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Almost every plastic object within arm's reach as you read this — the casing of your pen, the wrapper on your snack, the insulation on your phone charger, the bottle of water on your desk — is an addition polymer. Addition polymers are long-chain molecules built from small alkene monomers by repeated 1,2-addition across the C=C double bond, with no atoms gained or lost in the process. This lesson develops the chemistry of how monomers become polymers, how to identify a polymer's repeat unit from its structure, and how to draw a polymer given a monomer (and vice versa). We will tour the six addition polymers AQA expects you to recognise — polyethene, polypropene, PVC, polystyrene, PTFE and Perspex — and then build the bridge from molecular structure to bulk properties such as melting point, flexibility, crystallinity and chemical resistance. Finally we will evaluate the environmental footprint of these materials and the three main routes for end-of-life disposal: landfill, incineration, and recycling.
Spec mapping (AQA 7405): This lesson is the principal anchor for §3.3.12 (polymers — addition polymerisation only at AS-level; condensation polymers are §3.3.13 in the second-year organic-advanced course). It draws directly on lesson 4 of this course (alkenes — the monomer source) and on lesson 8 (the electrophilic-addition mechanism that initiates polymerisation). Forward links: §3.3.7 (organic synthesis with chiral catalysis — a natural extension to stereospecific polymerisation), §3.3.13 (condensation polymers — polyesters and polyamides), and §3.3.15 (organic analysis — characterising polymer structure by IR and NMR). Refer to the official AQA specification document for the exact wording of each section.
Assessment objectives: AO1 items include defining addition polymerisation, recalling the structures and uses of the six named polymers above, and identifying the monomer that gives rise to a given polymer (or vice versa). AO2 questions test the ability to draw a polymer given its monomer, draw the repeat unit, and calculate the molecular formula of n monomers polymerised together. AO3 questions reward students who can rationalise property differences from structural features (e.g. LDPE versus HDPE, isotactic versus atactic polypropene), evaluate competing disposal routes against environmental criteria, and link branching, side-group size and tacticity to bulk mechanical properties.
A polymer is a long-chain molecule built by joining many small molecules, called monomers, together. Addition polymerisation is the specific process by which alkene monomers — molecules containing a C=C double bond — combine to form a long saturated chain in which every atom of every monomer ends up in the polymer. There is no by-product; nothing is lost. Contrast this with condensation polymerisation (covered in §3.3.13 in the second-year course), in which each linkage step expels a small molecule, typically water or HCl. The atom economy of addition polymerisation is therefore exactly 100% — a useful synoptic link back to lesson 7 of the foundational organic course (yields and atom economy).
The general scheme for addition polymerisation of an unsymmetrical alkene CH₂=CHR is:
n CH₂=CHR → (−CH₂−CHR−)ₙ
The subscript n is a large integer — typically 10³ to 10⁵ — representing the degree of polymerisation. The repeating fragment inside the brackets is the repeat unit, and the brackets carry two free bonds (one at each end) signalling that the structure continues on either side. The C=C double bond of the monomer has been replaced by two new single C−C bonds, one to each neighbouring monomer.
Key Point: Addition polymerisation requires a C=C double bond in the monomer. Saturated molecules (alkanes, alcohols, carboxylic acids without C=C) cannot undergo addition polymerisation. They polymerise — if at all — by condensation, ring-opening or other mechanisms.
The mechanism in industry is usually free-radical addition initiated by trace oxygen, peroxides or other radical sources, but A-Level does not require the radical mechanism for polymers themselves. What you do need is the product: a long-chain saturated polymer with the repeat unit clearly identifiable.
Examiners frequently give a section of polymer chain and ask you to circle or draw the repeat unit. The strategy is:
For example, given a section of chain:
−CH₂−CHCl−CH₂−CHCl−CH₂−CHCl−
The pattern −CH₂−CHCl− recurs. The repeat unit is therefore −(CH₂−CHCl)− and the full polymer is written:
−(CH₂−CHCl)ₙ− or equivalently [−CH₂−CHCl−]ₙ
This is polychloroethene, commonly known as PVC. The monomer can be reconstructed by re-introducing the C=C: CH₂=CHCl (chloroethene, also called vinyl chloride).
Exam Tip: When asked to identify the monomer from a repeat unit, take the two backbone carbons in the repeat unit and re-introduce a double bond between them. Whatever substituents are present in the repeat unit stay where they are on those two carbons.
Given the monomer propene (CH₂=CHCH₃), draw the repeat unit of the resulting polymer.
Step 1: Identify the C=C. It is between C1 and C2.
Step 2: Open the C=C into a single bond. The C1 carries two H atoms (was CH₂), the C2 carries one H and one CH₃ group.
Step 3: Draw the two-carbon backbone fragment with the two free bonds and the substituents in place:
−(CH₂−CH(CH₃))ₙ−
This is polypropene. Note the convention: the smaller group (H) is shown on the first carbon, the larger group (CH₃) on the second; this is purely a drawing convention.
If 5000 molecules of styrene (C₆H₅−CH=CH₂, Mᵣ = 104.15) polymerise to form one polystyrene chain, what is the molecular formula and approximate molar mass of the chain?
This is a typical polymer molar mass; commercial polystyrene ranges from ~10⁵ to ~10⁶ g mol⁻¹ depending on the manufacturing conditions.
AQA specifies six addition polymers by name. The table below lists each one with its monomer, repeat unit, and principal applications.
| Polymer | Monomer | Repeat unit | Key uses |
|---|---|---|---|
| Polyethene (PE) | ethene CH₂=CH₂ | −(CH₂−CH₂)ₙ− | bags, bottles, pipes, sheeting |
| Polypropene (PP) | propene CH₂=CHCH₃ | −(CH₂−CH(CH₃))ₙ− | rope, carpet fibre, food containers |
| Polychloroethene (PVC) | chloroethene CH₂=CHCl | −(CH₂−CHCl)ₙ− | pipes, window frames, cable insulation |
| Polyphenylethene (PS) | phenylethene CH₂=CH(C₆H₅) | −(CH₂−CH(C₆H₅))ₙ− | packaging, disposable cups, insulation foam |
| PTFE (Teflon) | tetrafluoroethene CF₂=CF₂ | −(CF₂−CF₂)ₙ− | non-stick coatings, gaskets, plumbing tape |
| Perspex (PMMA) | methyl 2-methylpropenoate CH₂=C(CH₃)COOCH₃ | −(CH₂−C(CH₃)(COOCH₃))ₙ− | transparent sheet, aircraft canopies, optical lenses |
We now examine each polymer in more depth, drawing out the structural features that determine its properties.
Polyethene is the most-produced plastic in the world by mass — approximately 100 million tonnes per year globally. It is formed by addition polymerisation of ethene (C₂H₄), itself produced by cracking long-chain alkanes from crude oil (lesson 3 of this course). Despite being made from the simplest possible monomer, polyethene exists in two commercially important forms with strikingly different properties:
Low-density polyethene (LDPE) is produced at high pressure (~200 MPa) and high temperature (~300 °C) using oxygen or peroxide initiators. The chains are extensively branched because the free-radical chain-transfer steps create new branches. Branching prevents the chains from packing tightly, so LDPE has a low density (~0.92 g cm⁻³), a low melting point (~110 °C), and is soft and flexible. It is the polyethene of supermarket carrier bags, sandwich bags, cling film and squeezy bottles.
High-density polyethene (HDPE) is produced at low pressure (~3 MPa) and moderate temperature (~80 °C) using a Ziegler-Natta catalyst (typically TiCl₄ with an aluminium alkyl co-catalyst). This catalyst produces linear, unbranched chains that can pack tightly together. HDPE has a higher density (~0.95 g cm⁻³), a higher melting point (~135 °C), and is hard and rigid. It is the polyethene of milk bottles, drainpipes, washing-up bowls and chemical drums.
The contrast is a textbook example of how chain architecture alone — without any change in chemical composition — determines bulk properties. Both LDPE and HDPE have the same repeat unit −(CH₂−CH₂)ₙ− and the same elemental composition. They differ only in branching.
Polypropene is made from propene by addition polymerisation using a Ziegler-Natta or metallocene catalyst (developed by Karl Ziegler and Giulio Natta in the 1950s — see Going Further). The repeat unit −(CH₂−CH(CH₃))ₙ− carries a methyl side group on every other backbone carbon. The orientation of these methyl groups along the chain is called tacticity, and three forms exist:
Tacticity is a second example, alongside branching, of how the spatial arrangement of atoms (not their identity) determines macroscopic behaviour.
PVC is made from chloroethene (vinyl chloride, CH₂=CHCl). The chlorine atom is electronegative and polarises the polymer chain; PVC therefore has strong dipole-dipole intermolecular forces between chains, in addition to weak van der Waals forces. As-produced PVC is rigid — used for window frames, drainage pipes, vinyl flooring and credit cards.
A small quantity of a low-molar-mass additive called a plasticiser can be incorporated to make PVC flexible. The most familiar plasticisers are phthalate esters such as bis(2-ethylhexyl) phthalate (DEHP). Plasticiser molecules wedge themselves between the polymer chains, increasing the separation and weakening the chain-chain dipole-dipole forces. The polymer becomes softer and more flexible. Plasticised PVC is the material of cable insulation, garden hose, inflatable toys, hospital blood-bag tubing and waterproof clothing.
PVC contains chlorine, which is both a property advantage (Cl confers a degree of intrinsic fire-retardance — PVC is harder to ignite than polyethene) and an environmental problem (burning PVC releases HCl and, under poor combustion conditions, traces of chlorinated dioxins; see disposal below).
Polystyrene is made from phenylethene (styrene, C₆H₅−CH=CH₂). The bulky phenyl side group sits on every other backbone carbon. The aromatic ring prevents close packing of chains, so polystyrene is amorphous and brittle at room temperature. It has a glass-transition temperature of ~100 °C — above this it softens dramatically.
Two commercial forms dominate:
PTFE is made from tetrafluoroethene (CF₂=CF₂). Every C−H bond of polyethene is replaced by a C−F bond, and this single substitution transforms the polymer's behaviour. C−F bonds are very strong (~485 kJ mol⁻¹, compared to ~413 kJ mol⁻¹ for C−H), and the fluorine atoms sheath the carbon backbone almost completely. The result is:
PTFE is used for non-stick cookware coatings, plumbing tape, gaskets in chemical plant, surgical implants and high-frequency cable insulation. The discovery is attributed to Roy Plunkett at DuPont in 1938.
PMMA is made from methyl 2-methylpropenoate (methyl methacrylate, CH₂=C(CH₃)COOCH₃). The monomer carries both a methyl group and an ester group on the same carbon. The repeat unit is therefore −(CH₂−C(CH₃)(COOCH₃))ₙ−, with a quaternary carbon on every other position. PMMA is amorphous, transparent (transmitting ~92% of visible light, slightly better than glass), and resistant to UV degradation. It is sold under the trade names Perspex, Plexiglas and Lucite. Applications include aircraft canopies, museum display cases, optical lenses and dental fillings.
The six polymers above all share a saturated carbon backbone, yet their properties span a vast range. Three structural variables explain most of the differences:
Branched chains cannot pack closely. They form amorphous, low-density solids with low melting points and high flexibility. Unbranched (linear) chains pack tightly into semi-crystalline regions with higher density, higher melting point and higher strength. LDPE versus HDPE is the canonical example.
For polymers with asymmetric side groups, the spatial arrangement along the chain (isotactic, syndiotactic, atactic) determines whether the chains can crystallise. Isotactic chains pack into ordered crystalline regions; atactic chains remain amorphous. Polypropene is the classic illustration.
A fourth, more advanced, variable is molar-mass distribution: a polymer with a narrow distribution of chain lengths has more uniform mechanical properties than one with a broad distribution. The polydispersity index (Mw/Mn) quantifies this and is measured by gel permeation chromatography (see Going Further).
Plasticisers are small molecules added to a polymer to reduce inter-chain forces. They embed themselves between the long chains, increasing the distance between chains and reducing the dipole-dipole or van der Waals attraction. The polymer becomes softer, more flexible, and easier to mould. The classic case is flexible PVC, where typically 20-40% by mass of DEHP or a similar phthalate ester is incorporated. Plasticisers do not chemically react with the polymer; they simply act as a molecular lubricant between chains. Over time, plasticiser molecules can leach out of the polymer (into food, blood, or the environment), which is why phthalate-free alternatives are now preferred for medical devices and children's toys.
Addition polymers are non-biodegradable under normal environmental conditions. The C−C and C−H bonds of the saturated polymer backbone are resistant to microbial attack, and the chains are too large to diffuse into cells. A discarded polyethene bag will remain intact for hundreds of years in landfill. This single fact drives the entire end-of-life problem for addition polymers.
Four disposal routes are commonly considered.
Burying polymer waste is cheap but consumes land. The polymers persist for centuries and slowly release plasticiser or other additives into surrounding soil and groundwater. Modern landfills use impermeable liners and leachate collection, but the underlying problem — that the polymer is essentially permanent — is not solved, only contained.
Burning polymer waste recovers energy (typical calorific values are ~40 MJ kg⁻¹ for polyethene, comparable to fuel oil) and reduces volume by ~90%. The CO₂ and H₂O produced from polyethene combustion are no worse than the equivalent fossil-fuel burning. However:
Modern incinerators run at ~1000 °C with excess oxygen and multi-stage flue-gas scrubbing, addressing most of these issues, but the capital cost is high.
The polymer is collected, sorted, washed, chipped, melted and re-extruded into new product. This works well for clean, single-polymer streams such as PET bottles or HDPE milk containers. Limitations:
Higher-energy routes break the polymer back down to monomer or to low-molar-mass hydrocarbons that can be re-used as feedstock for new polymers or fuels. Methods include:
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