- Explain monomer, polymer, repeat unit, degree of polymerisation and stereoregularity, and calculate n and M.
- Tell polymerisation from polycondensation by the structure of the monomer and write their equations.
- Know the monomer, repeat unit, functional group and uses of the plastics, rubbers and fibres in the DİM programme.
- Calculate the mass and volume of monomer with a yield, the mass fraction of an element and the water released in polycondensation.
A water bottle, nylon tights, a car tyre, a plastic water pipe and a non-stick Teflon pan are all polymers: giant molecules formed when thousands of small molecules join into one chain. Chemists call them macromolecular (high-molecular) compounds. The 2026 DİM entrance exams had three tasks on it: a polymer’s functional group and type of reaction, the peptide bond, and the polymer of a CₓH₆ monomer. This lesson covers every polymer of the DİM programme.
Monomer, polymer, degree of polymerisation
Monomer: the low-molecular substance from which a polymer is made (CH₂=CH₂). Polymer: a substance made of macromolecules in which the same group of atoms repeats many times: –[CH₂–CH₂]ₙ–. The repeating group is the repeat (structural) unit, and the number of units n is the degree of polymerisation.
In a polymer sample the macromolecules have different lengths, so the molar mass and the degree of polymerisation are average values; polymers soften over a temperature range instead of melting sharply. In polymerisation the unit has the composition of the monomer (C₂H₄ → –C₂H₄–); in polycondensation it is lighter by the water split off.
- ndegree of polymerisation — the number of units (not the amount n(X))
- M(polymer)average molar mass of the polymer, g/mol
- M(unit)molar mass of the repeat unit, g/mol
Degree of polymerisation; conversely, M(polymer) = n · M(unit)
1) Polyethylene has an average molar mass of 42 000 g/mol. Find the degree of polymerisation.
2) The degree of polymerisation of polyvinyl chloride is 800. Calculate its average molar mass.
3) How many C atoms are in one macromolecule of polystyrene with M = 104 000 g/mol?
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2) Unit –CH₂–CHCl– (C₂H₃Cl): M = 24 + 3 + 35.5 = 62.5 g/mol → M(polymer) = 800 · 62.5 = 50 000 g/mol.
3) Unit –CH₂–CH(C₆H₅)– (C₈H₈): M = 8 · 12 + 8 = 104 g/mol → n = 104 000 / 104 = 1000. Each unit has 8 carbon atoms: 8 · 1000 = 8000.
The regular arrangement in space of the side groups of the chain (–CH₃ in polypropylene, –Cl in PVC, –C₆H₅ in polystyrene) — all on one side or alternating. Regular chains pack tightly and form crystalline regions, which raises density, strength and softening temperature. An irregular polymer is amorphous and weaker. Stereoregular polymers are made with special catalysts; in natural rubber all units have the cis structure.
Polymerisation and polycondensation
Monomer molecules with a multiple bond (C=C) joining together with no by-product: nCH₂=CH–CH₃ → –[CH₂–CH(CH₃)]ₙ–. The π bond of the double bond breaks, and the free valences form σ bonds with neighbouring molecules. The polymer has the same elemental composition as the monomer.
Monomers with at least two functional groups in the molecule (–OH, –COOH, –NH₂) joining into a polymer while a small by-product molecule (usually H₂O) is released: nH₂N–(CH₂)₆–COOH → –[NH–(CH₂)₆–CO]ₙ– + nH₂O. The composition of the polymer differs from that of the monomer.
| Feature | Polymerisation | Polycondensation |
|---|---|---|
| Monomer | has C=C: ethylene, styrene, isoprene | two functional groups: amino acid, dibasic acid + diol, phenol + formaldehyde |
| By-product | none | H₂O (sometimes HCl, NH₃) |
| Composition of the unit | same as the monomer | differs from the monomers |
| Main chain | carbon atoms only | O or N as well as C: –CO–O–, –CO–NH– |
- 1Rebuild the monomer
Put the double bond back between the two chain carbons of the unit: –CH₂–CHCl– → CH₂=CHCl.
- 2Look at the main chain
Carbon atoms only means polymerisation; –CO–O– or –CO–NH– inside the chain means polycondensation.
- 3Ignore the side group
A functional group in a side group does not change the type of reaction: the ester group of PMMA is in the side group, yet PMMA is made by polymerisation.
Which substances (pairs) undergo polycondensation?
I. terephthalic acid and ethylene glycol
II. propene
III. 7-aminoheptanoic acid
IV. methyl methacrylate
V. phenol and formaldehyde
A) I, II, III B) I, III, V C) II, IV D) III, IV, V E) V only
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I: two –COOH and two –OH → lavsan + H₂O; III: –NH₂ and –COOH → enant + H₂O; V: phenol + formaldehyde → phenol-formaldehyde resin + H₂O.
II and IV contain a C=C bond, so they polymerise (polypropylene, organic glass).
Correct answer: B.
A CₓH₃Cl molecule contains one double bond. Which polymer does this compound form?
A) –[CH₂–CH₂]ₙ– B) –[CH₂–CH(CH₃)]ₙ– C) –[CH₂–CHCl]ₙ– D) –[CF₂–CF₂]ₙ– E) –[CH₂–C(Cl)=CH–CH₂]ₙ–
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The monomer is vinyl chloride CH₂=CHCl. In polymerisation the π bond breaks: nCH₂=CHCl → –[CH₂–CHCl]ₙ– (polyvinyl chloride).
Option E is the polymer of chloroprene (C₄H₅Cl, two double bonds).
Correct answer: C.
Plastics
Plastics are polymer materials that can be shaped when heated. Macromolecules can be linear, branched (polyethylene made at high pressure) or network (cross-linked). Thermoplastics (PE, PP, PVC, PS, PMMA, Teflon, PET) are linear or branched: they soften on heating, harden on cooling and can be recycled. In thermosets (cured phenol-formaldehyde resin, ebonite) the chains are joined into a network: they do not soften and decompose at high temperature.
| Polymer | Monomer | Repeat unit | Properties and uses |
|---|---|---|---|
| Polyethylene (PE) | CH₂=CH₂ | –[CH₂–CH₂]ₙ– | film, bags, bottles, pipes |
| Polypropylene (PP) | CH₂=CH–CH₃ | –[CH₂–CH(CH₃)]ₙ– | stronger than PE; pipes, food boxes, ropes |
| Polyvinyl chloride (PVC) | CH₂=CHCl | –[CH₂–CHCl]ₙ– | water pipes, window frames, linoleum |
| Teflon (PTFE) | CF₂=CF₂ | –[CF₂–CF₂]ₙ– | resists even aqua regia; pan coatings |
| Polystyrene (PS) | C₆H₅–CH=CH₂ | –[CH₂–CH(C₆H₅)]ₙ– | foam packaging, disposable cups |
| Polymethyl methacrylate (organic glass) | CH₂=C(CH₃)–COOCH₃ | –[CH₂–C(CH₃)(COOCH₃)]ₙ– | transparent shatter-resistant «glass»; ester group in the side group |
| Phenol-formaldehyde resin | C₆H₅OH + HCHO | –[C₆H₃(OH)–CH₂]ₙ– | polycondensation, a thermoset; switches, pan handles |
Phenol-formaldehyde resin: nC₆H₅OH + nHCHO → –[C₆H₃(OH)–CH₂]ₙ– + nH₂O. Formaldehyde links the benzene rings with –CH₂– bridges; excess formaldehyde also links the chains, and the resin becomes a network.
- ω(E)mass fraction of element E in the polymer
- knumber of E atoms in one repeat unit
- M(unit)molar mass of the repeat unit, g/mol
The mass fraction is calculated from the unit and does not depend on n
1) Calculate the mass fraction (%) of chlorine in polyvinyl chloride.
2) Calculate the mass fraction (%) of fluorine in Teflon. Ar(F) = 19
3) A synthetic rubber has one Cl atom per unit, ω(Cl) = 40.1%. Formula of the unit and name of the rubber?
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2) C₂F₄: M = 24 + 4 · 19 = 100 g/mol → ω(F) = 76 / 100 · 100% = 76%.
3) M(unit) = 35.5 / 0.401 ≈ 88.5 g/mol; the hydrocarbon part is 88.5 − 35.5 = 53 → C₄H₅. The unit is C₄H₅Cl: the monomer is chloroprene CH₂=C(Cl)–CH=CH₂ and the polymer is chloroprene rubber –[CH₂–C(Cl)=CH–CH₂]ₙ–.
- ηyield of the polymerisation (as a fraction: 80% = 0.8)
- mmass, kg or g
Polymerisation has no by-product: in theory 1 kg of monomer gives 1 kg of polymer
1) With an 80% yield, what volume of ethylene (STP, m³) is needed to make 5.6 kg of polyethylene?
2) Vinyl chloride is made from acetylene: C₂H₂ + HCl → CH₂=CHCl (100% yield). With an 80% polymerisation yield, what volume of acetylene (STP, m³) is needed for 25 kg of PVC?
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2) m(CH₂=CHCl) = 25 / 0.8 = 31.25 kg → n = 31.25 / 62.5 = 0.5 kmol. By the equation n(C₂H₂) = 0.5 kmol → V = 0.5 · 22.4 = 11.2 m³.
Rubbers and vulcanised rubber
Natural rubber comes from the sap (latex) of the Hevea tree. It is a stereoregular polymer of isoprene (2-methylbuta-1,3-diene), cis-polyisoprene: nCH₂=C(CH₃)–CH=CH₂ → –[CH₂–C(CH₃)=CH–CH₂]ₙ–. Dienes add in the 1,4 positions: a new double bond forms in the middle of the unit, so rubber decolourises bromine water. The trans polymer of the same monomer, gutta-percha, is not elastic.
Synthetic rubbers. The first synthetic rubber made on an industrial scale came from S. V. Lebedev’s method: butadiene (buta-1,3-diene, divinyl) is made from ethanol and then polymerised with sodium as the catalyst:
2C₂H₅OH →(Al₂O₃, ZnO, t) CH₂=CH–CH=CH₂ + 2H₂O + H₂
nCH₂=CH–CH=CH₂ → –[CH₂–CH=CH–CH₂]ₙ– (butadiene rubber)
Isoprene rubber resembles natural rubber; chloroprene rubber (monomer CH₂=C(Cl)–CH=CH₂) resists petrol and oils; copolymerising butadiene with styrene gives butadiene–styrene rubber.
Heating rubber with sulfur. At the double bonds sulfur atoms link neighbouring chains with –S–S– bridges, forming a loose network. A little sulfur (a few percent) gives strong, elastic vulcanised rubber (tyres, hoses); a lot of sulfur (about 30% or more) gives hard, non-elastic ebonite.
54 kg of buta-1,3-diene was made from ethanol by Lebedev’s method (80% yield).
(1) Mass (kg) of ethanol used?
(2) By the equation, what volume of hydrogen (STP, m³) forms with the butadiene?
(3) The rubber has an average molar mass of 162 000 g/mol. Degree of polymerisation?
Mr(C₂H₅OH) = 46, Mr(C₄H₆) = 54
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(1) n(C₄H₆) = 54 / 54 = 1 kmol. In theory 2 kmol of ethanol are needed, in practice 2 / 0.8 = 2.5 kmol → m = 2.5 · 46 = 115 kg.
(2) 1 mol of H₂ is released per mol of butadiene: n(H₂) = 1 kmol → V = 22.4 m³.
(3) Unit –CH₂–CH=CH–CH₂– (C₄H₆, M = 54): n = 162 000 / 54 = 3000.
Fibres
Fibres are polymers in the form of long, thin, strong threads. Natural fibres: cotton, flax (cellulose), wool, natural silk (proteins). Artificial fibres are made by chemically processing a natural polymer: viscose and acetate fibre from cellulose (acetate is an ester of cellulose with acetic acid). Synthetic fibres are synthetic polymers: lavsan (a polyester), kapron, enant and nylon (polyamides).
| Fibre | Monomer(s) | Repeat unit | Group |
|---|---|---|---|
| Lavsan (PET) | HOOC–C₆H₄–COOH + HO–CH₂–CH₂–OH | –[CO–C₆H₄–CO–O–CH₂–CH₂–O]ₙ– | ester –CO–O– |
| Kapron (nylon-6) | H₂N–(CH₂)₅–COOH | –[NH–(CH₂)₅–CO]ₙ– | amide –CO–NH– |
| Enant (nylon-7) | H₂N–(CH₂)₆–COOH | –[NH–(CH₂)₆–CO]ₙ– | amide –CO–NH– |
| Nylon-6,6 | HOOC–(CH₂)₄–COOH + H₂N–(CH₂)₆–NH₂ | –[CO–(CH₂)₄–CO–NH–(CH₂)₆–NH]ₙ– | amide –CO–NH– |
Lavsan: nHOOC–C₆H₄–COOH + nHO–CH₂–CH₂–OH → –[CO–C₆H₄–CO–O–CH₂–CH₂–O]ₙ– + 2nH₂O. School textbooks present kapron as a polyamide made by polycondensation of ε-aminocaproic acid: nH₂N–(CH₂)₅–COOH → –[NH–(CH₂)₅–CO]ₙ– + nH₂O; industry makes it from the cyclic amide of this acid, caprolactam.
- kwater molecules released per repeat unit: kapron, enant, phenol-formaldehyde resin — 1; lavsan, nylon — 2
- 18M(H₂O), g/mol
n(H₂O) = k · n(unit); strictly kn − 1 molecules, negligible for large n
1) 38.4 kg of lavsan was made. Masses (kg) of water released and of terephthalic acid and ethylene glycol used? Mr(C₈H₆O₄) = 166, Mr(C₂H₆O₂) = 62
2) 25.4 kg of enant was made. Masses (kg) of water released and of 7-aminoheptanoic acid used?
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m(terephthalic acid) = 0.2 · 166 = 33.2 kg; m(ethylene glycol) = 0.2 · 62 = 12.4 kg. Check: 33.2 + 12.4 = 38.4 + 7.2.
2) Unit C₇H₁₃NO: M = 127 g/mol → n = 25.4 / 127 = 0.2 kmol; k = 1 → m(H₂O) = 0.2 · 18 = 3.6 kg; m(acid) = 0.2 · 145 = 29 kg.
- Ester group –CO–O–: lavsan (in the main chain), PMMA (in the side group), acetate fibre.
- Amide (peptide) group –CO–NH–: kapron, enant, nylon and proteins (see «Amines, amino acids and proteins»).
- Ether link C–O–C: starch and cellulose (between glucose residues).
- No heteroatoms: PE, PP, PS, butadiene and isoprene rubbers.
Which polymer that contains ester groups is made by polymerisation?
A) lavsan B) nylon C) polymethyl methacrylate D) polystyrene E) phenol-formaldehyde resin
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Nylon has amide groups; polystyrene has no heteroatoms; phenol-formaldehyde resin has no ester groups.
Correct answer: C.
Key points
- A polymer consists of a repeat unit repeated n times: n = M(polymer) / M(unit); M and n are average values.
- Polymerisation: monomers with C=C, no by-product (PE, PP, PVC, Teflon, PS, PMMA, rubbers). Polycondensation: monomers with two functional groups, water released (lavsan, kapron, enant, nylon, phenol-formaldehyde resin).
- Ester group: lavsan (polycondensation) and PMMA (polymerisation); amide group: kapron, enant, nylon and proteins; no heteroatoms: PE, PP, PS, butadiene and isoprene rubbers.
- Natural rubber is stereoregular cis-polyisoprene; Lebedev made butadiene from ethanol; vulcanisation turns rubber into soft vulcanised rubber (little S) or ebonite (much S).
- Thermoplastics (linear, branched) soften on heating and can be recycled; thermosets (network) do not soften. Recycling codes: 1 PET … 6 PS, 7 others.
Check yourself
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