force is the weak dispersion (London) force. (NESA's 2025 Q28 marking guideline treats PS as dispersion-only — don't add π-stacking for marks.)
   • Type 2 (expanded Styrofoam) — same polymer, gas-blown to ~95% air → very lightweight + excellent thermal/acoustic insulator (trapped air = poor heat conductor).
5. Polar bonds: none — phenyl is non-polar. Dispersion forces only.

📝 Sample exam answer (3 marks) — "Why is foam polystyrene used for disposable cups, but solid polystyrene used for CD cases?"

Both are made from the same polystyrene polymer (atactic, amorphous structure due to bulky phenyl side groups), but differ in processing. Foam polystyrene (Type 2 Styrofoam) is gas-expanded to ~95% air, making it very lightweight and an excellent thermal insulator (trapped air conducts heat poorly) — ideal for disposable hot-drink cups where the user's hand needs protection from heat. Solid (Type 1) polystyrene retains the amorphous transparency of the unfoamed polymer, giving rigid + transparent properties needed for CD cases (rigid to protect disc, transparent so the cover artwork is visible).

⚡ 30-sec recall: Bulky phenyl side group → chains can't pack closely → amorphous → glassy, brittle, transparent. IMF = dispersion only. Foam form = same polymer, ~95% air added.
⚠️ Top 2 mistakes: (1) Saying the phenyl ring provides H-bonds or dipole–dipole — phenyl is non-polar; PS has dispersion forces only. (2) Confusing styrene (the monomer CH₂=CHC₆H₅) with Styrofoam (the expanded form of polystyrene).

PTFE — Polytetrafluoroethylene (Teflon)

PTFE

1. Equation: n CF₂=CF₂ → −(CF₂−CF₂)_n−   (monomer: tetrafluoroethene — all 4 H of ethene replaced by F)
2. Use: non-stick cookware, plumber's tape, bearings, electrical insulation — needs heat resistance + low friction + inertness.
3. Properties: very low friction (phys) · very high m.p. (phys) · extremely chemically inert (chem) · insoluble in most solvents (chem).
4. Structure → Properties chain:
   • Symmetric F substitution on every C (all 4 H replaced by F) → individual C–F bonds polar BUT symmetric placement cancels the net dipole → no dipole-dipole forces between chains.
   • F atoms shield the carbon backbone + extremely strong C–F bonds (strongest single bond to carbon) → extremely chemically inert (resists almost all chemical attack).
   • F-rich surface has low electron-donating capacity for IMFs with other materials → very low friction (food doesn't stick; bearings glide).
   • Linear backbone + high crystallinity → tight packing → very high m.p. despite no dipole-dipole forces (enhanced dispersion from high F electron density).
5. Polar bonds: individual C–F bonds are polar, but symmetric substitution → no net dipole across the chain. Only enhanced dispersion forces (high F electron density). The shielded backbone is what makes PTFE so inert.

📝 Sample exam answer (3 marks) — "Why is PTFE used for non-stick cookware? Refer to its structure."

PTFE has fluorine atoms symmetrically placed on every carbon (all 4 H of ethene replaced). The C–F bonds are very strong (the strongest single bond to carbon), so PTFE is extremely chemically inert — it won't react with food acids or oils. The F-rich surface also has very low friction (electrons in C–F bonds are held tightly by F, so they don't interact strongly with other molecules) — food slides off easily. Tight chain packing gives a very high melting point, so PTFE survives high cooking temperatures. Inertness + low friction + heat resistance = ideal non-stick cookware coating.

⚡ 30-sec recall: All 4 H of ethene replaced by F → very strong C–F bonds → chemically inert + very high m.p. F-shielded surface → very low friction. Symmetric F substitution cancels net dipole.
⚠️ Top 2 mistakes: (1) Saying PTFE has strong dipole-dipole forces — individual C–F bonds are polar, but symmetric placement cancels the net dipole. (2) Forgetting that PTFE's inertness IS the feature: non-stick pans work because nothing sticks to or reacts with C–F.

1.5 Recycling Addition Polymers — background context, useful for extended-response framing

All 4 syllabus addition polymers (PE, PVC, PS, PTFE) are thermoplastics, so they can be melted and re-moulded rather than decomposed. This makes them recyclable in principle — though the practical recycling rate varies by polymer (PET and HDPE are widely recycled; PVC and PTFE are not).

2.1 Condensation Polymerisation — Mechanism

Condensation polymerisation is the process of forming a polymer by reacting bifunctional monomers together, with the elimination of a small molecule (almost always water) at each link.

How is this different from addition?

The Co-polymer Idea

Most condensation polymers are co-polymers — built from two different monomers joined alternately:

• Polyester (PET): a diacid (terephthalic acid) + a diol (ethylene glycol)
• Polyamide (Nylon-6,6): a diacid (adipic acid) + a diamine (hexamethylenediamine)
• Kevlar: aromatic diacid (terephthalic acid) + aromatic diamine (1,4-diaminobenzene)

A few condensation polymers use a single monomer that has both functional groups on the same molecule — for example, an amino acid (–NH₂ at one end, –COOH at the other) self-polymerises into a polypeptide. PHB (poly-3-hydroxybutyrate) works the same way.

🎯 The Master Pattern — every condensation polymer question follows this

Once you see this pattern, every NESA polyester/polyamide question (including unseen monomers like Kevlar or PHB) becomes the same problem. The diagrams below use abstract shapes (green rectangles + blue ellipses) for the carbon backbones — focus on the functional groups at the ends and what they form when they link.

The Functional-Group Rule

The "Cut-Across-the-Link" Technique — finding monomers from a polymer

The most common exam task is the reverse problem: given a polymer fragment, identify the monomer(s). Master this 3-step recipe:

Worked Example — Identify the monomers

Given a section of polymer with the structure:

⋯ –NH–(CH₂)₆–NH–C(=O)–(CH₂)₄–C(=O)–NH–(CH₂)₆–NH– ⋯

2.2 Polyester — Polyethylene Terephthalate (PET)

PET is the syllabus polyester. It's the world's most-produced condensation polymer (drinks bottles, polyester clothing, film) — and the one NESA tests most often (2020 HSC Q12, 2025 HSC Q16).

The Two Monomers

The Polymerisation Equation

n HOOC–C₆H₄–COOH  +  n HO–CH₂–CH₂–OH
↓
–[ O–C(=O)–C₆H₄–C(=O)–O–CH₂–CH₂ ]_n–  +  (2n – 1) H₂O

Each link eliminates one H₂O molecule. A chain of n repeating units has 2n–1 links → 2n–1 water molecules released. Critical for MW calculation — see §3.3.

Structure: what the chain looks like

The PET backbone is an alternation of rigid aromatic rings and short flexible –O–CH₂–CH₂–O– spacers, linked together by ester groups –C(=O)–O–. Two structural features dominate the properties:

• 4 Polar C=O bonds in the ester linkages → dipole–dipole IMFs between adjacent chains.
• Aromatic rings → π–π stacking between flat benzene rings of neighbouring chains (an enhanced form of dispersion force).

The combination is what makes PET so strong despite being only "weakly bonded" between chains. 2020 HSC Q12 directly tests this — the correct answer was: "Extensive dipole–dipole and dispersion forces exist between the polymer chains."

Properties

Uses (linked to properties)

• Drink bottles — transparent (when rapidly cooled), tough (high tensile strength), lightweight, food-safe, recyclable (code 1).
• Polyester clothing fibres — high strength + crystallinity + low water absorption (stays cool, dries fast).
• Magnetic tape, X-ray film, packaging films — flexible when thin, strong, transparent.
• Sportswear / fleece — when recycled (rPET), often spun into fibres for jackets and carpets.

2.3 Polyamide — Nylon-6,6

Nylon-6,6 is the syllabus polyamide. It's the textbook example of hydrogen-bonded chain attraction — the same mechanism that makes Kevlar (§3.1), proteins, and silk strong.

The Two Monomers

The Polymerisation Equation

The –COOH of adipic acid reacts with the –NH₂ of hexamethylenediamine, eliminating H₂O at each link. This is the same condensation reaction that joins amino acids into proteins.

n HOOC–(CH₂)₄–COOH  +  n H₂N–(CH₂)₆–NH₂
↓
–[ C(=O)–(CH₂)₄–C(=O)–N(H)–(CH₂)₆–N(H) ]_n–  +  (2n – 1) H₂O

Structure: H-bonding between chains

This is where Nylon-6,6 differs dramatically from PE/PVC/PS/PTFE/PET. Every amide group in the chain has both:

• A polar N–H (H is δ⁺, attached to electronegative N) → H-bond donor
• A polar C=O (O has lone pairs and is δ⁻) → H-bond acceptor

This means every monomer unit contributes 2 H-bond sites — the N–H of one chain forms a strong hydrogen bond with the C=O oxygen of the adjacent chain. Multiplied across thousands of repeat units, the cumulative IMF is enormous.

Properties

Uses (linked to properties)

• Stockings, hosiery — elastic (H-bonds re-form after stretch), strong (resists tearing), smooth surface.
• Fishing line, ropes, parachutes — extreme tensile strength + slight elasticity (absorbs shock).
• Carpet fibres — durable + crystalline (resists wear).
• Engineering plastics (gears, bearings) — high m.p. + chemical resistance + low friction.

2.4 Addition vs Condensation — Side-by-Side Comparison

You've now seen both types. NESA frequently asks you to identify a polymer's type from its structure or to "outline the difference" between addition and condensation (e.g., James Ruse 2019 Q21, NEAP 2021 Q17). Master this comparison.

Backbone Diagnostic — how to tell which type from a polymer structure

Molecular Weight — first preview

The most-tested polymer calculation in HSC and trials is finding the molecular weight of a polymer given the monomer(s) and n. The formula depends on the type:

2.5 The 5-Step Scaffold — Condensation Polymers

For condensation polymers, use the same 4-step scaffold from §1.4 but extend the structural checklist with one additional item — presence of polar bonds in the backbone — because condensation polymers always have polar C=O (ester or amide) groups in the backbone that drive much of their IMF behaviour.

Worked Example — Compare Nylon-6,6 and PET (HW Ex 2 Q19)

Per-Polymer 5-Step Summary Cards — for memorisation

The two syllabus condensation polymers each have a summary card following the scaffold above. Note how the 5^th step (polar bonds in backbone) is now the decisive factor — both have polar C=O groups, but only Nylon has the N–H donor needed for full hydrogen bonding.

Nylon-6,6 — Polyamide

Nylon

1. Equation: n HOOC–(CH₂)₄–COOH + n H₂N–(CH₂)₆–NH₂ → −[C(=O)–(CH₂)₄–C(=O)–N(H)–(CH₂)₆–N(H)]_n− + (2n−1) H₂O
2. Use: stockings, fishing line, ropes, carpet, engineering plastics — needs elasticity + extreme tensile strength + heat resistance.
3. Properties: very high tensile strength (phys) · elastic — H-bonds re-form after stretch (phys) · high m.p. ~265 °C (phys) · slightly hygroscopic (phys) · inert in everyday conditions (chem).
4. Structure → Properties chain:
   • Linear regular backbone with amide groups at every monomer → chains align in parallel → highly crystalline → high m.p. + opacity.
   • Every amide link has BOTH N–H (donor) AND C=O (acceptor) → strong hydrogen bonds between adjacent chains at every link → very high tensile strength (chains resist being pulled apart).
   • H-bonds break and re-form when the chain is stretched → elasticity (the polymer returns to original shape) — perfect for stockings/clothing.
   • Polar N–H + C=O groups can H-bond with water → slightly hygroscopic (Nylon absorbs some water from air; stockings stretch when wet).
5. Polar bonds: both C=O (acceptor) AND N–H (donor) in every amide link → the key feature that makes Nylon's H-bonding so strong. This is the Checklist-item-5 difference vs PET (PET has C=O but no N–H donor).

📝 Sample exam answer (4 marks) — "Explain, with reference to structure, why Nylon-6,6 is used for stockings."

Nylon-6,6 has a regular linear backbone with an amide group at every monomer. Each amide group contains both an N–H (H-bond donor) and a C=O (H-bond acceptor), so adjacent Nylon chains attract each other through strong hydrogen bonds at every link. This H-bond network gives Nylon very high tensile strength — stockings don't tear easily. Critically, when Nylon is stretched, the H-bonds break and re-form at new positions, then re-form again when the stretch is released — this gives elasticity, the spring-back behaviour essential for stockings to fit closely to the leg. Strength + elasticity together make Nylon ideal for stockings.

⚡ 30-sec recall: –CO–NH– amide links → N–H donor + C=O acceptor → strong inter-chain H-bonds at every link → high tensile strength + elastic recovery (H-bonds break and re-form on stretch).
⚠️ Top 2 mistakes: (1) Drawing N=C in the amide (a frequent textbook error) — the C–N bond is a single bond, and the C=O is the double bond. (2) Misclassifying Nylon as addition because of the C=O — apply the backbone test: amide links in the backbone = condensation polymer.

PET — Polyethylene Terephthalate

PET

1. Equation: n HOOC–C₆H₄–COOH + n HO–CH₂CH₂–OH → −[O–CH₂CH₂–O–C(=O)–C₆H₄–C(=O)]_n− + (2n−1) H₂O
2. Use: drink bottles, polyester clothing fibres, films (X-ray, magnetic tape) — needs rigidity + transparency + chemical resistance.
3. Properties: rigid + tough (phys) · transparent when rapidly cooled (phys) · stain-resistant — hydrophobic surface (phys) · high m.p. ~260 °C (phys) · inert to dilute acids/bases; vulnerable to strong base hydrolysis at the ester link (chem).
4. Structure → Properties chain:
   • Rigid aromatic benzene rings in backbone → chains cannot kink → align parallel → partially crystalline when slow-cooled, amorphous when rapidly cooled (transparent for bottles).
   • Polar C=O ester groups → dipole-dipole forces between chains → high m.p. (~260 °C) + rigidity. NO N–H donor → no full H-bonds → PET is rigid but not as elastic as Nylon.
   • π–π stacking between adjacent aromatic rings → extra dispersion-class attraction → boosts m.p. and tensile strength.
   • Polar groups buried inside the chain, only C–H/aromatic surface exposed → hydrophobic surface → stain-resistant + suitable for clothing fibres.
5. Polar bonds: C=O ester groups → dipole-dipole IMFs. NO N–H donor → no full H-bonds (this is why PET is rigid but less elastic than Nylon-6,6). Aromatic rings provide π–π stacking.

📝 Sample exam answer (4 marks) — "Why is PET used for drink bottles? Refer to its structure."

PET has a backbone of rigid aromatic benzene rings alternating with short flexible –O–CH₂CH₂–O– spacers, linked by polar ester groups (C=O). The polar C=O bonds create dipole-dipole forces between chains, and the aromatic rings provide additional π–π stacking attraction. Together, these strong inter-chain forces give PET rigidity (it holds the bottle shape under carbonation pressure) and high tensile strength (the bottle doesn't crack). When the bottle is manufactured by rapid cooling, PET solidifies in an amorphous form → transparent, so consumers can see the drink inside. Finally, the polar ester groups are buried inside the chain, leaving the hydrophobic surface to provide stain resistance and good food-contact safety. Rigid + transparent + tough = ideal drink bottle.

⚡ 30-sec recall: Aromatic rings + ester (–CO–O–) links in backbone → dipole-dipole + π–π stacking → rigid + high m.p. (~260 °C). Transparent when rapidly cooled (amorphous), partially crystalline when slow-cooled.
⚠️ Top 2 mistakes: (1) Calling PET an addition polymer because of C=C in the benzene rings — aromatic C=C is not a reactive alkene; the backbone has ester links → condensation. (2) Saying PET has H-bonds — no N–H or O–H donors in the backbone, so dipole-dipole + π–π only (this is why PET is rigid but less elastic than Nylon).

2.6 Molecular Weight Calculation ★ CORE SKILL

Calculating the molar mass (MW) of a polymer is directly examined at the HSC level — 2025 HSC Q16 was a single-step MW calculation worth 1 mark. The formula differs slightly between addition and condensation polymers because condensation loses water at every link.

→ Four full worked examples (addition + 1-monomer condensation + 2-monomer condensation + reverse n) live in §3.3. Same skill, deeper practice.

3.1 Kevlar — Aromatic Polyamide

Kevlar is an aromatic polyamide — sometimes called para-aramid. NESA tested it directly in 2025 HSC Q28 (4 marks), and Sydney Boys 2022 Q29 + Exam Choice 2021 Q5 covered it well before that. Kevlar isn't a syllabus dot point, but it's the standard "extension" question for high-performing students — and it's the perfect way to see how the 5^th knob (polar bonds) interacts with rigid aromatic structure.

The Two Monomers

Compare with Nylon-6,6 (§2.3): both are polyamides with diacid + diamine monomers. The only difference is the backbone. Nylon has flexible aliphatic –(CH₂)_n– chains between functional groups; Kevlar has rigid aromatic rings.

Repeating Unit

–[ N(H)–C₆H₄–N(H)–C(=O)–C₆H₄–C(=O) ]_n–  +  (2n − 1) H₂O

Why is Kevlar so strong? — three reinforcing factors

1. H-bonds between chains — same N–H + C=O mechanism as Nylon-6,6.
2. π-stacking between aromatic rings — the flat benzene rings of adjacent chains line up, adding extra dispersion-class attraction along the entire chain length.
3. Rigid planar backbone — aromatic rings don't rotate freely (unlike –(CH₂)_n– chains), so chains align perfectly straight. This means H-bonds and π-stacks fall into ideal geometry along every monomer unit — maximum IMF density per unit length.

The combined effect gives Kevlar extreme tensile strength — by weight, it is many times stronger than steel.

Uses (linked to properties)

• Body armour, bulletproof vests — extreme tensile strength + light weight + heat resistant.
• Aerospace components, sports equipment — strength + low density + chemical inertness.
• Ropes, sail fabrics, brake pads, fibre-optic cable cores — high-strength fibre wherever steel is too heavy.

Worked Example — 2025 HSC Q28 (4 marks)

3.2 Biopolymers — Extension Topic Tested by NESA in 2022 + 2024

Both questions are applications of the condensation polymer mechanism you already learned in §2.1–2.3. The same cut-across-the-link technique and the same MW formula from §3.3 apply directly. This is not new chemistry — it's the same tools applied to biological monomers.

📝 Case Study 1 — 2022 HSC Q18 · PHB Biopolymer MW Selection

Question analysis

• What's being tested: recognising the biopolymer as a condensation polymer (1-monomer self-condensation) and applying the MW formula (n × MW(monomer) − (n−1) × 18.016).
• Key clue in the diagram: the polymer section shows ester linkages (–C(=O)–O–) with a repeating –CH₃ side group → this is a polyester biopolymer (PHB).
• Formula to apply: the 1-monomer condensation formula from §3.3 (you already know it).

Solution — calculate all 4 options

Answer: D. Two further clues confirm D:

1. Option A's monomer contains a C=C double bond (the parallel lines in the A/B structures). That's an unsaturated monomer suitable for addition polymerisation — but the polymer section in the question is a condensation polymer with ester linkages. Even if A's monomer underwent self-condensation, it would not produce the PHB-style structure shown.
2. Option D's monomer is saturated with –OH at one end and –COOH at the other → precisely a 1-monomer condensation polyester building block. MW 90.078 g/mol = HOCH₂CH₂COOH (3-hydroxypropanoic acid) — the short-chain cousin of the actual PHB monomer.

📝 Case Study 2 — 2024 HSC Q14 · Silk + Glycine: Identifying the Second Monomer

Question analysis

• What's being tested: identifying the second amino acid residue in the silk polymer section — a direct application of the cut-across-the-link technique from §2.1.
• Key clue: the silk section shows two alternating residues. One is –CH₂– (glycine); the other is –CH(CH₃)– (an amino acid with a CH₃ side group).
• Requirement: the second monomer must also be an amino acid (it needs both –NH₂ and –COOH for self-condensation into a peptide chain).

Solution — apply cut-across-the-link

Cut across each peptide bond (–C(=O)–N(H)–) in the silk section, adding H to the N side and OH to the C=O side (the §2.1 technique). This recovers the two amino acid monomers:

• Residue 1 (–CH₂–): H₂N–CH₂–COOH = glycine ✓ (given)
• Residue 2 (–CH(CH₃)–): H₂N–CH(CH₃)–COOH = this is the second monomer we're looking for

Check each option:

Answer: B (alanine). Real silk is a polypeptide of glycine + alanine + several other amino acids.

Why we grouped these two questions together

3.3 Molecular Weight (MW) Calculation

This is the single most frequently-tested polymer calculation in HSC and trial papers. 2025 HSC Q16 tests it directly. NEAP (every year), QATS, CSSA, Strathfield, Meriden, James Ruse, Knox, Loreto, Exam Choice, and many more have rotated through it. Master the two formulae and you've collected a guaranteed mark.

The two formulae (recap from §2.4)

How to pick which formula — 30-second decision tree

⚠️ Sometimes a question says "n monomers of A and n monomers of B" — that's 2-monomer with total 2n monomers and (2n−1) water losses. Sometimes it says "n monomers, each with both functional groups" — that's 1-monomer with (n−1) water losses. Count the functional groups carefully.

Worked Example 1 — Addition Polymer (Polyethylene)

Worked Example 2 — 2025 HSC Q16 (1-Monomer Condensation Polyester)

Worked Example 3 — 2-Monomer Condensation (Nylon-6,6)

Worked Example 4 — Reverse Calculation (Find n)

Common pitfalls — checklist before submitting your answer

• ✅ Did I correctly identify addition vs condensation? (Look at the chain — any O or N in the backbone?)
• ✅ For condensation: did I count 1-monomer vs 2-monomer correctly? (1-monomer = (n−1) waters; 2-monomer = (2n−1) waters)
• ✅ Did I use the correct MW(monomer)? Re-check with H + C + N + O + Cl + F atomic masses.
• ✅ Does my answer have realistic magnitude? Polymers are typically 10⁴–10⁶ g/mol — if you got 10², you mis-identified the monomer; if you got 10⁸, you wrote (n−1) waters instead of (n−1)×18.

The Polymer Master Table — every polymer, every facet

One table to rule them all. Each row is one of the 9 polymers covered in this guide. Click any column header to sort; use the filter chips to show only addition or only condensation polymers. This is your one-page memorisation hub for the night before the exam.

IMF strength legend: 1 = dispersion only · 2 = dipole–dipole · 3 = H-bond. (Sort by IMF column to rank chains by attraction strength.)

Flashcards — Monomer ↔ Polymer ↔ Properties ↔ Uses

Click any card to flip. Front shows the polymer + monomer; back shows properties, uses, and the structural reason behind them. Drill these until you can recite both sides — this is the four-way mapping NESA asks you to "model and compare."

📋 The Polymers Cheat Sheet — print this for revision

Everything in one page. The same information lives in the 📋 floating panel (bottom-right) for quick reference, but this version is sized for printing and final-night review. Use Ctrl/Cmd + P to print just this section.

⏱ 2-Minute Pre-Exam Checklist core

Last 120 seconds before the bell. Re-read these two boxes — nothing else.

MCQ Quiz — 12 questions from NESA + Trials

Click the option you think is correct. You'll get immediate feedback + a short explanation citing where in the guide to revise. Your score appears at the end. Mix of NESA HSC 2020–2025 + selected high-yield trial items.

Extended Response — Model Answers (6 worked questions)

Each panel below is a worked extended-response question with full marker-friendly answer. Click to expand. Source mix: NESA HSC, CSSA, Sydney Boys, James Ruse, and SKY HSC College HW Exercises 1 & 2. Two more model answers already live elsewhere in this guide: §1.4 (LDPE vs HDPE) and §2.5 (Nylon vs PET).