5.5.5Unit 5. Transition Metals and Organic Nitrogen Chemistry
Polymerisation — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 35 of 43 Unit quiz
Section 1 of 5Two Ways to Build a Polymer
The type of monomer decides which route is possible.
Addition polymerisation
- Many alkene monomers join through their C=C bonds, with no other product,
- so the polymer has the same empirical formula as the monomer.
Condensation polymerisation
- Monomers with two functional groups join by forming a link and releasing a small molecule such as water or hydrogen chloride.
Repeat unit
- The smallest section that repeats along the chain;
- it is drawn in brackets with open bonds extending through the brackets at both ends.
Monomer requirement
- Addition needs a C=C double bond;
- condensation needs at least two reactive groups on each monomer, such as two –OH or two –COOH.
| Feature | Addition | Condensation |
|---|---|---|
| Monomer | Alkene with C=C | Two functional groups each |
| By-product | None | H₂O or HCl |
| Example | Poly(propene) | Polyester |
Section 2 of 5Addition Polymers
1
In the monomer the C=C double bond is made up of one sigma bond and one pi bond that can be opened.
2
Under high pressure with a catalyst, the pi bond breaks and each carbon forms a new single bond to a neighbouring monomer.
3
Repeating this many thousands of times gives a long saturated carbon chain with side groups such as –CH₃ or –Cl.
4
For propene, CH₃CH=CH₂, the repeat unit is –CH(CH₃)–CH₂–, so the polymer is poly(propene) with the formula (C₃H₆)ₙ.
5
Because the chain contains only strong C–C and C–H bonds, addition polymers are unreactive and do not biodegrade.
Section 3 of 5Condensation Polyesters
Reaction between an acid group and an alcohol group repeats at both ends of each monomer.
Monomers
- A dicarboxylic acid such as benzene-1,4-dicarboxylic acid reacts with a diol such as ethane-1,2-diol.
Ester link
- Each –COOH joins to an –OH, forming –COO– and releasing one molecule of water for every link made.
Product
- The polymer poly(ethylene terephthalate), often shortened to PET, is used in fibres and drinks bottles.
Equation form
- n HOOC–R–COOH + n HO–R′–OH → [–OC–R–COO–R′–O–]ₙ + (2n – 1) H₂O for a chain of n repeat units.
Section 4 of 5Properties and Disposal
Properties and Disposal
Addition polymers
- They are non-polar and chemically inert, so they persist in landfill unless recycled or burned for energy.
Polyesters
- The ester link can be hydrolysed by acid or alkali, so polyesters can be broken down more readily than addition polymers.
Recycling
- Sorting and reprocessing polymers reduces use of crude oil and avoids the carbon dioxide and toxic gases released by burning.
Burning
- Burning polymers such as PVC gives hydrogen chloride, so waste gases must be scrubbed to remove acidic gases.
Section 5 of 5Exam Checklist & Tips
5.5.5 – Key Points for the Exam
- Monomer A small molecule that joins to others to build a polymer.
- Repeat unit The section of the chain that repeats, shown in brackets with continuation bonds.
- Addition polymer A polymer formed by opening C=C bonds with no by-product.
- Condensation polymer A polymer formed by linking monomers and losing a small molecule each time.
- Polyester A condensation polymer whose monomers are joined by ester links.
Common Mistakes
- Leaving the double bond in the repeat unit of an addition polymer; the polymer chain is saturated.
- Writing the monomer inside the brackets instead of the repeat unit, which loses the chain ends.
- Forgetting that a condensation link releases water, so the equation is unbalanced.
- Drawing only the brackets without the extended bonds that show the chain continues.
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