Classification of Polymers

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Polymers are large molecules (macromolecules) built by linking many small monomer units through polymerisation reactions. The average number of repeat units in a chain is the degree of polymerisation (DP), given by DP = Mₙ / M₀, where Mₙ is the number-average molar mass of the polymer and M₀ is the molar mass of one repeat unit; rearranged as Mₙ = DP × M₀. % yield = (actual mass / theoretical mass) × 100.

Two principal routes exist: addition (chain-growth) polymerisation, which joins monomers containing a C=C double bond (e.g. ethene → poly(ethene), chloroethene → PVC, methyl 2-methylpropenoate → PMMA, tetrafluoroethene → PTFE) via initiation–propagation–termination using radicals, ions or Ziegler–Natta catalysts; and condensation (step-growth) polymerisation, where bifunctional monomers lose small molecules (H₂O or HCl) to give polyamides (nylon-6,6, nylon-6) and polyesters (PET).

High-yield UACE pointers: distinguish thermoplastics from thermosets, write repeat-unit structures for nylon-6,6 and PET, and explain how vulcanisation with sulphur hardens natural rubber.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Classification of Polymers

Polymers are sorted three ways. By source: natural (cellulose, starch, proteins, natural rubber, nucleic acids) vs synthetic (poly(ethene), nylon, PVC). By structure: linear (high-density poly(ethene), nylon), branched (low-density poly(ethene)), and cross-linked (vulcanised rubber, thermosets). By thermal behaviour: thermoplastics soften on heating and can be remoulded (poly(ethene), PVC, PET); thermosetting plastics harden permanently on first heating because cross-links form (phenol-formaldehyde, urea-formaldehyde, melamine resins). Elastomers stretch many times their length and recover (vulcanised rubber, neoprene).

Addition (Chain-Growth) Polymerisation

Monomers must carry a C=C bond. A free-radical initiator (e.g. benzoyl peroxide) generates a radical R•, which adds across the double bond in the initiation step. The new radical-end attacks another monomer in propagation, repeating thousands of times. Two growing chains meet in termination (combination or disproportionation), ending the chain. The same three steps apply to ionic and Ziegler–Natta (co-ordination) catalysts, the latter giving stereoregular products: isotactic (all substituents on one side), syndiotactic (alternating sides) and atactic (random).

Condensation (Step-Growth) Polymerisation

Each monomer has two (or more) reactive functional groups; every linkage expels a small molecule. Nylon-6,6 forms from 1,6-diaminohexane + hexanedioic acid, losing H₂O at each amide bond: –[NH–(CH₂)₆–NH–CO–(CH₂)₄–CO]–ₙ. Nylon-6 comes from caprolactam by ring-opening. PET arises from ethane-1,2-diol + benzene-1,4-dicarboxylic acid, producing –[O–CH₂CH₂–O–OC–C₆H₄–CO]–ₙ with H₂O loss. Phenol–formaldehyde and urea–formaldehyde resins are step-growth thermosets.

Worked Relationship

For a poly(ethene) sample with Mₙ = 56 000 g mol⁻¹, M₀ (C₂H₄) = 28 g mol⁻¹, so DP = 56 000 / 28 = 2000 repeat units.

Exam Patterns at UACE

Expect: (i) drawing repeat units from monomers, (ii) identifying polymerisation type from a given equation, (iii) comparing properties with structure, (iv) explaining vulcanisation using disulphide (–S–S–) cross-links between cis-polyisoprene chains, raising strength and elasticity.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Structure–Property Relationships

Crystallinity rises with chain regularity: HDPE is more crystalline than LDPE, giving higher density and melting point. Chain length (DP) directly raises tensile strength and viscosity. Cross-link density controls elasticity: lightly cross-linked rubbers stretch, while heavily cross-linked thermosets are rigid. Stereoregularity from Ziegler–Natta catalysis produces isotactic poly(propene) that packs tightly and crystallises, unlike atactic poly(propene) which is a tacky amorphous solid. Branching lowers density and melting point by preventing close chain packing.

Natural Polymers in Depth

Cellulose is β-1,4-linked glucose forming straight chains that hydrogen-bond into fibres (cotton, paper); starch uses α-1,4 (amylose) and α-1,4/α-1,6 (amylopectin) links, making it digestible by humans. Proteins are polyamides of α-amino acids joined by peptide bonds; their 3-D folding (secondary, tertiary, quaternary) defines enzyme function. Nucleic acids are polynucleotides of a sugar–phosphate backbone with nitrogen bases. Natural rubber is cis-1,4-polyisoprene, whose kinked chains permit great extension but slip past one another; vulcanisation with 1–5 % sulphur at ~140 °C forms short –S–S– bridges that lock chains into a network, converting the sticky latex into durable, elastic rubber used in tyres.

Reactions of Polymers

Polyesters and polyamides undergo hydrolysis (acid- or base-catalysed) breaking C–O or C–N links and regenerating monomers — a key recycling route. Saponification of PET with NaOH gives the diol and disodium terephthalate. Combustion of hydrocarbon polymers releases CO₂ and H₂O, raising environmental concerns that motivate biodegradable alternatives (polylactic acid, polyhydroxyalkanoates).

Common Traps

Confusing condensation (small molecule lost, monomers bi-/polyfunctional) with addition (no loss, C=C monomers). Writing the repeat unit of nylon-6,6 with the wrong amide orientation. Forgetting that vulcanisation uses sulphur cross-links, not sulphur atoms inserted into the backbone. Treating HDPE and LDPE as different monomers — both come from ethene; the difference is branching.

Practice Prompts

A polymer has M₀ = 84 g mol⁻¹ (monomer CH₂=CHCl) and Mₙ = 126 000 g mol⁻¹. Calculate DP and the mass of chloroethene needed to make 50 g of polymer at 80 % yield.
Draw the repeat unit of the polyester formed from HOCH₂CH₂OH and HOOC–C₆H₄–COOH, name the linkage formed, and state one property that makes this polymer suitable for drinks bottles.

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Classification of Polymers