Chemistry • Year 12 • Module 7 • Lesson 22
Condensation Polymers: Polyesters & Polyamides
Apply the mechanism of condensation polymerisation to real data, a comparison table, a monomer-identification task, and an Australian recycling context.
1. Compare addition and condensation polymerisation
Complete the comparison table below. For each feature, write the correct response for addition polymerisation (using polypropylene as the example) and for condensation polymerisation (using PET as the example). Use precise chemical notation where appropriate. 10 marks (1 per row)
| Feature | Addition polymerisation (polypropylene) | Condensation polymerisation (PET) |
|---|---|---|
| Monomer requirement | ||
| Number of monomers needed | ||
| By-product of polymerisation | ||
| Linkage type in backbone | ||
| Monomer formula (or name) | ||
| Polymer empirical formula compared to monomer? | ||
| Hydrolysable? | ||
| Environmental persistence | ||
| Industrial Australian example | ||
| Repeat unit notation (general form) |
2. Interpret recycling data — PET collection in Australia
The table below shows tonnes of PET plastic collected for recycling in Australia across five reporting periods, alongside the percentage converted back to usable material (conversion efficiency). 7 marks
| Year | PET collected (thousand tonnes) | Conversion efficiency (%) | Material recovered (thousand tonnes) |
|---|---|---|---|
| 2018 | 52 | 72 | 37.4 |
| 2019 | 58 | 74 | 42.9 |
| 2020 | 61 | 68 | 41.5 |
| 2021 | 67 | 75 | 50.3 |
| 2022 | 74 | 78 | 57.7 |
Data adapted from APCO (Australian Packaging Covenant Organisation), National Plastics Report 2022–23. Values rounded.
2.1 Describe the overall trend in PET collection from 2018 to 2022. Include figures to support your description. 2 marks
2.2 In 2020, conversion efficiency dropped to 68% despite collection volume increasing. Identify one possible reason for this, in the context of the chemical recycling of PET (hydrolysis or methanolysis to regenerate monomers). 2 marks
2.3 Explain, using the chemistry of PET’s ester linkages, why chemical recycling (regenerating monomers by hydrolysis) is possible for PET but not for polyethylene. 3 marks
3. Sequence the steps — identifying monomers from a polymer
The steps below describe how to identify the monomers from the condensation polymer [-O-(CH₂)&sub4;-O-CO-(CH₂)&sub4;-CO-]n, but they are in the wrong order. Write the correct step number (1–7) in the “Order” column. 7 marks
| Step description (shuffled) | Order |
|---|---|
| Add H to each oxygen end of the broken linkage to recover the diol monomer: HO-(CH₂)&sub4;-OH. | |
| Name the diacid monomer by IUPAC rules: hexanedioic acid (adipic acid), 6 carbons including both carbonyl carbons. | |
| Identify the linkage type by scanning the repeat unit for -COO- (ester) or -CO-NH- (amide). | |
| Name the diol monomer: butane-1,4-diol (4 carbons, -OH at positions 1 and 4). | |
| Add OH to each carbonyl carbon end of the broken linkage to recover the diacid monomer: HOOC-(CH₂)&sub4;-COOH. | |
| Confirm the polymer is a polyester and that the reaction by which it formed would have produced water as a by-product. | |
| Locate each ester (-COO-) bond and mark it as the bond to break; insert H₂O across each marked bond. |
4. Interpret a graph — degree of polymerisation and tensile strength
The graph below is a stylised model showing how tensile strength (MPa) changes with increasing degree of polymerisation (n) for PET polyester fibres. A minimum strength threshold of 200 MPa is needed for the textile industry (dashed line). 6 marks
Figure 4.1. Stylised model: tensile strength of PET fibre vs degree of polymerisation. Adapted from Carothers’ original work on polyester condensation and modern fibre industry data.
4.1 Describe the shape of the tensile strength curve as n increases from 0 to 200. 2 marks
4.2 At approximately what degree of polymerisation does PET fibre first become suitable for textile applications? Justify your answer with reference to the graph. 1 mark
4.3 Explain, at the molecular level, why a higher degree of polymerisation produces a stronger fibre. Your answer should refer to intermolecular forces between PET chains. 3 marks
5. Case study — CSIRO biodegradable polyester research
CSIRO researchers have investigated polyhydroxyalkanoates (PHAs) — a class of polyesters produced naturally by certain soil bacteria in Australia. PHAs are condensation polymers whose repeat unit is [-O-CHR-CH₂-CO-]n, where R is a side chain that varies with the bacterial species. Unlike PET, PHAs are fully biodegradable in soil and marine environments within months, because soil microorganisms and their esterase enzymes can hydrolyse the ester bonds efficiently. Current challenges include high production cost and lower tensile strength compared with PET. 5 marks
Q5. Using lesson content and the stimulus, explain why PHAs are biodegradable while conventional PET is not, and evaluate whether PHAs are a viable replacement for PET in Australian single-use drink bottles. In your response: (i) identify the ester linkage in both polymers and explain why it makes them hydrolysable in principle; (ii) explain why biodegradation is rapid for PHA but slow for PET; (iii) assess one advantage and one limitation of replacing PET with PHA in an Australian context.
Q1 — Comparison table (sample answers)
| Feature | Addition (polypropylene) | Condensation (PET) |
|---|---|---|
| Monomer requirement | C=C double bond per monomer | Two reactive end-groups per monomer (-OH + -COOH) |
| Number of monomers needed | One (propene) | Two (ethylene glycol + terephthalic acid) |
| By-product | None | Water (H₂O), 2 mol per repeat unit |
| Linkage in backbone | C-C (from C=C opening) | Ester (-COO-) |
| Monomer name/formula | Propene (CH&sub3;-CH=CH₂) | Ethylene glycol (HOCH₂CH₂OH) + terephthalic acid (HOOC-C&sub6;H&sub4;-COOH) |
| Empirical formula vs monomer | Same (all atoms retained) | Different (atoms lost in H₂O) |
| Hydrolysable? | No — C-C not cleaved by water | Yes — ester bonds cleaved under acid/base/enzymatic conditions |
| Environmental persistence | Very high; degrades only by UV | High, but ester bonds offer a slow hydrolysis pathway |
| Australian industrial example | Visy packaging, polypropylene bags (PACT Group) | PET bottles collected by Clean Up Australia; ANTA/Rebel Sport polyester sportswear |
| Repeat unit notation | [-CH(CH&sub3;)-CH₂-]n | [-O-CH₂CH₂-O-CO-C&sub6;H&sub4;-CO-]n |
Q2.1 — Trend in PET collection
PET collection increased steadily from 52,000 tonnes in 2018 to 74,000 tonnes in 2022 — an increase of approximately 42% over five years. [1 mark for overall increasing trend; 1 mark for citing specific figures.]
Q2.2 — Drop in conversion efficiency (2020)
Possible reasons include: (a) processing plant disruption (e.g. COVID-19 lockdowns reducing operational capacity); (b) increased contamination of the collected PET stream (other plastics mixed in that cannot be chemically recycled under the same conditions); or (c) if using hydrolysis — reaction conditions optimised less well, leaving incompletely hydrolysed polymer chains that cannot be directly re-used as monomers. Award 1 mark for any chemically sound reason + 1 mark for linking it explicitly to the chemistry (ester hydrolysis requires controlled conditions; contaminants disrupt the process).
Q2.3 — Why PET but not polyethylene can be chemically recycled (3 marks)
PET contains ester linkages (-COO-) [1]. The carbonyl carbon in the ester is electrophilic (electron-deficient) and can be attacked by water (a nucleophile) or by -OH in alkaline solution, cleaving the C-O bond and regenerating the carboxylic acid (-COOH) and alcohol (-OH) ends of the original monomers [1]. Polyethylene contains only C-C and C-H bonds; neither is electrophilic or susceptible to nucleophilic attack by water. There is no functional group present that water or an enzyme can attack to break the chain. Chemical recycling to original monomers is therefore chemically impossible for polyethylene [1].
Q3 — Correct step order
1: Identify the linkage type (scan for -COO- or -CO-NH-). 2: Locate each ester bond and mark it to break; insert H₂O. 3: Add H to each oxygen end → recover diol HO-(CH₂)&sub4;-OH. 4: Add OH to each carbonyl carbon end → recover diacid HOOC-(CH₂)&sub4;-COOH. 5: Name the diol: butane-1,4-diol. 6: Name the diacid: hexanedioic acid. 7: Confirm polymer type is polyester and by-product of formation was H₂O.
Q4.1 — Curve shape description
Tensile strength increases steeply at low degrees of polymerisation, then the rate of increase slows and the curve approaches a plateau (levelling off) at higher n values near 200. [1 mark for identifying steep initial rise; 1 mark for plateau / levelling off at high n.]
Q4.2 — Threshold n value
The 200 MPa textile threshold is first reached at approximately n ≈ 50, as shown by the marked intersection of the curve and the dashed line.
Q4.3 — Molecular explanation of higher strength at higher n (3 marks)
Longer polymer chains (higher n) have more repeat units and therefore a greater surface area over which intermolecular forces act between adjacent chains [1]. In PET, the dominant intermolecular forces are dispersion forces (London forces) and weak dipole-dipole interactions from the ester C=O and C-O dipoles; with longer chains, the cumulative magnitude of these forces is much greater [1]. More energy is therefore required to separate the chains during stretching, which manifests as higher tensile strength. At very high n, chain entanglement also contributes, physically preventing chains from slipping past each other [1].
Q5 — PHA vs PET case study (5 marks)
(i) Ester linkage in both: Both PHA and PET contain ester linkages (-COO-) in their polymer backbone [1]. Both can in principle be hydrolysed because the ester C-O bond is susceptible to nucleophilic attack by water (or OH¹¹), cleaving the chain and regenerating the diol/hydroxy acid and carboxylic acid monomers.
(ii) Why rapid for PHA, slow for PET: PHA has an aliphatic (flexible, non-aromatic) backbone that is accessible to microbial esterase enzymes whose active site is shaped to fit the PHA structure [1]. Soil and marine bacteria evolved to produce these enzymes specifically to break down PHAs as a carbon source. PET’s backbone contains a rigid benzene ring (from terephthalic acid), which creates a hydrophobic, crystalline surface that water and most enzymes cannot penetrate easily; PETase-producing bacteria exist but are rare and the reaction is extremely slow under ambient conditions [1].
(iii) Advantage and limitation for Australia: Advantage — PHA would eliminate persistent plastic pollution in Australian oceans and rivers; it would degrade within months, consistent with CSIRO’s Clean Oceans research goals [1]. Limitation — current production costs for PHA are much higher than PET (fermentation is expensive); PHA also has lower tensile strength, meaning bottles may be less robust for carbonated drinks that require high gas-barrier performance [1].