Condensation Polymers: Polyesters and Polyamides

Polymers come in two main flavours. Addition polymers — like polyethene, polypropene and PVC — are made from alkene monomers by opening C=C double bonds and joining them together; no by-product is formed. Condensation polymers are made by joining monomers via amide or ester linkages, losing a small molecule (usually water) at each linkage. The difference in chemistry has huge consequences for how the final polymer behaves: condensation polymers are almost always biodegradable or hydrolysable, while addition polymers are notoriously persistent in the environment.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.2.5 (a)–(b): structure, formation and uses of polyesters and polyamides.

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1. Condensation vs Addition Polymers — The Big Picture

1.1 Addition polymer review (from AS/GCSE)

$n \cdot CH_2 = CHR \longrightarrow -(CH_2-CHR)_n-$n⋅CH2​=CHR⟶−(CH2​−CHR)n​−

• Monomer: an alkene.
• Reaction: C=C opens and joins to the next monomer.
• By-product: None.
• Repeat unit: Identical to the monomer with the C=C now a C–C single bond.
• Examples: poly(ethene), poly(propene), PVC, polystyrene, PTFE.

1.2 Condensation polymer overview

• Monomers: Two monomers, each with two functional groups (a difunctional monomer). These can be a diol + dicarboxylic acid, a diamine + dicarboxylic acid, or a single monomer that has both groups (e.g. an amino acid).
• Reaction: The two monomers join by forming a new bond (ester or amide), eliminating a small molecule (water or HCl).
• By-product: Yes — water (or HCl if using an acyl chloride).
• Repeat unit: Contains one residue of each monomer, with the new ester/amide linkage in between.

graph LR
  A[Addition polymer] --> B[C=C opens, no by-product]
  A --> C[Polyethene, PVC, PTFE]
  D[Condensation polymer] --> E[Ester or amide forms, loses H2O]
  D --> F[Polyester, polyamide, polypeptides]

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2. Polyesters

A polyester has a repeating ester linkage –CO–O– in its backbone.

2.1 Monomer combinations

Polyesters can be made from:

• A diol + a dicarboxylic acid (loses water), or
• A diol + a diacyl chloride (loses HCl, goes faster), or
• A single monomer with both an –OH and a –COOH group (e.g. hydroxycarboxylic acids — loses water).

2.2 Terylene (polyethylene terephthalate, PET)

The most famous polyester is Terylene, also known as PET. It is made from:

• Monomer 1: Benzene-1,4-dicarboxylic acid (terephthalic acid) — HOOC–C₆H₄–COOH.
• Monomer 2: Ethane-1,2-diol (ethylene glycol) — HO–CH₂CH₂–OH.

Each ester linkage forms by condensation of one –COOH with one –OH, eliminating water:

$n HOOC-C_6H_4-COOH + n HO-CH_2CH_2-OH \longrightarrow -[CO-C_6H_4-CO-O-CH_2CH_2-O]_n- + 2n H_2O$nHOOC−C6​H4​−COOH+nHO−CH2​CH2​−OH⟶−[CO−C6​H4​−CO−O−CH2​CH2​−O]n​−+2nH2​O

The repeat unit is –CO–C₆H₄–CO–O–CH₂CH₂–O–, with one residue from each monomer and two ester linkages per repeat.

Uses of PET:

• Clothing fibres (Terylene, Dacron)
• Drinks bottles (PET is the number "1" recycled plastic)
• Photographic film
• Food packaging

Exam Tip: When asked to draw the repeat unit of PET, draw it with brackets showing where it repeats, and explicitly mark the "–O" at one end and "–CO" at the other so the examiner can see the polymer continues on both sides.

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3. Polyamides

A polyamide has a repeating amide linkage –CO–NH– in its backbone. The same logic as polyesters applies: two difunctional monomers join, losing water.

3.1 Monomer combinations

• Diamine + dicarboxylic acid (or diacyl chloride), or
• A single amino acid monomer (gives a peptide chain).

3.2 Nylon-6,6

Nylon-6,6 is the classic synthetic polyamide, made from:

• Monomer 1: Hexanedioic acid (adipic acid) — HOOC–(CH₂)₄–COOH. Six carbons.
• Monomer 2: Hexane-1,6-diamine — H₂N–(CH₂)₆–NH₂. Six carbons.

Hence the name "nylon-6,6" — six carbons in each monomer.

$n HOOC-(CH_2)_4-COOH + n H_2N-(CH_2)_6-NH_2 \longrightarrow -[CO-(CH_2)_4-CO-NH-(CH_2)_6-NH]_n- + 2n H_2O$nHOOC−(CH2​)4​−COOH+nH2​N−(CH2​)6​−NH2​⟶−[CO−(CH2​)4​−CO−NH−(CH2​)6​−NH]n​−+2nH2​O

The repeat unit is –CO–(CH₂)₄–CO–NH–(CH₂)₆–NH–.

Uses of nylon:

• Textile fibres (hosiery, carpets, parachutes)
• Engineering plastics (gears, bearings)
• Fishing lines, ropes, ties

3.3 Kevlar

Kevlar is a polyamide with an aromatic twist, made from:

• Monomer 1: Benzene-1,4-dicarboxylic acid — HOOC–C₆H₄–COOH.
• Monomer 2: Benzene-1,4-diamine — H₂N–C₆H₄–NH₂.

$n HOOC-C_6H_4-COOH + n H_2N-C_6H_4-NH_2 \longrightarrow -[CO-C_6H_4-CO-NH-C_6H_4-NH]_n- + 2n H_2O$nHOOC−C6​H4​−COOH+nH2​N−C6​H4​−NH2​⟶−[CO−C6​H4​−CO−NH−C6​H4​−NH]n​−+2nH2​O