Describe and explain the formation of addition polymers from alkene monomers, and relate the structure of common addition polymers (polyethene, polypropene, polyvinyl chloride, polystyrene, polytetrafluoroethene) to their properties through chain branching and crystallinity

What this dot point is asking

QCAA wants you to recognise an addition polymer from its monomer, draw the repeat unit (with brackets and the n subscript), and explain how chain structure (branching, crystallinity, substituent group) sets the bulk properties (softening point, density, flexibility, chemical resistance) of the polymer. The dot point feeds IA3 (product-design context) and EA Paper 1 / Paper 2 short-response items.

The answer

Addition polymerisation joins alkene monomers into long chains by breaking the C=C pi bond and forming new C-C sigma bonds between molecules. No atoms are lost; the repeat unit has the same molecular formula as the monomer.

General mechanism (overview)

Each monomer is an alkene (one or more C=C). Under suitable conditions (free-radical initiator, heat, pressure; or a coordination catalyst like Ziegler-Natta), the pi bonds open and link successive monomers:

Three implications:

1. The repeat unit is the monomer with the double bond opened. Always drawn with the substituent R on one carbon.
2. No by-product. Polymer mass equals total monomer mass.
3. Polymer chains are long but variable. n is typically 1,000 to 100,000. The number-average molar mass and the distribution affect properties but are not tested at QCAA Unit 4 level.

Five core addition polymers in the QCAA syllabus

For each, the repeat unit retains every atom of the monomer; you should be able to draw both directions (monomer to repeat, repeat to monomer) on demand.

How structure sets properties

Three structural levers control polymer properties: substituent group, chain branching, and chain regularity (which sets crystallinity).

Substituent group identity

Non-polar substituents (H, CH3, F) give non-polar chains. Intermolecular forces are dispersion only; chains slip past each other easily, so the polymer is flexible and resistant to acids and bases. Polar substituents (Cl) give dipole-dipole forces between chains and stiffer polymers.

• Polyethene: -H substituent. Dispersion forces only; flexible; low melting point.
• Polypropene: -CH3 substituent. Dispersion forces; the methyl branch lets stereoregular (isotactic) chains pack densely; somewhat stiffer than PE.
• PVC: -Cl substituent. Strong C-Cl dipole adds dipole-dipole forces; PVC chains pack tightly; rigid at room temperature unless plasticiser is added.
• Polystyrene: -C6H5 substituent. Large rigid phenyl group; chains cannot fold; brittle at room temperature.
• PTFE: -F substituents on every position. Strong C-F bonds; non-reactive; high melting point; very low coefficient of friction.

Chain branching

Branching reduces how tightly chains can pack. Two extreme cases for polyethene:

LDPE (low-density polyethene). Made by high-pressure free-radical polymerisation. Chain transfer creates short alkyl branches every 20 to 50 carbons. Branches prevent close packing; density 0.91 to 0.93 g/cm^3; crystallinity around 40 to 50 percent. Flexible, transparent, low softening point (about 105 to 115 degrees C). Used for plastic bags and squeeze bottles.

HDPE (high-density polyethene). Made by low-pressure Ziegler-Natta catalysis. Chains are essentially linear (few branches). Tight packing; density 0.94 to 0.97 g/cm^3; crystallinity around 60 to 80 percent. Stiff, opaque, higher softening point (about 130 to 135 degrees C). Used for milk crates, rigid pipes, fuel tanks.

The LDPE/HDPE comparison is the canonical QCAA structure-property question and recurs every two or three years.

Crystallinity

Polymer chains can adopt two arrangements:

• Amorphous regions. Chains are tangled and randomly oriented.
• Crystalline regions. Segments of adjacent chains align parallel and pack into ordered, denser regions.

A polymer is described as semi-crystalline if it has both. Higher crystallinity means more chain-chain contact, stronger total dispersion / dipole-dipole interaction, higher density, and higher melting / softening temperature.

Crystallinity rises when:

• Chains are linear (no branching).
• Substituents are small and symmetric.
• Stereoregularity is high (e.g. isotactic polypropene where all methyls are on the same side of the chain).

Crystallinity falls when:

• Chains are heavily branched.
• Substituents are bulky and irregularly placed (atactic polypropene).

Thermoplastic vs thermosetting (preview)

All five addition polymers in the QCAA list are thermoplastic: they soften on heating and can be reshaped repeatedly, because chains are not covalently cross-linked. Thermosetting polymers (Bakelite, epoxy resins) are condensation polymers with permanent cross-links and are discussed in the separate condensation-polymers dot point.

Property predictions for product design

Given a target application, identify the property that matters and choose the polymer:

QCAA IA3 research investigations often present an applied context (food packaging, medical devices, sportswear) and ask students to choose and justify a polymer in structural terms. Memorising the table is not enough; the markers want the bond-and-IMF reasoning.

Drawing polymers correctly

Repeat unit conventions

Bracketed unit, with bonds extending outside the bracket on both sides; subscript n outside the bracket; substituents drawn on the correct carbon. For PVC, the chlorine is on the same carbon as in the monomer; for polystyrene, the phenyl group is on the same carbon as in styrene.

Monomer to polymer conversion

Replace the C=C with two C-C single bonds, one going to the previous unit and one to the next. The 2H on the CH2 end stay; the substituents on the CHR end stay.

Polymer to monomer conversion

Identify a single repeat unit (everything between consecutive identical patterns), insert a double bond where the chain crosses out of the unit, balance hydrogens. A single repeat unit contains exactly two carbons for the listed Unit 4 polymers.

Common traps

Drawing the C=C in the repeat unit

The double bond is consumed in polymerisation; the repeat unit has only single bonds.

Forgetting the brackets and n subscript

A repeat unit without brackets is just a fragment. QCAA marks for both.

Mixing up LDPE and HDPE

LDPE is the branched, low-crystallinity one; HDPE is the linear, high-crystallinity one. The "L" and "H" refer to density, which tracks crystallinity.

Assuming all polymers are thermoplastic

All addition polymers in this list are. But condensation polymers can be either thermoplastic (nylon, PET) or thermosetting (Bakelite, epoxy).

Treating "polymer" and "plastic" as synonyms

Many polymers are not plastic (rubber, cellulose, proteins). The IA3 distinction matters when the question is about natural vs synthetic biomolecules.

Examples in context

Example 1. LDPE film at Logan packaging plant. Visy's Logan converting facility extrudes low-density polyethylene film from ethene monomer ($\text{CH}_2 = \text{CH}_2$) produced at Botany petrochemicals. Free-radical initiation breaks the $\text{C} = \text{C}$ double bond; propagation links monomers into chains of $10^4$ repeat units, $\text{(-CH}_2 \text{-CH}_2\text{-)}_n$. The high-pressure free-radical process favours branched chains, lowering crystallinity to about $50\%$ and giving LDPE its characteristic flexibility, low melting point of $115^{\circ}\text{C}$ and translucency. Branches prevent close packing of chains, so dispersion forces are weaker than the linear high-density variant used in pipes.

Example 2. PVC pipe in Gladstone water infrastructure. Gladstone Regional Council uses uPVC for cold-water reticulation, made by polymerising chloroethene ($\text{CH}_2 = \text{CHCl}$). The polar $\text{C-Cl}$ bond gives strong dipole-dipole forces between chains, lifting glass-transition temperature to $80^{\circ}\text{C}$ and increasing rigidity. Addition polymerisation here is the same radical mechanism as LDPE, but the chlorine atom on every second carbon creates a head-to-tail pattern with substantial polarity. Plasticised PVC (with phthalate additives that slip between chains and weaken interactions) is used for flexible hose; rigid uPVC for pressure mains.

Try this

Q1. Draw the repeating unit of polypropene from propene ($\text{CH}_3 \text{-CH} = \text{CH}_2$). State whether the polymer is saturated. [3 marks]

• Cue. $\text{(-CH}_2\text{-CH(CH}_3\text{)-)}_n$. Saturated: no remaining double bonds.

Q2. Polystyrene and polyethene have similar molar masses, yet polystyrene has a higher glass-transition temperature ($100^{\circ}\text{C}$ vs $-110^{\circ}\text{C}$). Explain. [3 marks]

• Cue. Bulky phenyl side group restricts chain rotation; stronger dispersion forces from larger electron cloud; harder to deform.

Q3. A petrochemical engineer compares HDPE and LDPE. (a) Describe the structural difference. (b) Predict which has higher density and tensile strength. (c) Suggest one application each in Queensland industry. [2+3+2 marks]

• Cue. (a) HDPE linear, LDPE branched. (b) HDPE higher both: tighter packing. (c) HDPE: water pipes; LDPE: cling film. ISMG application.