11.8 BETA Verified

Polymers

14 learning objectives • 5 core • 9 extended

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.

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

📊A row of ethene molecules with double bonds changing into a long chain of single-bonded carbons with trailing bonds at the ends, surrounded by square brackets with a subscript 'n'
.

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 ($\text{CO}\text{(g)}$).

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.

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:
    📊$—\text{NH—CH(R)—CO—NH—CH(R')—CO—}$ showing the amide/peptide bonds between different R-group units
    .

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): $\text{R—COOH} + \text{H}_2\text{N—R'} \rightarrow \text{R—CONH—R'} + \text{H}_2\text{O}$

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.

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.