Define polymers as large molecules built up from many smaller molecules called monomers

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Polymers – Definition and Basic Concepts (Cambridge IGCSE 0620)

1. Key Definitions

  • Monomer – a low‑molecular‑weight molecule that can react with other monomers to form a polymer.
  • Polymer – a high‑molecular‑weight macromolecule composed of many repeating units.
  • Repeat unit – the smallest structural fragment that repeats along the polymer chain; it is the part of the monomer that remains after polymerisation.
  • Polymerisation – the chemical reaction that joins monomers together. Two types are recognised in the syllabus:
    • Addition (chain‑growth) polymerisation
    • Condensation (step‑growth) polymerisation

2. Types of Polymerisation

2.1 Addition (chain‑growth) polymerisation

  • Typical monomers: unsaturated molecules containing a C=C double bond (e.g., ethene, propene).
  • Reaction steps:
    1. Initiation – formation of a reactive centre (radical, cation or anion) often by heat, light or a peroxide catalyst.
    2. Propagation – monomer units add one by one to the growing chain.
    3. Termination – two reactive chains combine or a chain‑transfer agent stops growth.
  • By‑product: none.
  • Typical reaction conditions: high pressure and temperature for simple alkenes (e.g., ethene at ~150 °C, 1–3 MPa) and/or a catalyst such as Ziegler‑Natta or free‑radical initiator.

2.2 Condensation (step‑growth) polymerisation

  • Typical monomers: two different molecules each bearing complementary functional groups (e.g., –OH and –COOH, –NH₂ and –COOH).
  • Reaction steps:
    1. Any two monomer molecules (or oligomers) can react; the reaction proceeds stepwise.
    2. Each step eliminates a small molecule, most often water (H₂O) or methanol (CH₃OH).
  • By‑product: a small molecule (commonly H₂O or CH₃OH).
  • Typical reaction conditions: moderate temperature (150–250 °C) and often an acid or base catalyst to speed up esterification or amidation.

3. Worked Deductions – From Monomer ⇄ Repeat Unit

3.1 Addition polymer (propene)

  1. Given monomer: propene, CH₂=CH‑CH₃
  2. Write the repeat unit (show the opened double bond and the connection points): –CH₂–CH(CH₃)–
  3. Given repeat unit: –CH₂–CH(CH₃)–
  4. Identify the monomer: propene (CH₂=CH‑CH₃)

3.2 Condensation polymer (nylon‑6,6)

  1. Given monomers:
    • hexamethylenediamine, H₂N‑(CH₂)₆‑NH₂
    • adipic acid, HOOC‑(CH₂)₄‑COOH
  2. Write the repeat unit (show the amide linkage and the loss of H₂O): –NH‑(CH₂)₆‑NH‑CO‑(CH₂)₄‑CO–
  3. Given repeat unit: –NH‑(CH₂)₆‑NH‑CO‑(CH₂)₄‑CO–
  4. Identify the monomers:
    • Diamine: hexamethylenediamine
    • Dicarboxylic acid: adipic acid

4. Comparison of Addition and Condensation Polymers

Feature Addition (chain‑growth) polymer Condensation (step‑growth) polymer
Typical repeat unit –CH₂–CH₂– (no functional groups are eliminated) –O–CH₂–CH₂–O–CO–C₆H₄–CO– (contains groups that remain after loss of H₂O)
By‑product None Small molecule (usually H₂O or CH₃OH)
Typical monomers Unsaturated molecules with a C=C double bond (e.g., ethene, propene) Two different monomers each bearing complementary functional groups (e.g., –OH & –COOH, –NH₂ & –COOH)
Reaction conditions High pressure/temperature; catalysts such as Ziegler‑Natta or peroxide initiators Moderate temperature; acid/base catalysts; removal of water or methanol drives the reaction forward
Typical properties Often very high molecular weight, thermoplastic, can be moulded when heated Often thermosetting or semi‑crystalline, may be more rigid; properties depend on the functional groups present

5. Examples of Common Polymers

  • Polyethylene (PE) – addition polymerisation of ethene; used for bags, bottles, containers.
  • Polypropylene (PP) – addition polymerisation of propene; used for packaging, textiles.
  • Polyethylene terephthalate (PET) – condensation polymerisation of ethylene glycol and terephthalic acid; used for drink bottles and fibres.
  • Nylon‑6,6 – condensation polymerisation of hexamethylenediamine and adipic acid; used for fibres and engineering plastics.
  • Proteins – natural polymers of amino‑acid monomers linked by peptide bonds (condensation, water released).
  • Starch – natural polymer of glucose monomers linked by α‑glycosidic bonds (condensation, water released).

6. Environmental Implications and Disposal (Syllabus Requirement)

Polymers are extremely useful but create several environmental challenges. Students should know the main disposal routes and their drawbacks:

  • Land‑fill burial – polymers decompose very slowly; they occupy space for centuries.
  • Incineration – burning releases toxic gases (CO, CO₂, HCl, dioxins) and contributes to air pollution.
  • Ocean accumulation – plastic debris persists in marine environments, harming wildlife.
  • Recycling – reduces the need for virgin material but is limited by polymer type, contamination and loss of mechanical properties.

7. Why Understanding Polymers Is Important

  • Predict the structure and properties of both synthetic and natural polymers.
  • Explain why different polymers behave differently (e.g., flexibility of PE vs. rigidity of PET).
  • Assess the environmental impact of plastic use and evaluate sustainable disposal strategies.
Suggested diagram: a schematic showing monomers joining to form a polymer chain, with separate panels for addition (no by‑product) and condensation (water released).

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