CONDENSATION POLYMERISATION is a type of polymerisation in which TWO DIFFERENT monomers, each with TWO functional groups (one at each end), join together with the LOSS OF A SMALL MOLECULE (usually water) at each new linkage. Many monomers join β long polymer chain + many small molecules.
(Paper 2 content for 4CH1 β designated P statements.)
The two functional groups required.
For condensation polymerisation, each monomer needs to react on BOTH ends β it must have TWO functional groups (one at each end of the molecule).
| Polymer type | Monomer 1 | Monomer 2 | Linkage formed | Small molecule lost |
|---|
| Polyester | Dicarboxylic acid (HOOC-X-COOH) | Diol (HO-Y-OH) | Ester (βCOOβ) | Water (HβO) |
| Polyamide | Dicarboxylic acid (HOOC-X-COOH) | Diamine (HβN-Y-NHβ) | Amide (βCONHβ) | Water (HβO) |
General polyester equation.
nHOOC-X-COOH+nHO-Y-OHββ(CO-X-COO-Y-O)nββ+2nH2βO
Each ester linkage requires:
- The βOH of the acid (provides the OH of water).
- The βH of the alcohol (provides the H of water).
- The remaining C(=O)βO bonds together as the ester linkage.
Each repeat unit has TWO ester linkages (one between the diacid and the diol on each side) β 2n water molecules eliminated for n diacid + n diol.
Specific example: Terylene (PET).
Terylene is the most common polyester, also called PET (poly(ethylene terephthalate)).
- Monomer 1: benzene-1,4-dicarboxylic acid (terephthalic acid) β HOOC-CβHβ-COOH (where CβHβ is the benzene ring with the two COOH groups in para position).
- Monomer 2: ethane-1,2-diol (ethylene glycol) β HO-CHβ-CHβ-OH.
Equation:
nHOOC-C6βH4β-COOH+nHO-CH2βCH2β-OHββ(CO-C6βH4β-COO-CH2βCH2β-O)nββ+2nH2βO
Repeat unit:
βCOβCβHββCOOβCHβCHββOβ (Γ n times)
βββ acid half βββ ββ alcohol half ββ
The benzene ring (from terephthalic acid) alternates with the ethylene glycol units, joined by ester linkages.
Uses of Terylene / PET.
- Clothing fibres: Terylene (UK name) is used in shirts, suits, trousers β often blended with cotton. Durable, easy-care, wrinkle-resistant, holds dye well.
- PET drinks bottles: clear, lightweight, shatter-resistant, BPA-free β for water, soft drinks, juice. Recycling code 1.
- Photographic film: Mylar (a PET film).
- Sails for yachts: high tensile strength, doesn't stretch.
- Conveyor belts and reinforcement fibres: heavy-duty applications.
- Polyester thread: sewing thread.
The fibre form is called Terylene (also Dacron in USA); the bottle form is called PET. Same chemical, different physical form depending on processing.
General polyamide equation.
nHOOC-X-COOH+nH2βN-Y-NH2βββ(CO-X-CONH-Y-NH)nββ+2nH2βO
Each amide linkage (βCONHβ) forms with loss of water:
- The βOH of the acid + the βH of the amine = HβO.
- The remaining βCOβ bonds to βNHβ to form the amide.
Specific example: Nylon-6,6.
The number '6,6' refers to the carbon counts in the two monomers β both have 6 carbons.
- Monomer 1: hexanedioic acid (adipic acid) β HOOC-(CHβ)β-COOH (6 carbons including the two COOH).
- Monomer 2: hexane-1,6-diamine β HβN-(CHβ)β-NHβ (6 carbons + 2 N).
Equation:
nHOOC-(CH2β)4β-COOH+nH2βN-(CH2β)6β-NH2βββ(CO-(CH2β)4β-CONH-(CH2β)6β-NH)nββ+2nH2βO
Uses of Nylon.
- Clothing: strong, durable synthetic fibre β used in stockings, swimwear, sportswear, jackets.
- Ropes and lines: high tensile strength + low stretch β fishing line, climbing rope, parachute cord.
- Carpets: wear-resistant synthetic fibre.
- Tyre cords: reinforcement in car/truck tyres.
- Toothbrush bristles: stiff but flexible.
- Engineering plastics: nylon gears, bearings (low friction, durable).
Nylon was invented in the 1930s by Wallace Carothers at DuPont β one of the great industrial chemistry breakthroughs of the 20th century.
Comparison: addition vs condensation polymerisation.
| Feature | Addition | Condensation |
|---|
| Monomer types | ONE (alkene) | TWO (each with 2 functional groups) |
| Functional group on monomer | C=C double bond | βCOOH + βOH (polyester) or βCOOH + βNHβ (polyamide) |
| Bond breaking | Ο bond of C=C | O-H of acid + H of alcohol/amine |
| Bond forming in polymer | CβC single | Ester (βCOOβ) or amide (βCONHβ) |
| Byproduct | NONE | Small molecule (usually HβO) per linkage |
| Atom economy | 100% | < 100% (atoms lost as water) |
| Polymer backbone | All carbon | Carbon + heteroatom links (O or N) |
| Biodegradable? | Usually NO (no functional groups for microbes) | Often YES (ester/amide can be hydrolysed) |
| Recyclable? | Yes (melt-and-mould thermoplastics) | Yes (can be chemically hydrolysed back to monomers + recycled) |
| Examples | PE, PP, PVC, PTFE, polystyrene | Terylene/PET, Nylon, polylactic acid, proteins, DNA, cellulose |
Natural condensation polymers.
Many of the most important biological molecules are CONDENSATION POLYMERS:
-
Proteins: amino acids join via PEPTIDE BONDS (a type of amide bond, βCONHβ) with loss of water. Each peptide bond forms between the βCOOH of one amino acid and the βNHβ of another. Proteins are essentially POLYAMIDES with side chains of varying chemistry.
-
DNA / RNA: nucleotides join via PHOSPHODIESTER BONDS with loss of water. Each phosphodiester bond is a double-ester between the phosphate group of one nucleotide and the hydroxyl of another.
-
CELLULOSE (plant cell walls): glucose units join via GLYCOSIDIC BONDS β each bond between the βOH of one glucose and the βOH of another, with loss of water. Cellulose is a condensation polymer of glucose.
-
STARCH (energy storage in plants): same as cellulose chemically (glucose monomers + glycosidic bonds + water lost) but different geometry.
-
PROTEINS-in-detail: silk, wool, gelatin, collagen β all natural polyamides made by living organisms.
So the chemistry of condensation polymerisation is universal β it operates both in industrial polyester / nylon factories AND in every living cell forming proteins, DNA, and structural carbohydrates.
Biodegradability β why condensation polymers can be greener.
The ester (βCOOβ) and amide (βCONHβ) linkages in condensation polymers can be HYDROLYSED by water, acid, alkali, or enzymes (e.g. esterases, proteases in microorganisms). This means:
- Some condensation polymers naturally biodegrade in the environment over months to years (e.g. polylactic acid PLA, polyhydroxyalkanoates PHA).
- Recycling can be done by CHEMICAL HYDROLYSIS β break the polymer back to its monomers, then re-polymerise.
- Biological polymers (proteins, DNA, cellulose) are inherently biodegradable β microbes have evolved enzymes to break them down.
Addition polymers (all-CβC backbone, no functional groups) lack this advantage β much harder to biodegrade or recycle chemically.
Polylactic acid (PLA) β a modern green polymer.
PLA is made from LACTIC ACID (CHβCH(OH)COOH), which has both βCOOH AND βOH on the SAME molecule. It can polymerise with itself:
nCH3βCH(OH)COOHββ(CH3βCH-COO)nββ+nH2βO
Lactic acid comes from FERMENTATION of corn starch or sugar cane β renewable!
PLA is:
- Bioderived β from plants, not crude oil.
- Biodegradable β composts in 6 months in industrial composters (slower in normal landfill).
- Versatile β used for food packaging (clear cups, biodegradable cutlery), 3D printing filament, medical sutures (dissolves in the body).
PLA is one of the leading 'green' polymer alternatives to traditional petrochemical plastics.
Edexcel exam tip.
Paper 2 questions on condensation polymerisation typically:
- Ask you to identify the two monomer types (diacid + diol for polyester; diacid + diamine for polyamide).
- Show a generic equation with the small molecule eliminated.
- Compare with addition polymerisation.
- Give an example polymer and its use.
Mark scheme keywords:
- 'Two different monomers'.
- 'Each monomer has two functional groups'.
- 'Small molecule eliminated' or 'water released'.
- 'Ester linkage (βCOOβ)' for polyesters.
- 'Amide linkage (βCONHβ)' for polyamides.
Use these in your answers.