2.4.6Additional lessons
Condensation Polymers (HL) — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 48 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Forming a Condensation Polymer
Condensation polymerisation differs from addition polymerisation because no carbon-carbon double bond is needed.
1
Each monomer carries two reactive functional groups, one at each end, so a chain can grow in both directions.
2
A group on one monomer reacts with a group on another monomer and a small molecule, usually water, is eliminated.
3
The new linkage joins the two monomers, but both ends still carry reactive groups, so the process repeats many times.
4
Because a small molecule leaves at every link, the polymer's empirical formula is not the sum of the monomers' formulas.
5
Repeated reaction builds a long chain whose repeat unit contains the linkage, such as an ester or amide group.
Section 2 of 5Polyesters and Polyamides
Polyester link
- Polyester link An ester link –COO– forms between a diol (–OH, –OH) and a dicarboxylic acid (–COOH, –COOH), releasing water at each link.
Polyamide link
- Polyamide link An amide link –CONH– forms between a diamine (–NH₂, –NH₂) and a dicarboxylic acid, again losing one water per link.
Example: PET
- Example: PET Ethane-1,2-diol and benzene-1,4-dicarboxylic acid give poly(ethylene terephthalate), a polyester widely used for bottles and fibres.
Example: nylon-6,6
- Example: nylon-6,6 Hexane-1,6-diamine and hexanedioic acid give nylon-6,6, in which each monomer contributes six carbon atoms to the chain.
Acyl chlorides
- Acyl chlorides Replacing the acid with a diacyl chloride makes the reaction faster, and HCl is released instead of water.
| Feature | Addition polymer | Condensation polymer |
|---|---|---|
| Monomer needs | C=C double bond | Two functional groups per molecule |
| By-product | None | Small molecule such as H₂O or HCl |
| Repeat unit | Same atoms as monomer | Fewer atoms than the monomers combined |
Section 3 of 5Amino Acids as Monomers
Amino Acids as Monomers
Structure
- Structure An amino acid contains both an amine group –NH₂ and a carboxyl group –COOH, so one type of molecule can polymerise alone.
Peptide bond
- Peptide bond The amide link formed between two amino acids is called a peptide bond, and the chain is a polypeptide.
Hydrolysis
- Hydrolysis Heating a condensation polymer with aqueous acid or alkali breaks the ester or amide links and regenerates the monomers.
Biodegradability
- Biodegradability Polyesters and polyamides can be broken down by hydrolysis, unlike most addition polymers with an inert carbon-only backbone.
Section 4 of 5Worked Example
Question Calculate the relative molecular mass of the repeat unit of nylon-6,6, formed from hexanedioic acid, HOOC(CH₂)₄COOH, and hexane-1,6-diamine, H₂N(CH₂)₆NH₂.
- Mᵣ of hexanedioic acid, C₆H₁₀O₄ = 146.14.
- Mᵣ of hexane-1,6-diamine, C₆H₁₆N₂ = 116.21.
- One repeat unit uses one molecule of each monomer and forms two amide links, so two H₂O (2 × 18.02 = 36.04) are lost.
- Repeat unit C₁₂H₂₂N₂O₂146.14 + 116.21 − 36.04 = 226.31.
Answer Mᵣ of the repeat unit ≈ 226.3
Section 5 of 5Exam Checklist & Tips
2.4.6 – Key Points for the Exam
- Condensation polymerisation Joining of monomers with loss of a small molecule at each new linkage.
- Repeat unit The smallest section of the chain that, when repeated, generates the whole polymer.
- Ester link The –COO– group joining a carboxylic acid residue to an alcohol residue.
- Amide link The –CONH– group joining a carboxylic acid residue to an amine residue.
- Peptide bond The amide link between two amino acid residues.
Common Mistakes
- Drawing a repeat unit with the linkage left open at both ends instead of showing the bonds that continue the chain.
- Forgetting that the small molecule is lost, so the repeat unit's mass is lower than the monomers' total.
- Mixing up which monomer supplies the carbonyl carbon when drawing an ester link.
- Saying condensation polymers need a C=C bond in the monomer.
Last updated:
\n