Which structural feature is present in LDPE made by the high-pressure free-radical route but is essentially absent from both HDPE and LLDPE?

Long-chain branches long enough to entangle with other chains

Short ethyl or butyl branches, a few per hundred backbone carbons, which break up the crystal packing

Long-chain branches long enough to entangle with other chains

A crystalline phase that coexists with an amorphous phase and sets the melting point of the solid

Isotactic placement of side groups along the chain, which lets the backbone pack into a lattice

A homopolymer polypropylene bumper cracks easily on a winter morning. Which explanation and remedy are correct?

The amorphous phase is near its Tg of about 0 °C and turns glassy; copolymerising with ethylene or adding ethylene–propylene rubber restores low-temperature toughness

The chains are atactic and cannot crystallise; switching to an isotactic grade removes the brittleness

The crystals melt at winter temperatures; a phthalate plasticiser is the standard way to keep them solid

PP has a Tm of only 60 °C, so a higher-density grade such as HDPE must replace it

Rigid PVC with no stabiliser is extruded at about 190 °C. The extrudate is brown and acrid fumes rise from the die. Which explanation and remedy are correct?

HCl is lost along the chain leaving conjugated C=C sequences, and the HCl speeds the loss; a heat stabiliser such as a calcium–zinc or organotin salt takes up HCl and replaces labile chlorine

The chain unzips back to vinyl chloride monomer; an antioxidant that stops free-radical oxidation prevents it

Peroxide residues crosslink the chains; running the extruder at 120 °C avoids the crosslinking

HCl is lost along the chain leaving conjugated C=C sequences, and the HCl speeds the loss; a heat stabiliser such as a calcium–zinc or organotin salt takes up HCl and replaces labile chlorine

The phthalate plasticiser oxidises; replacing it with extra plasticiser restores the colour

Which of the following statements about polystyrene and the acrylics are correct?

High-impact polystyrene is opaque because its grafted rubber particles, comparable in size to the wavelength of light, scatter light

PMMA unzips to a high yield of methyl methacrylate on strong heating

High-impact polystyrene is opaque because its grafted rubber particles, comparable in size to the wavelength of light, scatter light

Atactic polystyrene crystallises when cooled slowly from the melt, giving an opaque semicrystalline plastic

PMMA unzips to a high yield of methyl methacrylate on strong heating

Polyacrylonitrile is melt-spun into acrylic fibre because it softens far below its decomposition temperature

Why is nylon 6,6 made by first forming a 1 : 1 salt of hexamethylenediamine and adipic acid in water?

Crystallising the salt fixes equimolar diamine and diacid, and in step-growth any imbalance of end groups caps the attainable molar mass

The salt melts below 100 °C, so that the polycondensation can then be carried out at room temperature without any need to remove a by-product

The salt route forms no water at all, so there is no by-product to remove from the melt and the molar mass is not limited by stoichiometry

Adipic acid alone ring-opens to give a polyamide of high molar mass, and the diamine only acts as a catalyst for the ring-opening step

Crystallising the salt fixes equimolar diamine and diacid, and in step-growth any imbalance of end groups caps the attainable molar mass

A PET bottle is blown by stretching a nearly amorphous preform above its Tg. Why is the bottle both stronger and still clear?

Stretching aligns the chains and induces many very small crystallites, below the scale that scatters light, which raise strength and CO₂ barrier

Stretching crosslinks the chains into a network, and a network cannot scatter light, so the wall stays clear and strong

Stretching aligns the chains and induces many very small crystallites, below the scale that scatters light, which raise strength and CO₂ barrier

Stretching pushes the Tg below 0 °C, so the wall stays rubbery and transparent while the bottle is strengthened

Stretching removes the ester groups and leaves a clear polyethylene-like wall that is both tough and clear

Which of the following statements about engineering and fluorinated plastics are correct?

In ABS the polybutadiene particles supply toughness while the glassy SAN matrix supplies rigidity and chemical resistance

PTFE is processed on conventional extruders as a low-viscosity melt just above 327 °C

Polysulfone is a semicrystalline polymer with a melting point above 300 °C

A phenol–formaldehyde prepolymer is made with an acid catalyst at a formaldehyde : phenol ratio below 1. It is a brittle, fusible solid that does not cure by heat alone. What must be added to cure it?

Hexamethylenetetramine, which supplies the methylene bridges on heating

More phenol, to supply additional methylol groups

An organic peroxide, to start free-radical crosslinking

Hexamethylenetetramine, which supplies the methylene bridges on heating

Nothing, because the methylol groups already present will self-cure on heating

A DGEBA epoxy resin is cured with the stoichiometric amount of a polyamine. A formulator then adds 30 % extra hardener “to be sure of full cure”. What is the most likely result?

Unreacted amine ends are left, the network is less complete and Tg and chemical resistance fall

Every extra N–H forms one more crosslink, so the crosslink density and the Tg rise further than at the balance point

The extra amine acts as a catalyst, so the cure is faster and the final network properties are left unchanged

The extra amine evaporates during the cure, so the final network is identical to the stoichiometric one

In the production of flexible polyurethane foam with no added physical blowing agent, the gas that expands the foam comes from:

Reaction of isocyanate with water, which gives an amine and CO₂

Decomposition of the urethane link at the curing temperature, which releases CO₂

Reaction of isocyanate with the polyol, which releases CO₂ as the by-product of urethane formation

Evaporation of the short-chain diol used as chain extender

Reaction of isocyanate with water, which gives an amine and CO₂

A reusable, see-through medical container must survive many steam-sterilisation cycles at about 134 °C and resist hydrolysis. Which of these is the best choice?

Polysulfone: amorphous, hence transparent, with Tg near 185 °C and good hydrolytic stability

Bisphenol-A polycarbonate: transparent, but its Tg of about 147 °C is marginal and repeated steam hydrolyses it

PEEK: very stable, but semicrystalline and therefore opaque

PPS: very stable, but semicrystalline and therefore opaque

Which of the following statements about thermosetting resins are correct?

A resol, made with base catalyst at a formaldehyde : phenol ratio above 1, contains methylol groups and cures on heating without a separate hardener

Urea–formaldehyde resin has poorer hot-water resistance than melamine–formaldehyde resin

Unsaturated polyester cures when free radicals copolymerise the maleate or fumarate C=C bonds with the styrene diluent

A resol, made with base catalyst at a formaldehyde : phenol ratio above 1, contains methylol groups and cures on heating without a separate hardener

A novolac is made with base catalyst at a formaldehyde : phenol ratio above 1 and cures by itself on heating

Urea–formaldehyde resin has poorer hot-water resistance than melamine–formaldehyde resin

Unsaturated polyester cures when free radicals copolymerise the maleate or fumarate C=C bonds with the styrene diluent

Plastics: Commodity, Engineering, Thermoset and Specialty Polymers

This chapter answers the plastics half of Section XE5.3, “Synthesis, Manufacturing and Properties”, of XE5 Polymer Science and Engineering. XE5 is one of the nine optional sections of the Engineering Sciences (XE) paper, any two of which a candidate chooses in the exam. The heading reads: “Commodity and general-purpose thermoplastics: PE, PP, PS, PVC; Polyesters; Acrylic; PU polymers; Engineering Plastics: Nylon, PC, PBT, Polyphenylene oxide, ABS, Fluoropolymers; Thermosetting polymers: Polyurethane, PF, MF, UF, Epoxy, Unsaturated polyester, Alkyds; … Specialty plastics: PEK, PEEK, Polyphenylene sulfide, Polysulfone, Polyethersulfone, etc.” (the rubber and bio-polymer parts of the heading are in the next chapter). For each plastic the chapter gives the monomers, the route, the link between structure and properties, and the uses. The numericals are monomer and repeat-unit masses, by-product and yield on a mass basis, a short-chain-branch count and an epoxy–hardener ratio.

1. Polyethylene and polypropylene: the route sets the architecture

Polyethylene (PE), –CH₂–CH₂–, comes from ethylene, CH₂=CH₂. The chemistry is identical in every grade, yet the route decides the chain architecture, and architecture decides density, crystallinity and stiffness. Branches stop chains packing: more branching means lower crystallinity, density, melting point and stiffness, but better clarity and flexibility.

The three polyethylenes
Grade	Route	Architecture	Density, crystallinity, Tm	Typical uses
LDPE	Free-radical, very high pressure (order of 1000–3000 bar, 150–300 °C), oxygen or peroxide initiator, tubular or autoclave reactor	Long-chain and short-chain branches	About 0.915–0.93 g/cm³; about 50 % crystalline; Tm near 110 °C	Film, bags, extrusion coating, squeezable bottles
HDPE	Low pressure, Ziegler–Natta (TiCl₄ with an aluminium alkyl) or Phillips (chromium oxide on silica) catalyst	Linear, almost no branches	About 0.94–0.97 g/cm³; about 70–80 % crystalline; Tm near 130 °C	Bottles, drums, pipes, crates
LLDPE	Low pressure, Ziegler–Natta or metallocene; ethylene copolymerised with 1-butene, 1-hexene or 1-octene	Linear backbone with uniform short branches, no long branches	About 0.915–0.94 g/cm³	Stretch and packaging film, tough liners

LLDPE has the low density of LDPE from its short branches, but its linear backbone and lack of long branches give higher tensile, tear and puncture strength at the same density. LDPE’s long branches give higher melt strength and easier film blowing, so the two are often blended. Each comonomer unit adds one branch: ethyl from butene, butyl from hexene, hexyl from octene.

Polypropylene (PP), from propylene, is made with Ziegler–Natta catalyst (TiCl₄ on MgCl₂ with an aluminium alkyl) or metallocene. The isotactic chain, with its methyl groups all on one side, crystallises: Tm is about 165 °C and density about 0.90 g/cm³, the lowest of the commodity plastics, so PP is stiffer and more heat resistant than HDPE. Its Tg is near 0 °C, so the homopolymer turns brittle in the cold; copolymerising with ethylene (random copolymer for clarity, impact copolymer with an ethylene–propylene rubber phase) cures this. The tertiary C–H bond makes PP oxidise and photo-degrade faster than PE, so it needs stabilisers. Atactic PP is amorphous and tacky, and goes into adhesives. Uses: fibres and rope, battery cases, caps, bumpers, living hinges.

2. Polystyrene, PVC and the acrylics

Polystyrene (PS) comes from styrene by free-radical bulk or suspension polymerisation. The chain is atactic, so it cannot crystallise: PS is glassy and transparent, with Tg near 100 °C, hard but brittle because the bulky phenyl rings lock the chain. General-purpose PS (GPPS) makes cups, lab-ware and disc cases. High-impact PS (HIPS) is made by polymerising styrene with dissolved polybutadiene, giving grafted rubber particles of about a micron in a PS matrix: far tougher, but opaque because the particles scatter light. Expandable PS holds pentane in beads that are steam-expanded into foam for insulation and packaging. Syndiotactic PS, made with metallocene catalyst, is crystalline (Tm near 270 °C).

Poly(vinyl chloride) (PVC) is made from vinyl chloride by free-radical polymerisation, mostly by suspension (also emulsion for paste resin, and bulk). It is atactic and only slightly crystalline, with Tg near 80 °C. The polar C–Cl bond and 57 % chlorine by mass give flame resistance, chemical resistance and a density near 1.4 g/cm³. Unplasticised (rigid) PVC makes pipes and window profiles; plasticised PVC, softened by a high-boiling ester such as a phthalate, makes cable sheathing, flooring and hose. Vinyl chloride monomer is a carcinogen, so residual monomer is stripped.

⚠️ PVC is processed close to where it starts to decompose

Melt processing at about 170–200 °C is near the onset of dehydrochlorination. Allylic and tertiary chlorine defects release HCl, the loss then “unzips” along the chain to leave conjugated C=C sequences (yellow to black), and the HCl catalyses further loss. Stabilisers (lead, calcium–zinc or organotin salts) mop up HCl and replace labile chlorine. This is why PVC is never sold without a stabiliser package.

Acrylics. Poly(methyl methacrylate) (PMMA) is made from CH₂=C(CH₃)COOCH₃ by free-radical bulk polymerisation (cell casting between glass plates gives sheet) or suspension polymerisation (beads for moulding). It is atactic and amorphous, Tg near 105 °C, with about 92 % light transmission and excellent weathering; it is harder and more scratch-resistant than PC, but far less impact resistant. Uses: glazing, lenses, signs, light guides, bone cement. On strong heating it unzips back to monomer, which is the basis of its chemical recycling. Polyacrylonitrile (PAN) has strong dipole–dipole attraction between nitrile groups, so it decomposes before melting and is solution-spun (from DMF or DMAc) into acrylic fibre, by convention at least 85 % acrylonitrile; it is also the main precursor of carbon fibre. Soft acrylate esters (ethyl, butyl) with low Tg go into adhesives, paints and latex coatings.

3. Polyesters, nylons and polycarbonate: step-growth engineering plastics

These are made by step-growth reactions, so high molar mass needs near-complete conversion, an exact 1 : 1 balance of functional groups and removal of the small-molecule by-product (Carothers: Xₙ = 1/(1 − p), so p = 0.99 gives only 100). Their polar, often aromatic backbones give high Tg and Tm, which is why they are the classic engineering plastics.

Polyesters, polyamides and polycarbonate
Polymer	Monomers	Route	Tg, Tm	Properties and uses
PET	Terephthalic acid (or dimethyl terephthalate) + ethylene glycol	Esterification (water out) or ester interchange (methanol out) to the bis(hydroxyethyl) ester, then melt polycondensation near 280 °C under vacuum with an antimony catalyst; solid-state polymerisation for bottle grade	Tg 70–80 °C; Tm 255–265 °C	Fibre, bottles (stretch blow moulding crystallises it by strain, keeping it clear), film; good CO₂ barrier
PBT	Terephthalic acid or DMT + 1,4-butanediol	Melt polycondensation, as for PET	Tg 40–50 °C; Tm about 225 °C	The flexible –(CH₂)₄– spacer lets it crystallise fast, so mould cycles are short; connectors, gears, glass-filled auto parts
Nylon 6	ε-Caprolactam	Hydrolytic ring-opening: a little water opens some rings, then amine ends add lactam, near 250 °C	Tm about 220 °C	Fibre, tyre cord, mouldings; takes up more water than nylon 6,6
Nylon 6,6	Hexamethylenediamine + adipic acid	Equimolar “nylon salt” made in water (fixes the 1 : 1 ratio), then polycondensation at 270–280 °C, water removed	Tm about 265 °C	Fibre, gears, bearings, cable ties, glass-filled auto parts
PC (bisphenol A)	Bisphenol A + phosgene (or diphenyl carbonate)	Interfacial polycondensation (aqueous NaOH / CH₂Cl₂, NaCl by-product) or melt transesterification (phenol out)	Amorphous; Tg about 147 °C	Clear, outstanding impact strength: helmets, glazing, optical discs; notch-sensitive, stress-cracks in solvents

Structure explains the table. Aromatic rings (PET, PC) stiffen the chain and raise Tg; the aliphatic spacer of PBT lowers Tg and speeds crystallisation. Amide groups hydrogen-bond, which raises Tm but makes nylons take up moisture. PC’s bulky bisphenol-A unit prevents crystallisation, which is why it is both transparent and very tough.

⚠️ Dry before moulding; and wet nylon is a different material

Molten PET, PBT, PC and the nylons are hydrolysed by moisture, which cuts chains and lowers molar mass, so pellets are dried before melting. In service, absorbed water plasticises nylon: Tg and modulus fall, impact strength rises and dimensions swell. Test data must therefore state “dry as moulded” or “conditioned”.

4. Polyphenylene oxide, ABS and the fluoropolymers

Polyphenylene oxide (PPO, also PPE) is poly(2,6-dimethyl-1,4-phenylene oxide), made from 2,6-dimethylphenol by oxidative coupling with O₂ and a copper–amine catalyst, with water as by-product. Its Tg is near 210 °C and it absorbs little water, with good dielectric properties and dimensional stability, but the neat melt is too viscous to process. PPO is miscible with polystyrene in all proportions, so PPO/HIPS blends have one Tg and an adjustable heat resistance; this pair is the standard example of a miscible blend (Chapter 3).

ABS is not a copolymer of three monomers in one chain but a two-phase system: a glassy styrene–acrylonitrile (SAN) matrix with polybutadiene rubber particles (about 0.1–1 µm) to which SAN is grafted. It is made by emulsion grafting of styrene and acrylonitrile onto polybutadiene latex, or by bulk/suspension grafting, often followed by blending with extra SAN. Each monomer has a job: acrylonitrile gives chemical resistance and stiffness, butadiene gives toughness (the rubber particles trigger crazing and shear yielding, absorbing impact energy, even at low temperature), styrene gives rigidity, gloss and easy processing. ABS is amorphous, with Tg of the SAN phase near 105 °C. The residual C=C bonds of the rubber make it weather poorly. Uses: appliance housings, car interiors, pipes, luggage, toy bricks.

Fluoropolymers owe their behaviour to the C–F bond, one of the strongest single bonds in organic chemistry, and to fluorine’s tight, low-polarisability sheath around the chain.

PTFE (tetrafluoroethylene, free-radical polymerisation in water under pressure): highly crystalline, Tm about 327 °C, density about 2.2 g/cm³, chemically inert, very low friction, excellent dielectric, usable to about 260 °C. The melt viscosity is so high that it does not flow, so PTFE is processed by compacting and sintering powder, like a ceramic. Uses: seals, gaskets, bearings, non-stick coatings, cable insulation.
FEP (TFE with hexafluoropropylene): the CF₃ side groups lower crystallinity and melt viscosity, so it is melt-processable (Tm about 260 °C) at some cost in service temperature.
PVDF (–CH₂–CF₂–): polar, melt-processable (Tm about 170 °C), tough, UV resistant and piezoelectric in its β phase; used in chemical-plant linings, architectural coatings, membranes and battery binders.

5. Polyurethane and the thermosetting polymers

Polyurethane (PU) forms by polyaddition, with no by-product: –NCO + HO– → –NH–CO–O–. The monomers are a di-isocyanate (TDI, MDI, or aliphatic HDI for light-stable coatings) and a polyol (a polyether, resistant to hydrolysis and flexible, or a polyester, stronger and more oil resistant), with amine or tin catalysts. The chain is a segmented block copolymer: soft polyol segments with hard, hydrogen-bonded urethane segments from the isocyanate and a short chain extender such as 1,4-butanediol. Linear PU is thermoplastic (TPU, spandex fibre); a polyol or isocyanate of functionality above 2 gives a crosslinked thermoset. Foams use water as blowing agent: –NCO + H₂O → –NH₂ + CO₂, and the amine goes on to form urea links. Flexible foam (TDI, polyether triol) is for cushioning; rigid foam (polymeric MDI) is for insulation; PU also makes coatings, adhesives and elastomer wheels.

Thermosets compared
Resin	Monomers	Chemistry and cure	Properties and uses
PF	Phenol + formaldehyde	Resol: base catalyst, F : P above 1, methylol groups, self-curing on heating. Novolac: acid catalyst, F : P below 1, no methylols, needs hexamethylenetetramine (“hexa”) to cure. Water is the by-product.	Heat, flame and electrical resistance, brittle and dark; switches, laminates, brake linings, foundry binders, plywood glue
UF	Urea + formaldehyde	Mono- and dimethylolureas form under mild alkali, then condense under acid with a latent acid catalyst (e.g. ammonium chloride)	Colourless, hard, cheap; poor hot-water resistance (reversible hydrolysis); particle board and plywood adhesive, moulded fittings; formaldehyde emission
MF	Melamine (2,4,6-triamino-1,3,5-triazine) + formaldehyde	Up to six methylol groups per melamine, then condensation to a dense network	Harder, more heat- and water-resistant than UF; decorative laminates, tableware, automotive paint crosslinker
Epoxy	Bisphenol A + epichlorohydrin (NaOH) gives DGEBA	Cured by ring-opening addition of a hardener: amines (ambient or heat), anhydrides (heat), catalytic imidazoles or BF₃ complexes. No volatile released.	Excellent adhesion (polar –OH and ether groups), low shrinkage, chemical resistance, but brittle; adhesives, coatings, composite matrix, laminates for circuit boards
Unsaturated polyester	Maleic anhydride (the C=C), phthalic anhydride, propylene glycol; dissolved in styrene	Polycondensation to a prepolymer, then free-radical copolymerisation with the styrene (peroxide, with cobalt accelerator at room temperature) crosslinks through the C=C	Cheap, easy hand lay-up; boats, tanks, pipes, moulded panels; more saturated phthalate lowers crosslink density (less rigid, tougher); styrene emission
Alkyd	Polyol (glycerol, pentaerythritol) + phthalic anhydride + fatty acids or drying oil	Oil-modified polyester; the unsaturated fatty chains crosslink by air oxidation (autoxidation) with metal driers	Long oil (above 60 %) is flexible and brushable, short oil (below 40 %) is hard; paints, enamels, varnishes

🧠 Epoxy stoichiometry in one line

Epoxide equivalent weight EEW = M/2 for pure DGEBA (M = 340 g/mol gives EEW = 170 g/eq; commercial liquid resins are about 180–190). For an amine, the amine-hydrogen equivalent weight AHEW = M/(number of N–H hydrogens). Stoichiometric hardener = 100 × AHEW / EEW parts per hundred resin (phr). Fewer hydrogens per kilogram of hardener means more hardener is needed.

6. Specialty high-performance plastics: PEEK, PEK, PPS, PSU, PES

These plastics keep stiffness and strength at 150–250 °C and resist chemicals. All have aromatic rings joined by short links (ether, ketone, sulfide, sulfone). The rings give high Tg; the ether and sulfide links give just enough rotation to allow melt processing; and the stronger the link, the higher the heat resistance. Most are made by nucleophilic aromatic substitution (aryl chloride or fluoride displaced by a phenoxide or sulfide) in a dipolar solvent at high temperature, with a salt as by-product.

Specialty plastics
Polymer	Monomers and route	Tg, Tm	Properties and uses
PEEK	4,4′-Difluorobenzophenone + hydroquinone, with K₂CO₃ or Na₂CO₃ in diphenyl sulfone at about 300 °C or above (HF as the salt)	Tg about 143 °C; Tm about 343 °C	Semicrystalline; use near 250 °C; resists chemicals, hydrolysis and wear; aerospace parts, implants, bearings, carbon-fibre composites
PEK	Same family, with one ether link per ketone (more ketone than PEEK)	Both higher than PEEK	The ketone link is stiffer than ether, so Tg and Tm rise with the ketone fraction, as does the difficulty of processing
PPS	p-Dichlorobenzene + sodium sulfide in N-methylpyrrolidone, about 250 °C, under pressure; NaCl by-product	Tg about 90 °C; Tm about 280 °C	Semicrystalline, inherently flame retardant, almost no solvent below about 200 °C, dimensionally stable; connectors, pump parts, hot-gas filter fabric
PSU	Bisphenol A disodium salt + 4,4′-dichlorodiphenyl sulfone in a dipolar aprotic solvent; NaCl out	Amorphous; Tg about 185 °C	Transparent (amber), steam-sterilisable, hydrolysis-stable; medical devices, hot-water fittings, ultrafiltration and dialysis membranes
PES	4,4′-Dichlorodiphenyl sulfone + 4,4′-dihydroxydiphenyl sulfone (or the self-condensing chloro-hydroxy monomer); NaCl out	Amorphous; Tg about 225 °C	No bisphenol-A isopropylidene unit, so more heat resistant than PSU; membranes, aircraft interiors, medical trays

7. Numericals: monomer masses, by-products and ratios

By-product and yield on a mass basis. Write the balanced equation, then use molar masses (H 1.008, C 12.011, N 14.007, O 15.999, Na 22.990, S 32.06, Cl 35.45 g/mol). For nylon 6,6, one repeat unit –NH(CH₂)₆NH–CO(CH₂)₄CO– contains two amide links, so each repeat unit releases two H₂O.

Nylon 6,6: salt = 116.21 + 146.14 = 262.35 g/mol; repeat unit C₁₂H₂₂N₂O₂ = 226.32 g/mol; water = 2 × 18.015 = 36.03 g/mol. From 100 kg of salt at full conversion: polymer = 100 × 226.32/262.35 = 86.3 kg, water = 13.7 kg.
PPS: C₆H₄Cl₂ (147.0) + Na₂S (78.04) → [C₆H₄S]ₙ (108.16 per unit) + 2 NaCl (116.88). Salt per kilogram of polymer = 116.88/108.16 = 1.08 kg: this route makes more salt than polymer by mass.

Counting units and branches. The repeat unit of PET, –OC–C₆H₄–COO–CH₂CH₂–, is C₁₀H₈O₄ = 192.2 g/mol, so a chain of Mₙ = 19,200 g/mol has about 100 repeat units. In LLDPE every monomer unit, ethylene or comonomer, puts two carbons in the backbone, so 1000 backbone carbons are 500 units; with a mole fraction x of comonomer, the branches per 1000 backbone carbons are 500x. At 6 mol % hexene that is 30 butyl branches.

Key takeaways

Polyethylene grades differ only in architecture: LDPE (free-radical, high pressure, long and short branches), HDPE (Ziegler–Natta or Phillips, linear) and LLDPE (copolymer with butene, hexene or octene, short branches only). Branching lowers crystallinity, density, Tm and stiffness.
Isotactic PP crystallises (Tm about 165 °C, density about 0.90); atactic PS, PVC and PMMA are amorphous. PVC needs stabilisers because it dehydrochlorinates near its processing temperature; HIPS and ABS are rubber-toughened two-phase systems.
PET, PBT, nylons and PC are step-growth polymers: they need exact stoichiometry, by-product removal and drying before melting. Aromatic rings raise Tg, flexible spacers speed crystallisation, amides raise Tm but absorb water.
Thermosets: resol (base, F : P above 1) self-cures, novolac (acid, F : P below 1) needs hexa; UF and MF are amino resins (MF the more durable); epoxy cures by ring-opening addition with no volatile (hardener phr = 100 × AHEW / EEW); unsaturated polyester cures with styrene; alkyds dry by autoxidation.
PEEK, PEK, PPS, PSU and PES are aromatic polymers made by nucleophilic aromatic substitution; ketone and sulfone links raise Tg and Tm, ether and sulfide links allow melt processing, and the by-product is a salt (or HF taken up by carbonate).

Practice questions (16)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

Which structural feature is present in LDPE made by the high-pressure free-radical route but is essentially absent from both HDPE and LLDPE?
1. Short ethyl or butyl branches, a few per hundred backbone carbons, which break up the crystal packing
2. Long-chain branches long enough to entangle with other chains
3. A crystalline phase that coexists with an amorphous phase and sets the melting point of the solid
4. Isotactic placement of side groups along the chain, which lets the backbone pack into a lattice
Show answer
Answer: B — Long-chain branches long enough to entangle with other chains
Free-radical “back-biting” and chain transfer at very high pressure give LDPE both short and long branches. The long branches are what raise melt strength, and HDPE (Ziegler–Natta or Phillips) and LLDPE (linear backbone) lack them. Short branches are the tempting answer, but LLDPE is made with them on purpose. All three polyethylenes are semicrystalline, and polyethylene has no side group whose tacticity could be defined.
A homopolymer polypropylene bumper cracks easily on a winter morning. Which explanation and remedy are correct?
1. The amorphous phase is near its Tg of about 0 °C and turns glassy; copolymerising with ethylene or adding ethylene–propylene rubber restores low-temperature toughness
2. The chains are atactic and cannot crystallise; switching to an isotactic grade removes the brittleness
3. The crystals melt at winter temperatures; a phthalate plasticiser is the standard way to keep them solid
4. PP has a Tm of only 60 °C, so a higher-density grade such as HDPE must replace it
Show answer
Answer: A — The amorphous phase is near its Tg of about 0 °C and turns glassy; copolymerising with ethylene or adding ethylene–propylene rubber restores low-temperature toughness
Isotactic PP has Tg near 0 °C, so at or below that the amorphous regions stop absorbing impact energy; an ethylene comonomer or a dispersed ethylene–propylene rubber phase fixes this. The atactic explanation confuses PP with PS: commercial PP is isotactic and highly crystalline. PP melts near 165 °C, not 60 °C, and plasticisers are not how PP is toughened.
Rigid PVC with no stabiliser is extruded at about 190 °C. The extrudate is brown and acrid fumes rise from the die. Which explanation and remedy are correct?
1. The chain unzips back to vinyl chloride monomer; an antioxidant that stops free-radical oxidation prevents it
2. Peroxide residues crosslink the chains; running the extruder at 120 °C avoids the crosslinking
3. HCl is lost along the chain leaving conjugated C=C sequences, and the HCl speeds the loss; a heat stabiliser such as a calcium–zinc or organotin salt takes up HCl and replaces labile chlorine
4. The phthalate plasticiser oxidises; replacing it with extra plasticiser restores the colour
Show answer
Answer: C — HCl is lost along the chain leaving conjugated C=C sequences, and the HCl speeds the loss; a heat stabiliser such as a calcium–zinc or organotin salt takes up HCl and replaces labile chlorine
PVC degrades by dehydrochlorination, an autocatalytic zip reaction that forms coloured polyene sequences and releases HCl. A stabiliser that neutralises HCl and stabilises defect chlorines is the remedy. Unzipping to monomer is the PMMA behaviour and is the tempting wrong answer, and an antioxidant alone does not stop HCl loss. The compound is rigid PVC, so it has no plasticiser, and PVC cannot be melted at 120 °C.
Which of the following statements about polystyrene and the acrylics are correct?
1. High-impact polystyrene is opaque because its grafted rubber particles, comparable in size to the wavelength of light, scatter light
2. Atactic polystyrene crystallises when cooled slowly from the melt, giving an opaque semicrystalline plastic
3. PMMA unzips to a high yield of methyl methacrylate on strong heating
4. Polyacrylonitrile is melt-spun into acrylic fibre because it softens far below its decomposition temperature
Show answer
Answer: A — High-impact polystyrene is opaque because its grafted rubber particles, comparable in size to the wavelength of light, scatter light; C — PMMA unzips to a high yield of methyl methacrylate on strong heating
Rubber particles of about a micron scatter visible light, so HIPS is opaque, and PMMA depolymerises thermally back to monomer. Atactic PS has no regular structure and stays amorphous, and PAN does not melt because its strong nitrile–nitrile dipole interactions hold it together until it decomposes, so it is spun from solution.
Why is nylon 6,6 made by first forming a 1 : 1 salt of hexamethylenediamine and adipic acid in water?
1. The salt melts below 100 °C, so that the polycondensation can then be carried out at room temperature without any need to remove a by-product
2. The salt route forms no water at all, so there is no by-product to remove from the melt and the molar mass is not limited by stoichiometry
3. Adipic acid alone ring-opens to give a polyamide of high molar mass, and the diamine only acts as a catalyst for the ring-opening step
4. Crystallising the salt fixes equimolar diamine and diacid, and in step-growth any imbalance of end groups caps the attainable molar mass
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Answer: D — Crystallising the salt fixes equimolar diamine and diacid, and in step-growth any imbalance of end groups caps the attainable molar mass
With two different monomers of functionality 2, the Carothers relation requires equal numbers of NH₂ and COOH groups; an excess of either leaves chain ends of one kind and stops growth early. The salt crystallises as a precise 1 : 1 compound, so weighing it fixes the balance. The polymerisation is not a room-temperature process (it runs near 270–280 °C), water is released and must be removed, and adipic acid is not a ring-opening monomer; that description belongs to nylon 6 from caprolactam.
Nylon 6,6 is made by melt polycondensation of the 1 : 1 salt of hexamethylenediamine (M = 116.21 g/mol) and adipic acid (M = 146.14 g/mol). Each repeat unit formed releases two molecules of water (M = 18.015 g/mol). For complete conversion of 100 kg of salt, how many kilograms of water are released? Give the answer to one decimal place.
Numerical answer — type the value.
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Answer: 13.7
Salt per mole of repeat unit = 116.21 + 146.14 = 262.35 g. Water per mole of repeat unit = 2 × 18.015 = 36.03 g. Fraction of mass lost = 36.03/262.35 = 0.1373, so 100 kg of salt releases 100 × 0.1373 = 13.7 kg of water (and leaves 86.3 kg of polymer).
A PET bottle is blown by stretching a nearly amorphous preform above its Tg. Why is the bottle both stronger and still clear?
1. Stretching crosslinks the chains into a network, and a network cannot scatter light, so the wall stays clear and strong
2. Stretching aligns the chains and induces many very small crystallites, below the scale that scatters light, which raise strength and CO₂ barrier
3. Stretching pushes the Tg below 0 °C, so the wall stays rubbery and transparent while the bottle is strengthened
4. Stretching removes the ester groups and leaves a clear polyethylene-like wall that is both tough and clear
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Answer: B — Stretching aligns the chains and induces many very small crystallites, below the scale that scatters light, which raise strength and CO₂ barrier
Orientation by stretching induces strain-induced crystallisation. The crystallites are far smaller than the wavelength of visible light and so do not scatter it, while aligned chains and crystals raise strength, stiffness and gas barrier. Slow cooling of PET without stretching gives large spherulites, which are opaque. Stretching does not crosslink PET, lower its Tg or remove its ester groups.
Which of the following statements about engineering and fluorinated plastics are correct?
1. Polyphenylene oxide is miscible with polystyrene in all proportions, so PPO/PS blends show a single Tg
2. In ABS the polybutadiene particles supply toughness while the glassy SAN matrix supplies rigidity and chemical resistance
3. PTFE is processed on conventional extruders as a low-viscosity melt just above 327 °C
4. Polysulfone is a semicrystalline polymer with a melting point above 300 °C
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Answer: A — Polyphenylene oxide is miscible with polystyrene in all proportions, so PPO/PS blends show a single Tg; B — In ABS the polybutadiene particles supply toughness while the glassy SAN matrix supplies rigidity and chemical resistance
PPO and PS form a miscible pair, and ABS is a rubber-toughened two-phase plastic, so the statements on PPO/PS and on ABS are correct. PTFE has such a high melt viscosity that it does not flow even above 327 °C, so it is sintered, not extruded as a melt. Polysulfone is amorphous, with Tg near 185 °C and no melting point.
A phenol–formaldehyde prepolymer is made with an acid catalyst at a formaldehyde : phenol ratio below 1. It is a brittle, fusible solid that does not cure by heat alone. What must be added to cure it?
1. More phenol, to supply additional methylol groups
2. An organic peroxide, to start free-radical crosslinking
3. Hexamethylenetetramine, which supplies the methylene bridges on heating
4. Nothing, because the methylol groups already present will self-cure on heating
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Answer: C — Hexamethylenetetramine, which supplies the methylene bridges on heating
This is a novolac: acid catalysis with excess phenol gives methylene-linked oligomers with no methylol groups, so heating alone cannot crosslink it. Hexamethylenetetramine (“hexa”) decomposes to supply the extra formaldehyde-equivalent carbon and ammonia and links the phenolic rings. The statement that methylol groups already present will self-cure describes a resol, which is made with base at F : P above 1 and does self-cure. Phenol, a peroxide or a free-radical route are not how PF resins crosslink.
A DGEBA epoxy resin is cured with the stoichiometric amount of a polyamine. A formulator then adds 30 % extra hardener “to be sure of full cure”. What is the most likely result?
1. Unreacted amine ends are left, the network is less complete and Tg and chemical resistance fall
2. Every extra N–H forms one more crosslink, so the crosslink density and the Tg rise further than at the balance point
3. The extra amine acts as a catalyst, so the cure is faster and the final network properties are left unchanged
4. The extra amine evaporates during the cure, so the final network is identical to the stoichiometric one
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Answer: A — Unreacted amine ends are left, the network is less complete and Tg and chemical resistance fall
Once every epoxide group has reacted, surplus amine has nothing to bond with. It remains as dangling amine ends that plasticise the network and lower Tg and chemical resistance, and the same loss occurs with too little hardener. More crosslinks cannot form because each crosslink needs an epoxide group, and the extra N–H groups have none left to meet. Primary and secondary amines add stoichiometrically, so they are not catalysts, and the polyamine is not volatile enough to leave.
A liquid epoxy resin has an epoxide equivalent weight EEW = 200 g/eq. It is cured with diethylenetriamine (DETA, H₂N–CH₂CH₂–NH–CH₂CH₂–NH₂, M = 103.17 g/mol), which has 5 reactive N–H hydrogens per molecule. Use AHEW = M/(number of N–H hydrogens) and stoichiometric hardener (phr) = 100 × AHEW/EEW. How many parts of DETA per hundred parts of resin are needed? Give the answer to one decimal place.
Numerical answer — type the value.
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Answer: 10.3
AHEW = 103.17/5 = 20.63 g/eq. phr = 100 × 20.63/200 = 10.3. A common slip is to divide by 3 (the nitrogen count) instead of 5 (the N–H count), which gives 17.2 phr and an under-crosslinked, hardener-rich product.
In the production of flexible polyurethane foam with no added physical blowing agent, the gas that expands the foam comes from:
1. Decomposition of the urethane link at the curing temperature, which releases CO₂
2. Reaction of isocyanate with the polyol, which releases CO₂ as the by-product of urethane formation
3. Evaporation of the short-chain diol used as chain extender
4. Reaction of isocyanate with water, which gives an amine and CO₂
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Answer: D — Reaction of isocyanate with water, which gives an amine and CO₂
Water reacts with –NCO to form an unstable carbamic acid that loses CO₂ and leaves an amine, which then reacts with more isocyanate to form urea links. The isocyanate–hydroxyl reaction is a polyaddition that forms the urethane link with no by-product, which is the tempting wrong choice. The urethane link is stable at curing temperatures, and a chain extender is not volatile enough to act as a blowing agent.
Poly(phenylene sulfide) is made from p-dichlorobenzene (C₆H₄Cl₂, M = 147.0 g/mol) and sodium sulfide (Na₂S, M = 78.04 g/mol): n C₆H₄Cl₂ + n Na₂S → [C₆H₄S]ₙ + 2n NaCl. Take the repeat unit C₆H₄S as 108.16 g/mol and NaCl as 58.44 g/mol. How many kilograms of NaCl are formed per 100 kg of polymer? Give the answer to the nearest whole number.
Numerical answer — type the value.
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Answer: 108
Each repeat unit (108.16 g) is accompanied by 2 NaCl = 116.88 g. Per 100 kg of polymer: 100 × 116.88/108.16 = 108.06, i.e. 108 kg. The mass balance checks: 147.0 + 78.04 = 225.04 = 108.16 + 116.88. The salt exceeds the polymer in mass.
A reusable, see-through medical container must survive many steam-sterilisation cycles at about 134 °C and resist hydrolysis. Which of these is the best choice?
1. Polysulfone: amorphous, hence transparent, with Tg near 185 °C and good hydrolytic stability
2. Bisphenol-A polycarbonate: transparent, but its Tg of about 147 °C is marginal and repeated steam hydrolyses it
3. PEEK: very stable, but semicrystalline and therefore opaque
4. PPS: very stable, but semicrystalline and therefore opaque
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Answer: A — Polysulfone: amorphous, hence transparent, with Tg near 185 °C and good hydrolytic stability
Transparency needs an amorphous polymer, and 134 °C steam with hydrolysis resistance needs a high Tg and hydrolytically stable links, which polysulfone provides. PC is transparent but is attacked by repeated hot steam and has a marginal Tg. PEEK and PPS are tough and heat resistant but their crystallinity makes them opaque, so they fail the stated requirement.
An LLDPE is made from ethylene and 1-octene, with 4 mol % of the monomer units being octene. Assume each monomer unit, ethylene or octene, contributes two carbons to the backbone, and each octene unit carries one hexyl branch. How many hexyl branches are there per 1000 backbone carbons?
Numerical answer — type the value.
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Answer: 20
1000 backbone carbons ÷ 2 carbons per unit = 500 monomer units. Octene units = 0.04 × 500 = 20, and each carries one hexyl branch, so there are 20 branches per 1000 backbone carbons. The six remaining carbons of each octene form the hexyl side chain and are not counted among the 1000 backbone carbons.
Which of the following statements about thermosetting resins are correct?
1. A resol, made with base catalyst at a formaldehyde : phenol ratio above 1, contains methylol groups and cures on heating without a separate hardener
2. A novolac is made with base catalyst at a formaldehyde : phenol ratio above 1 and cures by itself on heating
3. Urea–formaldehyde resin has poorer hot-water resistance than melamine–formaldehyde resin
4. Unsaturated polyester cures when free radicals copolymerise the maleate or fumarate C=C bonds with the styrene diluent
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Answer: A — A resol, made with base catalyst at a formaldehyde : phenol ratio above 1, contains methylol groups and cures on heating without a separate hardener; C — Urea–formaldehyde resin has poorer hot-water resistance than melamine–formaldehyde resin; D — Unsaturated polyester cures when free radicals copolymerise the maleate or fumarate C=C bonds with the styrene diluent
A resol carries methylol groups and self-cures, MF is more hot-water resistant than UF, and UP cures by free-radical copolymerisation with styrene through the maleate/fumarate C=C. The novolac statement is wrong in both respects: a novolac is made with acid catalyst at F : P below 1 and needs hexamethylenetetramine to cure.