HSC Exam Practice

Sample response. Condensation polymerisation is a reaction in which bifunctional monomers react repeatedly, with loss of a small molecule (usually water, or HCl for acid-chloride routes) at each bond formed, producing a polymer chain. Requirements: (1) the monomer must be bifunctional — it must carry two reactive groups, one at each end, so the chain can grow from both ends; (2) those reactive groups must be capable of reacting with each other (e.g. -OH reacting with -COOH, or -NH₂ reacting with -COOH).

Marking notes. 1 mark for defining condensation polymerisation as involving loss of a small molecule at each bond formed; 1 mark for identifying the bifunctionality requirement (two reactive groups per monomer); 1 mark for stating that the two groups must be capable of reacting with each other to form a covalent bond (e.g. -OH + -COOH).

Sample response. Monomer 1: ethylene glycol (ethane-1,2-diol, HOCH₂CH₂OH) — carries two hydroxyl groups (-OH), one at each end. Monomer 2: terephthalic acid (benzene-1,4-dicarboxylic acid, HOOC-C&sub6;H&sub4;-COOH) — carries two carboxylic acid groups (-COOH), one at each end. The -OH group from ethylene glycol reacts with the -COOH group from terephthalic acid, forming an ester linkage. By-product: water (H₂O); 2 mol produced per repeat unit.

Marking notes. 1 mark for correctly naming (or giving the formula of) both monomers; 1 mark for identifying the functional groups (-OH on diol, -COOH on diacid); 1 mark for stating water as the by-product (accept “2 mol H₂O per repeat unit” for full credit).

Sample response. Step 1: Identify the linkage — the -CO-NH- group is an amide linkage, so this is a polyamide. [1] Step 2: Locate each amide bond and insert H₂O across it: add H to the N end → -NH₂; add OH to the C=O end → -COOH. [1] Step 3: Fragment 1 — H₂N-(CH₂)&sub6;-NH₂ = hexane-1,6-diamine (six carbons, -NH₂ at each end). [1] Fragment 2 — HOOC-(CH₂)&sub4;-COOH = hexanedioic acid (six carbons total including two carbonyl carbons, -COOH at each end). [1] This is Nylon 6,6.

Marking notes. 1 mark for identifying amide linkage and classifying as polyamide; 1 mark for correctly describing the H₂O insertion rule (H to N, OH to C=O); 1 mark for recovering hexane-1,6-diamine (correct name or formula); 1 mark for recovering hexanedioic acid (correct name or formula, including noting six carbons total).

Sample response. Addition polymerisation requires monomers with a C=C double bond (pi bond); the pi bond opens and monomers add together — no by-product is produced and the polymer has the same empirical formula as the monomer; the linkage is C-C throughout. Condensation polymerisation requires bifunctional monomers each carrying two reactive end-groups (e.g. -OH and -COOH, or -NH₂ and -COOH); the groups react with loss of a small molecule (H₂O or HCl) at each step; the linkage formed is ester (-COO-) or amide (-CO-NH-).

Marking notes. 1 mark for monomer functional group requirement (C=C for addition vs two reactive end-groups for condensation); 1 mark for by-product (none for addition; H₂O or HCl for condensation); 1 mark for linkage type (C-C for addition vs ester/amide for condensation).

Sample response. Within each Nylon 6,6 chain, the repeat unit is held together by strong covalent amide bonds (-CO-NH-), which are not easily broken at normal temperatures [1]. Between adjacent chains, the N-H group of one amide unit forms hydrogen bonds (N-H···O=C) with the C=O group of an adjacent chain’s amide unit [1]. These interchain hydrogen bonds are extensive (every amide unit can hydrogen bond) and require significant thermal energy to break; the collective strength of the network raises the melting point to ~265 °C [1].

Marking notes. 1 mark for strong covalent amide bonds within chains; 1 mark for identifying N-H···O=C hydrogen bonding between chains; 1 mark for explaining that the extensive H-bond network requires high energy to disrupt, producing a high melting point.

Sample response. Chemical recycling of PET works by hydrolysis: water (or methanol in methanolysis) is added across each ester linkage (-COO-) in the chain. The ester C-O bond is broken by nucleophilic attack of water on the electrophilic carbonyl carbon, regenerating the two original monomers: ethylene glycol (HOCH₂CH₂OH) and terephthalic acid (HOOC-C&sub6;H&sub4;-COOH). These can be purified and repolymerised.

Marking notes. 1 mark for naming hydrolysis and identifying ester bonds as the bonds broken; 1 mark for naming both monomers regenerated (ethylene glycol and terephthalic acid, or equivalent formulas).

Sample response (a). Nylon 6,6 shows the greatest mass loss, increasing consistently from 0% at week 0 to approximately 18% at week 8, with a roughly linear increase after week 2. PET shows a moderate and gradual mass loss, reaching approximately 8% at week 8; the rate appears to slow slightly in the later weeks. LDPE shows negligible mass loss throughout all 8 weeks, remaining essentially at 0% across the entire period.

Marking notes (a). 1 mark for correctly describing Nylon’s trend (consistently increasing, greatest loss) with a value; 1 mark for correctly describing PET’s trend (moderate, gradual) with a value; 1 mark for correctly describing LDPE as essentially unchanged / near zero.

Sample response (b). Both Nylon 6,6 and PET contain hydrolysable linkages, but their susceptibility differs. Nylon’s amide bonds (-CO-NH-) are hydrolysed more readily than PET’s ester bonds (-COO-) under acidic conditions because: (i) the N-H proton is more easily protonated under acid conditions, activating the amide carbonyl toward nucleophilic attack; (ii) the aliphatic, flexible backbone of Nylon 6,6 is more accessible to water molecules than the semi-crystalline, aromatic PET structure, in which the rigid benzene rings create hydrophobic surfaces that resist water penetration. Hence Nylon loses mass faster (18% vs 8% at week 8).

Marking notes (b). 1 mark for identifying both as hydrolysable via their respective linkage types; 1 mark for explaining Nylon is more susceptible (amide bonds + aliphatic flexibility / accessibility); 1 mark for explaining PET is less susceptible (ester bonds less reactive under acid + benzene ring creates crystalline barrier).

Sample response (c). LDPE contains only C-C and C-H bonds throughout its backbone — it has no ester, amide, or other heteroatom-containing functional group. Dilute H₂SO&sub4; (and water) cannot attack C-C bonds because there is no electrophilic carbonyl or other susceptible site for nucleophilic attack. Without a hydrolysable linkage, mass loss cannot occur via chemical degradation under these conditions. Only UV photodegradation could degrade LDPE, and that mechanism is absent in a closed acidic bath.

Marking notes (c). 1 mark for identifying that LDPE has only C-C/C-H bonds with no hydrolysable functional group; 1 mark for explaining that acid/water cannot attack C-C bonds (no electrophilic site) — hence no mass loss.

Sample response. The claim that condensation polymers pose a “different and less tractable” environmental persistence problem than addition polymers is partially valid but overstated. A nuanced structural analysis is required to reach a defensible judgement.

Addition polymers such as polyethylene contain only C-C and C-H bonds in their backbones. These bonds are not hydrolysable by water, acid, base, or most enzymes. The sole degradation pathway in the environment is UV-induced photolysis and mechanical fragmentation into microplastics, which does not mineralise the carbon. Once in marine or soil environments, addition polymer fragments can persist for centuries. This makes addition polymers a severe and largely intractable environmental persistence problem.

Condensation polymers such as PET (polyester) and Nylon 6,6 (polyamide) contain ester (-COO-) and amide (-CO-NH-) linkages respectively. Both are susceptible in principle to hydrolysis: the ester C-O bond and the amide C-N bond can be cleaved by water under acidic or alkaline conditions, or by specific microbial enzymes. This means a chemical degradation pathway exists that does not exist for addition polymers. The discovery of PETase-producing bacteria (Ideonella sakaiensis, 2016) confirms biological degradation of PET is chemically possible, and CSIRO research into biodegradable polyesters (PHAs) demonstrates that the ester linkage motif can support complete mineralisation when the polymer backbone is tailored for enzymatic accessibility.

However, in practice, the persistence of PET and Nylon 6,6 in Australian ocean and riverine environments remains very high over human-relevant timescales. At ambient temperature and near-neutral ocean pH, hydrolysis rates are extremely slow, and the crystalline, hydrophobic surfaces of both polymers limit enzyme access. Clean Up Australia data show PET fragments accumulating in beach surveys alongside LDPE fragments — both classes persist for decades under field conditions.

The claim is therefore partially valid in that condensation polymers are structurally “different” from addition polymers (they have hydrolysable linkages), but “less tractable” is incorrect. On the contrary, condensation polymers are actually more tractable than addition polymers in principle, because a chemical degradation pathway exists. The deeper environmental problem — the enormous volume of plastic entering ocean systems — applies to both classes. Neither class should be characterised as “more tractable” in practice until biodegradable alternatives and chemical recycling infrastructure are deployed at sufficient scale.

Marking criteria.

• 1 mark — Identifies addition polymer backbone (C-C only) and explains why it cannot be hydrolysed (no electrophilic carbonyl / heteroatom in backbone).
• 1 mark — Identifies condensation polymer linkages (ester in PET, amide in Nylon) and explains that both are hydrolysable in principle.
• 1 mark — Explains the mechanism of hydrolysis for at least one linkage type (nucleophilic attack on ester/amide C=O; C-O or C-N bond cleaved).
• 1 mark — Acknowledges that in practice, PET and Nylon persist for long periods in ocean/soil environments because hydrolysis is very slow at ambient conditions (semi-crystalline structure, low pH variation, limited enzyme access).
• 1 mark — References at least one named Australian environmental initiative or research program (e.g. Clean Up Australia PET beach data; CSIRO biodegradable polyester research; Australia’s National Plastics Plan; PET kerbside recycling via state programs).
• 1 mark — Identifies a pathway for biological degradation of condensation polymers (PETase bacteria, esterases, proteases) that does not exist for addition polymers.
• 1 mark — Reaches an explicit, justified conclusion that evaluates the claim — agreeing that condensation polymers are structurally different but challenging the claim that they are “less tractable” (they are actually more tractable in principle because a degradation pathway exists).
• 1 mark — Quality throughout: uses precise chemical terminology (hydrolysis, ester, amide, C-C backbone, nucleophilic, electrophilic carbonyl, crystallinity, enzymatic) and avoids unsupported generalisations.