Biomolecules and Polymers

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Biomolecules are organic compounds (C, H, O, N, P, S) made by living cells — chiefly carbohydrates, proteins, lipids, and nucleic acids. Polymers are large macromolecules built by joining many small monomers through covalent bonds, either by addition (no by-product) or condensation (eliminates H₂O or HCl). The degree of polymerisation is given by DP = M(polymer) ÷ M(monomer), and every condensation linkage loses 18 g mol⁻¹ of water.

Four biomolecule families: carbohydrates, proteins, lipids (not true polymers), nucleic acids.
Two polymerisation routes: addition (e.g. polythene from CH₂=CH₂) vs condensation (e.g. nylon-6,6, terylene, bakelite).
High-yield MCQ traps: lipids are esters, not polymers; rubber needs vulcanisation with sulphur; nylon-6,6 needs two different monomers.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Monomers and Bonds

Each biomolecule class has a characteristic monomer and a characteristic linkage. Monosaccharides (e.g. glucose, C₆H₁₂O₆, M = 180 g mol⁻¹) join by glycosidic bonds to form disaccharides (sucrose, maltose, lactose — C₁₂H₂₂O₁₁) and polysaccharides such as starch and cellulose, whose empirical repeat unit is (C₆H₁₀O₅)ₙ. Amino acids join through peptide bonds (–CO–NH–) to give polypeptides and proteins. Nucleotides link via phosphodiester bonds to build DNA and RNA. Triglycerides form when glycerol reacts with three fatty acids through ester bonds — note that this produces a small molecule, not a repeating-monomer chain, so lipids are not classed as true polymers.

Polymerisation Mechanisms

In addition (chain-growth) polymerisation, unsaturated monomers like ethylene, vinyl chloride, and styrene open their C=C bond under a free-radical initiator (initiation → propagation → termination). No small molecule is lost, so the repeat unit retains the monomer’s atoms: polyethylene is –(CH₂–CH₂)ₙ– with M(monomer) = 28 g mol⁻¹.

In condensation (step-growth) polymerisation, bifunctional monomers react with loss of H₂O, HCl, or another small molecule. Nylon-6,6 forms from adipic acid (HOOC–(CH₂)₄–COOH) and hexamethylenediamine (H₂N–(CH₂)₆–NH₂); terylene/Dacron from terephthalic acid + ethylene glycol; bakelite from phenol + formaldehyde. Each linkage formed means 18 g mol⁻¹ of water is eliminated, which is why M(polymer) < n × M(monomer).

Class	Monomer(s)	Linkage	Example product
Polysaccharide	Glucose	Glycosidic	Starch, cellulose
Protein	α-amino acids	Peptide (–CO–NH–)	Casein, enzymes
Nucleic acid	Nucleotides	Phosphodiester	DNA, RNA
Lipid (ester)	Glycerol + fatty acid	Ester	Triglyceride
Addition polymer	Alkene	C–C (no by-product)	PVC, polythene
Condensation polymer	Diacid + diamine/diol	Amide/ester (–H₂O)	Nylon-6,6, terylene

Exam Pattern Pointers

ECAT Chemistry has ~3% weight on this cluster, usually yielding 1–2 conceptual MCQs. Expect questions that ask you to identify the polymerisation type from a given monomer, pick the correct linkage (peptide, glycosidic, ester, phosphodiester), or name the monomers of nylon, terylene, or bakelite. Numerical items are rarer but may ask DP or mass-loss in a condensation reaction.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Worked Example — Degree of Polymerisation

A polyethylene sample has M(polymer) = 56,000 g mol⁻¹ and M(monomer ethylene) = 28 g mol⁻¹.

DP = 56,000 ÷ 28 = 2000. So the chain contains roughly 2000 repeat units of –CH₂–CH₂–. The same formula works for any addition polymer. For condensation polymers, divide by (M(monomer) − 18) because water (18 g mol⁻¹) leaves each linkage; for nylon-6,6 the effective repeat-unit mass is M(adipic acid) + M(hexamethylenediamine) − 18 = 146 + 116 − 18 = 244 g mol⁻¹.

Classification of Plastics by Behaviour

Thermoplastics (PVC, polythene) are linear or lightly branched, soften on heating, and can be remoulded. Thermosets (bakelite, melamine) have extensive cross-links set during curing; further heating decomposes them rather than melting. Elastomers (natural rubber) have sparse cross-links that allow reversible stretching — introduced by vulcanisation with 1–3% sulphur, which bridges polyisoprene chains at the allylic positions.

Common Mistakes to Avoid

Confusing polythene’s formula — it is –(CH₂–CH₂)ₙ– or (C₂H₄)ₙ, never simply C₂H₄ (that is ethylene, the monomer).
Calling lipids polymers — they are tri-esters, not macromolecular chains of one repeating unit.
Forgetting that condensation loses mass — every linkage formed releases H₂O (18 g mol⁻¹), so DP ≠ M(polymer)/M(monomer) for step-growth polymers.
Treating rubber as already tough — raw latex is soft and sticky until sulphur cross-links form during vulcanisation.

Practice Prompts

Identify the polymerisation type and write the repeat unit for the polymer formed from CH₂=CH–Cl.
Nylon-6,6 is made from adipic acid and hexamethylenediamine. Calculate the mass of water released when 1.00 mol of each monomer forms the repeating amide linkage.

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Biomolecules and Polymers