Classification of Polymers (2)

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your UACE Chemistry paper.

Polymers are giant molecules (macromolecules) built by linking many small monomer units. Polymerisation is the reaction that joins them through covalent bonds.

Addition polymerisation: monomers containing C=C (e.g. ethene, propene, vinyl chloride, styrene) join without loss of any small molecule. Initiator, propagation, termination.
Condensation polymerisation: monomers with two functional groups (e.g. −OH and −COOH, or −NH₂ and −COOH) link with elimination of H₂O or HCl; produces polyesters (terylene) and polyamides (nylon 6,6).
Degree of polymerisation: n = M(polymer) / M(monomer), dimensionless.
Classification by behaviour: thermoplastics (soften on heating: polythene, PVC) versus thermosets (irreversibly harden: Bakelite, melamine).

UACE hot tip: a 3% weightage topic still drops a 10-mark structured question on naming the polymer from its monomer, drawing the repeat unit, and explaining one property–structure link. Memorise –CH₂–CHR– as the repeat-unit pattern for addition polymers.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Classification of Polymers

Origin	Examples	Structural class	Examples
Natural	Starch, cellulose, proteins, natural rubber	Linear	Polythene, nylon
Synthetic	Polythene, PVC, perspex, terylene, Bakelite	Branched	Low-density polythene (LDPE)
Semi-synthetic	Cellulose nitrate, cellulose acetate	Cross-linked	Vulcanised rubber, Bakelite

Polymers are also classified by response to heat: thermoplastics have weakly held chains that slide past each other when heated (reversible), while thermosetting plastics form a 3D network of covalent cross-links that cannot remelt (irreversible). Elastomers like natural rubber stretch and recoil because of flexible, lightly cross-linked chains.

Addition Polymerisation

A free-radical chain mechanism converts C=C to C–C–C–C. Steps:

Initiation: initiator (e.g. organic peroxide) → R• + R•
Propagation: R• adds to monomer, new radical attacks next monomer — chain grows.
Termination: two radicals combine or disproportionate, ending the chain.

For chloroethene (vinyl chloride), the repeat unit is –CH₂–CHCl– and the polymer is PVC. For propene: –CH₂–CH(CH₃)– gives poly(propene). For tetrafluoroethene: –CF₂–CF₂– gives PTFE (Teflon).

Condensation Polymerisation

A dicarboxylic acid + a diol → polyester + H₂O. Example: terephthalic acid + ethane-1,2-diol → terylene (Dacron). A diacyl chloride + diamine → polyamide + HCl. Example: hexanedioyl chloride + 1,6-diaminohexane → nylon 6,6. Amino acids (H₂N–CHR–COOH) condense to polypeptides with loss of water between −NH₂ and −COOH groups.

Worked Calculation

Polythene sample has M = 140,000 g mol⁻¹; monomer (ethene) M = 28 g mol⁻¹.

n = 140,000 / 28 = 5,000 repeat units.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

How Structure Determines Properties

Chain length raises melting point and tensile strength because longer chains have greater cumulative London dispersion forces and more chain entanglement. Branching (as in LDPE from free-radical polymerisation at high pressure) prevents close packing, lowering density, crystallinity and softening temperature. Cross-linking locks chains in place: vulcanised rubber (rubber + S at ~140 °C) gains disulphide bridges that increase elasticity and stop the polymer from flowing — Goodyear’s 1839 discovery.

Exam-Specific Traps

Balancing: a condensation equation for nylon 6,6 from hexanedioic acid + 1,6-diaminohexane must show (n − 1) H₂O eliminated, not n. Many candidates write n incorrectly.
Repeat unit vs monomer: never redraw the C=C in the repeat unit; that double bond has reacted. Repeat unit shows only the saturated backbone carbons.
Biodegradability: polyesters like PHBV and polylactic acid (PLA) are biodegradable; polythene, PVC, and polystyrene are not — examiners test whether you can link polarity of backbone to microbial attack.

Common Mistakes

Mistake	Correction
Treating natural rubber as a thermoplastic	It is an elastomer; it flows unless vulcanised
Writing degree of polymerisation with M in kg mol⁻¹	Always convert to g mol⁻¹ to keep n dimensionless and realistic (~10³–10⁵)
Saying “addition polymerisation produces water”	Only condensation does; addition involves no by-product
Drawing Perspex repeat unit with C=C intact	Methyl methacrylate’s C=C has opened; show –CH₂–C(CH₃)(COOCH₃)–

Exam Strategy

On UACE Paper 1 (2 hr 30 min, 100 marks = 100%), allocate ≤6 minutes to this topic’s question. Read the stem for whether it is asking structure–property, named example, or equation-writing, then answer in bullet points or a labelled diagram — markers reward every correct keyword (cross-link, repeat unit, initiator, thermoplastic).

Practice Prompts

Draw the repeat unit of the polymer formed when methyl 2-methylpropenoate (methyl methacrylate) polymerises, and name the product. State one use.
Terylene is made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. Write the condensed polymerisation equation (showing the by-product) and state the type of linkage formed.

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Classification of Polymers (2)