Chemistry • Year 12 • Module 7 • Lesson 21
Addition Polymers
Apply polymer structure and properties to data tables, real Australian industrial contexts, and multi-step reasoning about intermolecular forces.
1. Interpret polymer properties data
The table below lists five common addition polymers with selected physical properties. Study the data, then answer the questions. 9 marks
| Polymer (abbreviation) | Monomer | Repeat unit | Density (g cm−3) | Melting point (°C) | Key application (Australia) |
|---|---|---|---|---|---|
| Polyethylene — LDPE | Ethene (CH2=CH2) | (-CH2-CH2-)n | 0.91–0.93 | ~110 | Plastic bags, cling film (flexible packaging) |
| Polyethylene — HDPE | Ethene (CH2=CH2) | (-CH2-CH2-)n | 0.94–0.97 | ~130 | Milk crates, piping, detergent bottles (Qenos, Altona VIC) |
| Polypropylene (PP) | Propene (CH2=CHCH3) | (-CH2-CH(CH3)-)n | 0.90–0.91 | ~165 | Food containers, car parts, rope |
| Poly(vinyl chloride) — PVC | Chloroethene (CH2=CHCl) | (-CH2-CHCl-)n | 1.30–1.45 | ~80 (softens) | Plumbing pipes, electrical cable insulation (Vinyl Council of Australia) |
| Poly(tetrafluoroethylene) — PTFE | Tetrafluoroethene (CF2=CF2) | (-CF2-CF2-)n | 2.14–2.20 | ~327 | Non-stick cookware coatings, plumber’s tape (Australian consumer goods) |
Data sources: Polymer Properties Database; Vinyl Council of Australia. Qenos Pty Ltd (Altona, VIC) is Australia’s largest polyethylene producer.
1.1 LDPE and HDPE have the same repeat unit but different densities and melting points. Using IMF reasoning, explain why HDPE has both a higher density and a higher melting point than LDPE. 3 marks
1.2 PTFE has by far the highest density (2.14–2.20 g cm−3) of the five polymers listed. Explain this high density using the structure of the PTFE repeat unit. 2 marks
1.3 PP has the lowest density of the five polymers but a higher melting point than HDPE. Account for this seemingly unexpected result. 2 marks
1.4 From the table, identify which polymer is most suitable for use as underground drinking water pipes in Australia and justify your choice using two data points and one structural reason. 2 marks
2. Interpret graph — degree of polymerisation and molecular mass
The graph below shows the relationship between degree of polymerisation (n) and molecular mass (g mol−1) for polyethylene, polypropylene, and PVC. 7 marks
Calculated from Mpolymer = n × Mrepeat unit. PE = 28 g mol−1; PP = 42 g mol−1; PVC = 62.5 g mol−1 per repeat unit. Data are theoretical.
2.1 Which polymer reaches a molecular mass of 500 000 g mol−1 at the lowest value of n? Read the value of n from the graph and explain why this polymer reaches this mass at a lower n than the others. 2 marks
2.2 A sample of HDPE manufactured by Qenos at their Altona, VIC plant has a molecular mass of 280 000 g mol−1. Use the graph (or calculation) to determine the degree of polymerisation n for this sample. Show your working. 2 marks
2.3 Predict the slope of a line for PTFE (repeat unit mass 100 g mol−1) if it were added to this graph. Would it be steeper or shallower than the PVC line? Justify your prediction. 2 marks
2.4 Explain why polymer samples of the same substance can have a range of molecular masses rather than a single exact value. 1 mark
3. Compare addition and condensation polymerisation
Complete the two-column comparison table. Leave a cell blank only if you genuinely cannot determine the entry from lesson content. 8 marks
| Feature | Addition polymerisation | Condensation polymerisation (preview) |
|---|---|---|
| Functional group required in monomer | Two complementary functional groups (e.g. —OH + —COOH) | |
| By-product produced? | Yes — small molecule (e.g. H2O or HCl) | |
| Empirical formula of polymer vs monomer | Different — atoms lost in by-product | |
| Example polymer | Nylon-6,6; PET polyester | |
| Biodegradability of backbone | Ester/amide bonds can be hydrolysed by microbial enzymes | |
| Chain architecture can be branched? | Yes, but less common | |
| Australian industry example | PET beverage bottles | |
| Recyclable by melting? | Depends on type (thermoplastic polyesters: yes; thermoset epoxies: no) |
4. Case study — Qenos and Australian polyethylene
5 marks
Stimulus. Qenos Pty Ltd operates Australia’s only polyethylene manufacturing plant at Altona, Victoria, making it the nation’s largest producer of this polymer. The plant produces both LDPE and HDPE grades from the same ethene monomer feedstock, using different catalyst systems and reactor conditions. LDPE is produced at high pressure (~1500 atm) using free-radical initiators (organic peroxides), yielding a branched product used in flexible packaging. HDPE is produced at lower pressure using a Ziegler–Natta catalyst, yielding a linear product used in rigid piping and storage containers. Annual Australian consumption of polyethylene exceeds 600 000 tonnes.
4.1 Both LDPE and HDPE are produced from ethene at Qenos. Write the equation for the addition polymerisation of ethene using correct repeat-unit notation. 1 mark
4.2 Using structural chemistry, explain why the same monomer produces two polymers with different physical properties at Qenos. 2 marks
4.3 Predict one environmental consequence of Australia consuming 600 000+ tonnes of polyethylene per year, using specific chemical reasoning rather than a general statement. 2 marks
Q1.1 — LDPE vs HDPE density and melting point (3 marks)
Both LDPE and HDPE have the same repeat unit and therefore identical per-unit chemistry. The difference is chain architecture: HDPE chains are linear and can align parallel, maximising the surface contact between chains. This maximises cumulative London (dispersion) forces [1]. Greater dispersion forces means more energy is required to separate chains — hence a higher melting point (~130°C vs ~110°C for LDPE) [1]. Closer chain packing also puts more mass per unit volume — higher density (0.94–0.97 vs 0.91–0.93 g cm−3) [1]. LDPE branches prevent close alignment, so both properties are lower.
Q1.2 — PTFE high density (2 marks)
The PTFE repeat unit (-CF2-CF2-)n replaces all four hydrogen atoms (M = 1) with fluorine atoms (M = 19). Fluorine is a much heavier atom than hydrogen, so each repeat unit has a much larger mass than the corresponding polyethylene unit (100 g mol−1 vs 28 g mol−1) [1]. PTFE chains also pack efficiently because fluorine atoms are only slightly larger than hydrogen, so chain–chain spacing is not significantly increased while mass per unit is greatly increased — producing a very high density [1].
Q1.3 — PP: lowest density but higher melting point than HDPE (2 marks)
The CH3 side groups on every second backbone carbon of PP prevent the chains from packing as closely as linear HDPE chains — hence the lower density [1]. However, the methyl groups also increase the mass per repeat unit (42 vs 28 g mol−1 for PE) and the larger surface area of each chain segment generates stronger London dispersion forces between adjacent chains overall, producing a higher melting point (~165°C). Accept also: the isotactic arrangement of methyl groups in commercial PP creates some crystalline regions that require more thermal energy to disrupt [1].
Q1.4 — Most suitable polymer for underground water pipes (2 marks)
Most suitable: HDPE. Data points: density 0.94–0.97 g cm−3 (rigid, structurally strong) and melting point ~130°C (well above typical soil temperatures, will not soften). Structural reason: linear chains provide mechanical rigidity and resistance to deformation under soil pressure; the C–C backbone does not react with water [1+1]. Accept PVC with valid justification (high density, used in Australian plumbing as noted in the table).
Q2.1 — Lowest n for 500 000 g mol−1 (2 marks)
PVC reaches 500 000 g mol−1 at the lowest n (approximately n = 8000 from the graph) [1]. PVC has the largest repeat unit mass (62.5 g mol−1) of the three polymers — so each additional unit contributes more mass, and fewer repetitions are needed to accumulate 500 000 g mol−1 [1]. Calculation check: 500 000 / 62.5 = 8000. ✓
Q2.2 — Degree of polymerisation for HDPE sample (2 marks)
n = Mpolymer / Mrepeat unit = 280 000 / 28 = 10 000 [1 for working, 1 for correct answer]. This can also be read from the graph: the PE line reaches 280 000 at n = 10 000 (right edge of graph). Unit: dimensionless (degree of polymerisation is a pure number).
Q2.3 — PTFE slope prediction (2 marks)
Steeper than PVC. PTFE’s repeat unit mass is 100 g mol−1 (vs 62.5 for PVC), so each increment in n adds 100 g mol−1 to molecular mass — a steeper slope [1]. The PTFE line would reach 500 000 g mol−1 at only n = 5000, compared with n = 8000 for PVC [1].
Q2.4 — Range of molecular masses (1 mark)
In any polymerisation batch, individual chains terminate at different times during the propagation step — some chains are short (terminate early) and some are long (propagate for many more steps before termination). The result is a distribution of chain lengths and therefore a distribution of molecular masses rather than a single exact value. The average molecular mass is reported.
Q3 — Comparison table
Functional group in monomer: C=C double bond. By-product: No by-product — all atoms from all monomers incorporated into polymer. Empirical formula of polymer vs monomer: Same — no atoms lost. Example polymer: Polyethylene, PVC, PTFE, PP, polystyrene (any valid addition polymer). Biodegradability: C–C backbone is not hydrolysable by microbial enzymes — persists hundreds to thousands of years. Chain branching: Yes — LDPE is a major example of a branched addition polymer. Australian industry example: Qenos Altona (LDPE/HDPE); PVC plumbing pipes (Vinyl Council of Australia); PTFE non-stick cookware. Recyclable by melting: Yes — most addition polymers (PE, PP, PVC, PS) are thermoplastics and can be re-melted and remoulded.
Q4.1 — Polymerisation equation (1 mark)
n CH2=CH2 → (-CH2-CH2-)n [1 mark — must have n on left, correct repeat unit notation with square brackets and subscript n on right].
Q4.2 — Same monomer, different properties (2 marks)
At Qenos, high-pressure free-radical conditions cause branching approximately every 50 backbone carbons in LDPE, because the chain radical can abstract a hydrogen from its own chain and continue growing from that point [1]. Ziegler–Natta catalyst at lower pressure produces linear, unbranched HDPE chains because the active site geometry allows only sequential head-to-tail monomer insertion [1]. Branched chains (LDPE) pack poorly → weaker dispersion forces → flexible, lower density/melting point. Linear chains (HDPE) pack closely → stronger dispersion forces → rigid, higher density/melting point.
Q4.3 — Environmental consequence (2 marks)
The C–C backbone of polyethylene contains no hydrolysable linkages (no ester, amide, or glycosidic bonds), so microbial hydrolase enzymes cannot break it down [1]. 600 000+ tonnes of polyethylene per year that is not recycled accumulates in landfill or the environment, potentially for hundreds to thousands of years. UV radiation and mechanical abrasion break it into microplastics (<5 mm) that enter waterways and food chains — detected in marine organisms and human blood [1].