Polymers

1. Overview

Polymers are high-molecular-weight compounds fundamental to both modern industry (plastics, fibers) and biological life (proteins, DNA). This topic explores how small molecules (monomers) link together through chemical reactions to form long chains, and the environmental impact of these synthetic materials.

Key Definitions

• Monomer: A small, reactive molecule that can be joined together with other similar molecules to form a polymer.
• Polymer: A large molecule (macromolecule) built up from many smaller units called monomers.
• Addition Polymerisation: A reaction where many unsaturated monomers (containing C=C double bonds) join together to form a polymer as the only product.
• Condensation Polymerisation: A reaction where monomers join together to form a polymer with the elimination of a small molecule, such as water or hydrogen chloride.
• Repeat Unit: The specific part of the polymer chain that is repeated over and over.

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Core Content

Formation of Poly(ethene)

Poly(ethene) is formed through addition polymerisation. Ethene molecules (monomers) contain a C=C double bond. Under high pressure and in the presence of a catalyst, the double bond "breaks" or opens up, allowing the carbon atoms to bond to adjacent ethene molecules.

Word Equation:
ethene → poly(ethene)

Symbol Equation:
$n\text{C}_2\text{H}_4\text{(g)} \rightarrow \text{-[CH}_2\text{-CH}_2\text{]}_n\text{-(s)}$

Plastics and Disposal

Plastics are synthetic polymers. Because most plastics are non-biodegradable (cannot be broken down by microbes), their disposal presents significant environmental challenges:

• Landfill sites: Plastics take hundreds of years to decompose, leading to the rapid filling of landfill sites and loss of land.
• Accumulation in oceans: Marine animals can mistake plastic for food or become entangled in it, leading to death and the entry of microplastics into the food chain.
• Burning (Incineration): While burning plastics can produce energy, it releases toxic gases. For example, PVC releases hydrogen chloride gas ($\text{HCl}\text{(g)}$), and incomplete combustion can produce toxic carbon monoxide ($\text{CO}\text{(g)}$).

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Extended Content (Extended Curriculum Only)

Addition vs. Condensation Polymerisation

Feature	Addition Polymerisation	Condensation Polymerisation
Monomer type	Contains C=C double bond (alkenes)	Contains two functional groups per molecule
Number of products	Only the polymer	Polymer + a small molecule (e.g., $\text{H}_2\text{O}$)
Chain linkage	Carbon-carbon single bonds	Ester or Amide linkages

Deducing Structures

• From Monomer to Polymer (Addition): Replace the C=C with a C—C bond, extend bonds out the sides, and place brackets with 'n'.
• From Monomer to Polymer (Condensation): Remove an —OH from the carboxylic acid and an —H from the alcohol/amine to form water.

How to Deduce a Monomer from a Polymer (and vice versa)

Addition polymers (like poly(ethene), poly(propene)):

• Polymer → Monomer: Find the repeat unit in the brackets. Replace the single C-C bond in the backbone with a C=C double bond. Remove the continuation bonds.
• Monomer → Polymer: Open the C=C double bond to form two single bonds. These become the continuation bonds that link to the next repeat unit.

Condensation polymers (like nylon, PET):

• Two different monomers react. One has two $-COOH$ groups (dicarboxylic acid), the other has two $-NH_2$ groups (diamine, for polyamides) or two $-OH$ groups (diol, for polyesters).
• Each time two monomers join, a small molecule is lost, usually water ($H_2O$).
• To find the monomers from a polymer: look for the amide link ($-CONH-$) or ester link ($-COO-$), and break the chain there. Add back the $-OH$ and $-H$ that were lost as water.

The exam often gives you a polymer section and asks you to draw the monomer, or vice versa. Practise both directions.

Polyamides (e.g., Nylon)

Formed from a dicarboxylic acid and a diamine.

• Linkage: Amide linkage (—CONH—).
• Monomers: $\text{HOOC-R-COOH}$ and $\text{H}_2\text{N-R'-NH}_2$.
• Equation:
  $n\text{HOOC-X-COOH (l)} + n\text{H}_2\text{N-Y-NH}_2 \text{(l)} \rightarrow \text{-[OC-X-CONH-Y-NH]}_n\text{-(s)} + 2n\text{H}_2\text{O(l)}$

Polyesters (e.g., PET)

Formed from a dicarboxylic acid and a diol.

• Linkage: Ester linkage (—COO—).
• Monomers: $\text{HOOC-R-COOH}$ and $\text{HO-R'-OH}$.
• Note: PET can be chemically broken down back into its monomers and re-polymerised, making it highly recyclable.

Proteins: Natural Polyamides

Proteins are natural polymers made from amino acid monomers.

• General structure of amino acid: $\text{H}_2\text{N-CH(R)-COOH}$.
• They contain both an amine group ($-\text{NH}_2$) and a carboxylic acid group ($-\text{COOH}$).
• Structure of Protein:

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Key Equations

1. General Addition Polymerisation:
$n(\text{C=C}) \rightarrow \text{-[C-C]}_n\text{-}$
(Where $n$ is a large number representing the many monomers)

2. Formation of an Ester Linkage (PET):
$\text{R-COOH} + \text{HO-R'} \rightarrow \text{R-COO-R'} + \text{H}_2\text{O}$

3. Formation of an Amide Linkage (Nylon/Protein):
$\text{R-COOH} + \text{H}_2\text{N-R'} \rightarrow \text{R-CONH-R'} + \text{H}_2\text{O}$

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Common Mistakes to Avoid

• ❌ Wrong: Including the C=C double bond in the repeat unit of an addition polymer.
• ✅ Right: The polymer chain must only contain C—C single bonds.
• ❌ Wrong: Forgetting to draw the "continuation bonds" (the lines extending outside the brackets).
• ✅ Right: Ensure bonds extend through the square brackets to show the chain continues.
• ❌ Wrong: Forgetting the $H_2O$ molecule in condensation equations.
• ✅ Right: Always include $+ 2n H_2O$ (or similar) when showing the formation of a condensation polymer from $n$ monomers.

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Exam Tips

• Command Words: If asked to "Draw the repeat unit," do not include the 'n'. If asked to "Draw the polymer," include the brackets and the 'n'.
• Structure Identification: Look at the linkage. If you see —CONH—, it is a polyamide (Nylon or Protein). If you see —COO—, it is a polyester (PET).
• Contexts: Expect questions about why burning PVC is dangerous—always mention "toxic gases" like hydrogen chloride ($\text{HCl}$).
• Typical Tasks: You will often be given a complex-looking monomer and asked to draw the polymer. Simply identify the C=C bond, turn it into a single bond, and draw everything else exactly as it was attached to those two carbons.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Define the term 'polymer'. [2]

(b) Poly(ethene) is a polymer formed from ethene monomers. State two environmental problems associated with the disposal of poly(ethene). [2]

(c) State whether poly(ethene) is formed via addition or condensation polymerisation. [1]

Worked Solution:

(a)

1. A polymer is a large molecule.
   This is the first part of the definition.

2. Formed from many smaller repeating units called monomers.
   This completes the definition.

How to earn full marks:

• Must mention both 'large molecule' and 'monomers'.
• 'Repeating units' or similar wording is needed.

(b)

1. Disposal in landfill sites takes up space.
   This identifies one environmental problem.

2. Burning poly(ethene) can produce toxic gases.
   This identifies a second environmental problem.

How to earn full marks:

• Any two of: landfill space, accumulation in oceans, toxic gases from burning.
• Must be specific, e.g., 'pollution' is not sufficient.

(c)

1. $\boxed{\text{Addition polymerisation}}$
   States the correct type of polymerisation.

How to earn full marks:

• Must state 'addition'.

Common Pitfall: Make sure you know the definitions of key terms like "polymer" and "monomer" precisely. Also, be specific when describing environmental problems; vague answers won't get the mark.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) Draw the structure of the repeat unit of the polymer formed from the following monomer: $CH_2=CH(CH_3)$. [3]

(b) State one difference between addition and condensation polymerisation. [1]

(c) Describe the disposal of plastics in landfill sites. [2]

Worked Solution:

(a)

1. Correctly shows the breaking of the double bond.

2. Correctly shows the connection of the carbons to neighboring repeat units.

3. The repeat unit should be enclosed in brackets with 'n' outside.

How to earn full marks:

• Correct repeat unit structure with single bonds between the carbon atoms.
• Correctly shows the side chain ($CH_3$).
• 'n' outside the bracket.

(b)

1. In addition polymerisation, all the atoms of the monomer are used to form the polymer, whereas in condensation polymerisation, some atoms are lost to form a small molecule like water.
   States a correct difference.

How to earn full marks:

• Must mention the fate of the atoms of the monomer in each type of polymerisation.
• Accept other valid differences, such as the loss of a small molecule in condensation polymerisation, or the number of monomers involved.

(c)

1. Plastics are buried in large holes in the ground called landfill sites.
   Describes the process.

2. This takes up space and plastics are non-biodegradable, so they remain in the landfill for a long time.
   Explains the issue of space and non-biodegradability.

How to earn full marks:

• Correctly describes the process of burying plastics.
• Mentions the issue of space and/or non-biodegradability.

Common Pitfall: When drawing repeat units, remember to show the bond extending from each end of the unit, indicating that it continues to link to other units. Also, be clear about the fate of atoms during polymerisation.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

(a) Nylon is a polyamide formed from a dicarboxylic acid and a diamine. Draw the structure of nylon, showing at least two repeat units, formed from the following monomers:

• Hexanedioic acid: $HOOC(CH_2)_4COOH$
• 1,6-diaminohexane: $H_2N(CH_2)_6NH_2$ [4]

(b) Describe the key features of the polymerisation reaction that forms nylon. [3]

(c) State one use of nylon. [1]

Worked Solution:

(a)

1. Shows the correct linkage.

2. Shows the correct structure of the hexanedioic acid section.

3. Shows the correct structure of the 1,6-diaminohexane section.

4. Shows correct linking of at least two repeat units.

How to earn full marks:

• Correct amide linkage (–CO–NH–).
• Correct hexanedioic acid section ($-(CH_2)_4-$).
• Correct 1,6-diaminohexane section ($-(CH_2)_6-$).
• At least two repeat units linked correctly.

(b)

1. This is a condensation polymerisation reaction.
   Identifies the type of polymerisation.

2. A small molecule, water, is eliminated during the reaction.
   States that water is eliminated.

3. The monomers join together via amide linkages.
   States the type of linkage.

How to earn full marks:

• Must mention condensation polymerisation.
• Must state that water is eliminated.
• Must state amide linkages.

(c)

1. $\boxed{\text{Clothing}}$ / $\boxed{\text{Rope}}$ / $\boxed{\text{Stockings}}$ / $\boxed{\text{Parachutes}}$ / $\boxed{\text{Tents}}$
   States a correct use.

How to earn full marks:

• Any valid use of nylon.

Common Pitfall: When drawing condensation polymers like nylon, pay close attention to the linkage between the monomers (amide linkage in this case). Make sure you're removing the correct atoms to form the water molecule.

Exam-Style Question 4 — Extended Response [10 marks]

Question:

(a) Define the term 'monomer'. [1]

(b) PET (polyethylene terephthalate) is a polyester formed from a dicarboxylic acid and a diol.
(i) State the type of polymerisation that forms PET. [1]
(ii) State one advantage of being able to convert PET back into its monomers and re-polymerise it. [1]

(c) The diagram shows the structure of the monomers used to make PET.

Draw the structure of PET, showing at least two repeat units. [4]

(d) Compare and contrast addition and condensation polymerisation. [3]

Worked Solution:

(a)

1. A monomer is a small molecule that combines with many other monomers to form a polymer.
   Provides a correct definition.

How to earn full marks:

• Must mention that it is a small molecule.
• Must mention that it combines with other monomers to form a polymer.

(b)(i)

1. $\boxed{\text{Condensation polymerisation}}$
   States the correct type of polymerisation.

How to earn full marks:

• Must state 'condensation'.

(b)(ii)

1. $\boxed{\text{Reduces the amount of plastic waste}}$ / $\boxed{\text{Reduces reliance on crude oil}}$ / $\boxed{\text{Conserves resources}}$
   States a valid advantage.

How to earn full marks:

• Any valid advantage related to waste reduction, resource conservation, or reduced reliance on crude oil.

(c)

1. Shows the correct linkage.

2. Shows the correct structure of the terephthalic acid section.

3. Shows the correct structure of the ethane-1,2-diol section.

4. Shows correct linking of at least two repeat units.

How to earn full marks:

• Correct ester linkage (–CO–O–).
• Correct terephthalic acid section (–C6H4–).
• Correct ethane-1,2-diol section (–CH2–CH2–).
• At least two repeat units linked correctly.

(d)

1. Addition polymerisation involves only one type of monomer, while condensation polymerisation involves two different types of monomers.
   States a difference.

2. Condensation polymerisation involves the elimination of a small molecule, such as water, while addition polymerisation does not.
   States another difference.

3. Both addition and condensation polymerisation produce large molecules called polymers.
   States a similarity.

How to earn full marks:

• Must state a valid difference regarding the number of monomers.
• Must state a valid difference regarding the elimination of a small molecule.
• Must state a similarity, such as both forming polymers or both requiring monomers.

Common Pitfall: Be careful with the ester linkage in polyesters like PET. It's easy to get the orientation of the carbonyl group wrong. Also, remember that a comparison requires both similarities and differences.