Lesson 14: Polymers and Materials Chemistry

A polymer is a molecule built from thousands of repetitions of a small unit, the monomer. Polymers dominate modern materials — packaging, clothing, medical devices, aircraft — because their properties can be tuned by choosing the monomer, the chain length and the architecture. This lesson covers the two great polymerisation mechanisms, the calculations that describe chain size, and the structure–property logic that materials chemists design with.

Addition polymerisation

In addition polymerisation, alkene monomers open their C=C bonds and link into a chain with no by-product; the polymer's repeat unit is the monomer minus the double bond. Ethene gives poly(ethene); chloroethene (vinyl chloride, CH₂=CHCl) gives PVC; tetrafluoroethene gives PTFE (Teflon). Conditions differ by process: low-density poly(ethene) (LDPE) forms at high pressure with radical initiators, giving branched chains that cannot pack closely — hence low density and flexibility. High-density poly(ethene) (HDPE) forms with Ziegler–Natta catalysts at mild conditions, giving straight chains that pack and crystallise — rigid, strong, milk-bottle material. Same monomer, different architecture, different product: polymer chemistry in one sentence. The length of chains is measured by the degree of polymerisation (DP): the number of repeat units per chain.

Worked example 1: degree of polymerisation

A sample of PVC has an average molar mass of 125,000 g/mol. The repeat unit, −[CH₂−CHCl]−, has the same composition as the monomer C₂H₃Cl: Mr = (2 × 12.01) + (3 × 1.008) + 35.45 = 62.50. DP = 125,000 ÷ 62.50 = 2000 repeat units per chain. In a real sample every chain differs in length — polymerisation is statistical — so 125,000 g/mol is an average, and materials scientists routinely quote two different averages (number-average and weight-average) whose ratio measures the spread. A polymer's properties depend on that spread as well as on the average.

Condensation polymerisation

In condensation polymerisation, monomers join by eliminating a small molecule, usually water. Polyesters form from a dicarboxylic acid and a diol (PET — the drinks-bottle and fleece polymer — from terephthalic acid and ethane-1,2-diol). Polyamides form from a diamine and a dicarboxylic acid: nylon-6,6 from hexamethylenediamine and adipic acid, with two water molecules lost per repeat unit.

Worked example 2: repeat-unit mass of nylon-6,6

Hexamethylenediamine, C₆H₁₆N₂: Mr = (6 × 12.01) + (16 × 1.008) + (2 × 14.01) = 72.06 + 16.13 + 28.02 = 116.21. Adipic acid, C₆H₁₀O₄: 72.06 + 10.08 + 64.00 = 146.14. Each repeat unit joins one of each and ejects 2H₂O (2 × 18.02 = 36.03). Repeat-unit mass = 116.21 + 146.14 − 36.03 = 226.32 g/mol. The contrast with Worked example 1 is the conceptual point: addition polymers keep every atom (atom economy 100%, Lesson 15), while condensation polymers pay a water tax per link — mass and money out the door as by-product.

Properties: thermoplastics, thermosets, and intermolecular forces

Thermoplastics (PE, PVC, PET) have chains held to each other only by intermolecular forces; they soften on heating and can be remoulded — and recycled. Thermosets (epoxy resins, Bakelite) are cross-linked into a single giant network during curing; they char rather than melt and cannot be recycled by remoulding. Chain stiffness, polarity and crystallinity tune the details: polar chains attract more strongly (nylon's hydrogen bonds between chains give fibres their famous strength — you pull against intermolecular forces aligned along the fibre), while plasticisers inserted between PVC chains let them slide, converting rigid pipe polymer into flexible tubing.

In the lab: making nylon at an interface

The "nylon rope trick" draws a continuous fibre from the interface between a diamine solution and an acyl chloride solution. As a quantitative exercise, students dry and weigh the fibre — and here uncertainty dominates everything. The fresh fibre is swollen with solvent and trapped monomer; its mass on day one overstates polymer by 20–30%. Drying to constant mass (weigh, dry, reweigh over days) is the only honest route, and patience becomes an experimental technique. The balance contributes ±0.01 g on perhaps 0.30 g of dry polymer — 3% — but fibre-to-fibre variability (how steadily the student draws, how thick the rope) swamps the balance entirely: replicate fibres scatter ±15%. The lesson generalises across this course's practicals — know which uncertainty is biggest before reaching for a fancier instrument. Safety quantification matters too: hexamethylenediamine is corrosive and the acyl chloride lachrymatory, so the practical is scaled in millilitres precisely because polymer chains form readily at tiny scale.

Chemistry in the real world

Kevlar, a polyamide cousin of nylon, aligns hydrogen-bonded chains into fibres stronger than steel by weight — bulletproof vests are intermolecular forces, engineered. Biodegradable polymers such as PLA (polylactic acid, a polyester) exploit the condensation link: ester bonds hydrolyse back to lactic acid in compost, while addition polymers' all-carbon backbones resist every enzyme nature offers — the chemistry behind plastic persistence in oceans.

Common mistakes

• Drawing the repeat unit with the double bond retained. In addition polymerisation the C=C is consumed; the repeat unit shows single bonds continuing through.
• Forgetting the second water in nylon-type repeats: a dicarboxylic acid plus diamine loses 2H₂O per repeat unit (one at each link).
• "Thermosets melt if heated enough." They decompose/char; cross-links prevent chain sliding at any temperature.
• Confusing monomer and repeat unit masses for condensation polymers — they differ by the eliminated small molecules (Worked example 2).

Exam Experience

1. Which monomer polymerises to give PVC? A) CH₂=CH₂ B) CH₂=CHCl C) CF₂=CF₂ D) CH₃CH=CH₂ Answer: B. Poly(chloroethene) comes from chloroethene (vinyl chloride); A gives poly(ethene), C gives PTFE, D gives poly(propene).

2. Nylon-6,6 is a condensation polymer because: A) it is made in a laboratory B) a small molecule (water) is eliminated as each link forms C) it has a high melting point D) its monomers contain nitrogen Answer: B. The classification is mechanistic: condensation eliminates a small molecule per bond formed; addition does not.

3. A poly(propene) sample has an average molar mass of 210,000 g/mol. Its repeat unit C₃H₆ has Mr = 42.08. The degree of polymerisation is approximately: A) 500 B) 5000 C) 50,000 D) 4200 Answer: B. DP = 210,000 ÷ 42.08 = 4990 ≈ 5000. A divides by 420 through a decimal slip; D forgets that the repeat mass already accounts for the atoms.

Constructed response (5 marks). Perspex (PMMA) is made by addition polymerisation of methyl methacrylate, CH₂=C(CH₃)COOCH₃ (C₅H₈O₂, Mr = 100.12). (a) Calculate the degree of polymerisation of a PMMA sample of average molar mass 350,000 g/mol. (b) Draw/describe the repeat unit, showing which bonds change during polymerisation. (c) A condensation polymer of similar molar mass is made from monomers of combined mass 250.18 g/mol losing 2H₂O per repeat. Calculate its repeat-unit mass and DP, and comment on the comparison. Scoring guide: (a) 1 mark: repeat mass = monomer mass = 100.12 (addition, nothing lost); 1 mark: DP = 350,000 ÷ 100.12 = 3496 ≈ 3.50 × 10³. (b) 1 mark: C=C becomes C−C single bonds with the chain continuing through both carbons; methyl and ester groups unchanged as side groups. Deduct if the double bond survives in the drawing. (c) 1 mark: repeat = 250.18 − 36.03 = 214.15; 1 mark: DP = 350,000 ÷ 214.15 = 1634, with a comment that fewer, heavier repeats make the same total mass (or equivalent reasoning).

Fibres, elastomers and engineering polymers

Architecture continues past addition-versus-condensation. Fibres (nylon, polyester, Kevlar) are drawn after extrusion, stretching chains into alignment so intermolecular forces act cooperatively along the fibre — strength from orientation, not bond type. Elastomers (rubber) are the opposite design: coiled chains with sparse cross-links (vulcanisation's sulfur bridges), stretching by uncoiling and snapping back because the cross-links remember the shape. Add many cross-links and the elastomer becomes a rigid thermoset (ebonite) — one chemistry, three materials, tuned by cross-link density. Engineering polymers such as polycarbonate (safety goggles, aircraft windows) and PEEK (aerospace brackets) pay for aromatic rings in the backbone with stiffness, heat resistance and price.

Recycling: the chemistry of second lives

Resin codes sort the waste stream by polymer: PET (1) and HDPE (2) recycle profitably because they are thermoplastics in huge, clean volumes; PVC (3) contaminates other streams with chlorine chemistry; thermosets and mixed laminates defeat remelting entirely and mostly go to energy recovery or landfill. Mechanical recycling degrades chains (each melt cycle shortens them — DP falls, properties sag), so bottles become fleece become insulation: *downcycling*, not a loop. Chemical recycling attacks the bond types of this lesson directly: condensation polymers can be hydrolysed back to monomers (PET glycolysis is industrial practice) because their links were made by reversible condensation; addition polymers resist, needing high-temperature pyrolysis to crack their carbon backbones. Design-for-recycling is thus a Lesson 14 decision made at the monomer stage: choose links your grandchildren can undo.

Kevlar and PLA: two polymers, two philosophies

Kevlar (a para-aramid) shows where condensation chemistry reaches when orientation is perfected: rigid, rod-like chains hydrogen-bond into sheets so cooperative that the fibre's tensile strength exceeds steel's at one fifth of the density — spun from concentrated sulfuric acid solution because only that aggressive solvent can hold the chains apart long enough to align them. PLA (polylactic acid) argues the opposite design brief: an ordinary polyester whose ester links hydrolyse in industrial compost within weeks, polymerised from fermented plant starch — a condensation polymer deliberately built with an undo button (Lesson 14's recycling section). Between them they frame the modern materials question, which is no longer "can we make it?" but "what is its whole life for?" — performance, service, and afterlife are all written into the monomer choice.