# Addition polymers

> Chemistry · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9701-u24-addition-polymers/

This sub-topic covers the formation, structure, drawing of repeating units, identification of monomers, and common uses of addition polymers, a core organic topic regularly assessed in CIE A-Level Chemistry papers.

**Prerequisites:** [Alkene structure and addition reactions](https://www.owlsprep.com/study/cie-9701-u14-alkenes/)

## Learning objectives

- Describe the formation of addition polymers from alkene monomers
- Draw repeating units of addition polymers from given monomers
- Identify the original monomer from the repeating unit of an addition polymer
- Recall common uses of major commercial addition polymers

## 1. What is Addition Polymerization?

**Addition Polymerization** — A reaction where many small alkene-based monomer molecules add together to form a large polymer chain, with no small by-products produced.

*Example:* Ethene monomers joining to form polyethene

Unlike condensation polymerization, addition polymerization only requires monomers with at least one carbon-carbon double bond. All atoms from the original monomers are retained in the final polymer, with no elimination of small molecules like water or hydrogen chloride.

**Worked example:** Show the formation of polypropene from propene monomers and write its repeating unit.

1. 1. Start with the structure of the propene monomer:
2. $$CH_2=CH(CH_3)$$
3. 2. Break the carbon-carbon double bond to create open bonding sites on each carbon:
4. $$\bullet CH_2-CH(CH_3) \bullet$$
5. 3. Link the activated units together repeatedly to form a long polymer chain:
6. $$...-CH_2-CH(CH_3)-CH_2-CH(CH_3)-CH_2-CH(CH_3)-...$$
7. 4. Draw the simplified repeating unit for exam use:
8. $$\ce{-[CH2-CH(CH3)]_n-}$$

> **Exam tip:** Always draw bonds extending through the brackets of a repeating unit to show it connects to adjacent units. Examiners regularly penalize missing outer bonds.

## 2. Drawing Repeating Units From Monomers

For any alkene monomer with the general formula $CH_2=CHR$, where R is any substituent group (alkyl, aryl, halogen etc.), the repeating unit follows a consistent pattern: the double bond breaks, R stays attached to its original carbon, and the two carbons from the double bond form the backbone of the repeating unit.

> **Memory Hook**
>
> Double bond breaks, R stays put, link them out!

**Worked example:** Draw the repeating unit of poly(vinyl chloride) (PVC) from its monomer $CH_2=CHCl$.

1. 1. Locate the carbon-carbon double bond in the monomer, confirm the Cl substituent is attached to one carbon of the double bond.
2. 2. Break the double bond to form two open single bonds on the backbone carbons.
3. 3. Keep the Cl substituent attached to its original carbon, enclose the two-carbon backbone in brackets, add extending bonds and n subscript.
4. Final correct repeating unit:
5. $$\ce{-[CH2-CH(Cl)]_n-}$$

**Check your understanding**

Test your understanding:

1. What is the correct repeating unit of polystyrene, formed from monomer $CH_2=CH(C_6H_5)$?

   - A: $\ce{-[CH2-CH(C6H5)]_n-}$
   - B: $\ce{-[CH(C6H5)-CH(C6H5)]_n-}$
   - C: $\ce{-[CH2-CH2-C6H5]_n-}$

   *Why:* Correct! The phenyl group stays attached to the original carbon from the double bond.

## 3. Identifying Monomers From Repeating Units

A common exam question asks you to work backwards from a given repeating unit to find the original alkene monomer. The process is simply the reverse of polymerization: split the polymer backbone between the bonds connecting adjacent repeating units, then reform the double bond between the two carbons of the backbone.

**Worked example:** Identify the monomer of the addition polymer with repeating unit $\ce{-[CH2-C(CH3)2]_n-}$.

1. 1. Split the backbone between the carbon atoms that connect to adjacent repeating units, leaving you with the fragment $CH_2-C(CH_3)_2$.
2. 2. Reform the double bond between these two carbons, which was the original double bond in the monomer.
3. 3. The final monomer is:
4. $$CH_2=C(CH_3)_2 (2-methylpropene)$$

> **Exam tip:** The molecular formula of the monomer is identical to the molecular formula of the repeating unit for addition polymers. Use this to check your answer if you are unsure.

## 4. Common Addition Polymers and Uses

CIE exams regularly test recall of the names, monomers and common uses of the major commercial addition polymers, as shown in the table below:

| Polymer Name | Monomer Name | Common Use |
| --- | --- | --- |
| Polyethene (PE) | Ethene | Plastic bags, food packaging |
| Polypropene (PP) | Propene | Ropes, crates, synthetic fibres |
| Poly(vinyl chloride) (PVC) | Chloroethene (vinyl chloride) | Pipes, window frames, electrical insulation |
| Polystyrene (PS) | Phenylethene (styrene) | Packaging foam, disposable containers |
| Polytetrafluoroethene (PTFE) | Tetrafluoroethene | Non-stick coatings, low-friction bearings |

## Common pitfalls

- **Wrong:** Drawing a repeating unit without extending bonds through the brackets
  - Why it fails: Examiners require this to show the repeating unit connects to the rest of the chain, and will deduct marks for missing bonds
  - Correct: Always draw a single bond extending out of each side of the brackets, and add the n subscript
- **Wrong:** Moving the substituent R when drawing the repeating unit, e.g. writing polypropene as $\ce{-[CH2-CH2-CH2]_n-}$
  - Why it fails: Confuses addition polymer backbone structure with straight-chain alkanes, forgetting R stays attached to the original double bond carbon
  - Correct: Keep all substituents in their original position relative to the two double bond carbons after breaking the double bond
- **Wrong:** Stating addition polymerization produces a small by-product like water
  - Why it fails: Confuses addition polymerization with condensation polymerization, a common exam trick question
  - Correct: Addition polymerization produces no by-products; all atoms from monomers end up in the polymer chain
- **Wrong:** Identifying a monomer with more than one double bond from a 2-carbon repeating unit
  - Why it fails: Incorrectly splitting the polymer backbone over more than two carbons
  - Correct: For addition polymers from monalkene monomers, split the backbone every 2 carbons then reform the double bond between those two carbons
- **Wrong:** Writing the monomer of PTFE as $CH_2=CF_2$
  - Why it fails: Misremembering the structure of this common addition polymer
  - Correct: PTFE is made from tetrafluoroethene $CF_2=CF_2$, with all four positions on the double bond substituted with fluorine

## Cheatsheet

| Task | Step-by-Step Rule |
| --- | --- |
| Draw repeating unit from monomer | 1. Locate C=C in monomer 2. Break double bond to single 3. Add extending bonds to each end 4. Enclose in brackets, add n |
| Find monomer from repeating unit | 1. Split chain between adjacent units 2. Reform C=C between 2 backbone carbons 3. Result is the monomer |
| Key facts | No by-products formed, all monomers have C=C, molecular formula of monomer = formula of repeating unit |
| Common polymers | PE: ethene → polyethene, PVC: chloroethene → PVC, PTFE: CF₂=CF₂ → PTFE |

## What's next

Addition polymers are the foundational polymer type in the CIE A-Level polymers unit, and the core skills of drawing repeating units and identifying monomers are required for all subsequent polymer topics. Most CIE exam papers include at least one question testing these skills, so mastering this sub-topic guarantees you easy, guaranteed marks on exam day. This knowledge also sets you up to compare addition polymers with condensation polymers, the second major polymer class, and prepares you for questions on polymer properties, biodegradability and disposal which are often tested in extended response questions.

- [Condensation Polymers](https://www.owlsprep.com/study/cie-9701-u24-condensation-polymers/)
- [Amino acids, proteins and DNA](https://www.owlsprep.com/study/cie-9701-u25-overview/)
- [Amino acids](https://www.owlsprep.com/study/cie-9701-u25-amino-acids/)

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