Polymers.

HookThe stockings that started riots

ModelTwo functional groups each — and something small falls out

Addition polymerisation (section 3.3.4) needs one monomer and one trick: the C=C bond opens and the monomers chain up with nothing left over — poly(ethene) is its own only product, atom economy 100%. Condensation polymerisation runs on a different logic. Each monomer must carry two functional groups, one at each end, so that every new bond leaves a fresh reactive end for the chain to keep growing — and every bond formed expels a small molecule, usually water.

Pair a dicarboxylic acid with a diol and each join is an esterification: the –COOH and –OH condense to an ester linkage, –COO–. The flagship is Terylene, the fibre and bottle plastic PET, made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol: \(n\,\text{HOOC-C}_6\text{H}_4\text{-COOH} + n\,\text{HO-CH}_2\text{CH}_2\text{-OH} \rightarrow [\text{-OC-C}_6\text{H}_4\text{-CO-O-CH}_2\text{CH}_2\text{-O-}]_n + 2n\,\text{H}_2\text{O}\). Two ester linkages form per repeating unit, so 2n water molecules leave the equation.

Pair the dicarboxylic acid with a diamine instead and each join is an amide linkage, –CONH– : that is a polyamide. And the two monomers need not be different molecules at all. An amino acid carries an –NH₂ at one end and a –COOH at the other, so amino acids self-condense into polyamides — which is exactly what a protein is (section 3.3.13). Likewise 2-hydroxypropanoic acid (lactic acid) carries –OH and –COOH and self-condenses to the polyester PLA, a name that will matter when we reach disposal.

One practical variant earns marks: swap the dicarboxylic acid for its diacyl dichloride and the condensation runs faster at room temperature, expelling HCl instead of water — the demonstration where nylon is drawn as a continuous thread from the interface of two immiscible solutions uses exactly this chemistry. Same linkage, different small molecule; check which reagent the question gave you before you write the by-product.

CaseNylon-6,6 and Kevlar — the linkage plus the hydrogen bonds between chains

Carothers' polymer is nylon-6,6, and the name is a recipe: hexane-1,6-diamine (six carbons) condensed with hexanedioic acid (six carbons). The equation is the polyamide template: \(n\,\text{H}_2\text{N}(\text{CH}_2)_6\text{NH}_2 + n\,\text{HOOC}(\text{CH}_2)_4\text{COOH} \rightarrow [\text{-NH}(\text{CH}_2)_6\text{NH-OC}(\text{CH}_2)_4\text{CO-}]_n + 2n\,\text{H}_2\text{O}\). Note the middle of hexanedioic acid contributes only four CH₂ groups — its other two carbons are the carboxyl carbons that end up inside the amide linkages. Copying six CH₂ groups into both halves of the repeat unit is the classic lost drawing mark.

What makes a polyamide worth rioting over is not the chain but what happens between chains. Every amide linkage carries an N–H and a C=O; the δ+ hydrogen on one chain's nitrogen hydrogen-bonds to the carbonyl oxygen of its neighbour. Stretch nylon as it solidifies and the chains align, locking a ladder of hydrogen bonds down the fibre — strength you can hang a parachutist from. Polyesters cannot do this: an ester linkage has no N–H, so Terylene chains attract only through permanent dipole–dipole and van der Waals forces — still a useful fibre, but the melting points and tensile strengths sit below the equivalent polyamide, and AQA loves asking why.

Push the logic to its limit and you get Kevlar, invented by DuPont chemist Stephanie Kwolek in 1965: a polyamide where both monomers are benzene-1,4-disubstituted — benzene-1,4-dicarboxylic acid and benzene-1,4-diamine. The aromatic rings make each chain flat and rigid, the chains stack like uncooked spaghetti, and the hydrogen bonds form in ordered sheets. Weight for weight, the result is about five times stronger than steel — bulletproof vests, Formula 1 survival cells and undersea cables are woven from a condensation reaction you can write in one line.

MechanismThe biodegradability divide — the linkage decides the lifespan

Turn a poly(ethene) bag over in your mind: the backbone is nothing but C–C and C–H σ bonds — strong, and, more importantly, non-polar. There is no δ+ carbon anywhere for a nucleophile to attack, so water, hydroxide ions, acids and the enzymes of decomposer organisms all slide off it. Polyalkenes are chemically inert and therefore non-biodegradable: estimates for how long a poly(ethene) bag survives in landfill run to centuries, and they are estimates only because none has yet been around long enough to finish rotting.

Condensation polymers carry their weak point in every repeat unit. The ester and amide linkages are polar — the carbonyl carbon sits δ+ between electronegative oxygens (or oxygen and nitrogen) — so water can attack it. Hydrolysis is condensation run backwards: slow with water alone, faster with acid or alkali and heat, and catalysed at ambient temperature by microbial enzymes. That is the precise sense in which polyesters and polyamides are biodegradable, and the exam wants the polarity argument, not a vague appeal to 'weaker bonds'.

The products depend on the conditions, and mark schemes are strict about it. Boil Terylene with aqueous sodium hydroxide and the ester links cleave to give ethane-1,2-diol and the disodium salt of benzene-1,4-dicarboxylic acid — an alkaline flask leaves carboxylic acids deprotonated. Heat nylon-6,6 with hydrochloric acid and the amide links (tougher than esters, so harsher conditions) give hexanedioic acid and the diammonium salt of hexane-1,6-diamine — an acidic flask protonates the amines. Match every product's protonation state to the pH of the reagent you were given.

Chemists now design the weakness in deliberately. PLA — the self-condensed polyester of lactic acid from block one, made by fermenting corn starch — is engineered to hydrolyse in industrial composting at around 60 °C. Renewable feedstock in, compost out: a polymer with a planned funeral.

DataLandfill, incinerator, recycling plant — pricing the three exits

The world now makes roughly 400 million tonnes of plastic a year, and every tonne eventually faces the same three doors. The exam's favourite extended response asks you to weigh them, so weigh them with numbers.

Landfill is cheap and demands no sorting — and that is the entire case for it. Addition polymers persist underground for centuries, sites are filling, and burying a polymer buries the crude oil embodied in it. UK landfill tax exists precisely to push waste towards the other two doors.

Incineration with energy recovery exploits the fact that polymers are solid petroleum: poly(ethene) releases about 46 MJ kg⁻¹ — half as much again as typical coal at roughly 30 — and the UK now burns more of its plastic waste than it recycles. The costs: the carbon is fossil, so the CO₂ is a genuine addition to the atmosphere, and chlorinated polymers such as PVC release corrosive HCl on combustion. Modern energy-from-waste plants answer with chemistry you can cite — flue gases held above 850 °C to destroy toxic products such as dioxins, then scrubbed with alkaline calcium oxide or carbonate to neutralise the acidic HCl.

Mechanical recycling — sort (PET is resin code 1, HDPE 2), wash, chip, remelt — saves both the material and the oil that made it, but it has two honest problems. Different polymers do not mix, so contaminated batches make weak products; and each heating cycle shortens chains, so quality ratchets downwards: bottle to fleece to carpet fill, a slide known as downcycling.

The elegant fourth option belongs to condensation polymers alone. Because their linkages hydrolyse, they can be chemically recycled: depolymerised back to monomers — PET plants already run this on used bottles — then repolymerised to virgin-quality plastic, a genuinely closed loop. Polyalkenes, with no linkage to unpick, have no such undo button. The linkage you drew in block one turns out to be the single most important fact about the polymer's afterlife.

VocabularyKey terms the mark scheme pays for

TrapsMisconceptions that cost marks

ExamWhat examiners want