Organic Chemistry – Polymers (IGCSE 0620, Section 11.8)
Learning Objective
Students will be able to:
- Define polymer, monomer, repeat unit and linkage.
- Describe the **differences** between addition (chain‑growth) and condensation (step‑growth) polymerisation.
- Identify repeat units and linkages from given monomers (and the reverse).
- Deduce the structure of a polymer from its monomers and vice‑versa.
- Explain why most plastics are polymers and discuss the main environmental issues associated with them.
Key Concepts
- Polymer (macromolecule): A very large molecule composed of many repeating structural units.
- Monomer: The small, reactive molecule that joins with others during polymerisation.
- Repeat unit: The smallest portion of the polymer chain that repeats throughout the molecule.
- Linkage: The type of covalent bond that joins repeat units (e.g. C–C, –O–C(=O)–, –NH–C(=O)–).
Types of Polymerisation
1. Addition (Chain‑Growth) Polymerisation
- Monomer requirement: contains a carbon–carbon double bond (C=C) such as ethene, CH₂=CH₂.
- Mechanism: three distinct stages
- Initiation: a radical, cation or anion is generated (often by a peroxide or azo compound) and adds to a monomer, creating an active chain‑end.
- Propagation: the active chain‑end repeatedly adds fresh monomer units, lengthening the chain.
- Termination: two active chains combine or a chain‑end is quenched, ending growth.
- By‑product: none – the polymer is essentially the monomer with the double bond removed.
- Typical polymers: polyethylene (PE), poly‑vinyl chloride (PVC), polystyrene (PS), polypropylene (PP).
2. Condensation (Step‑Growth) Polymerisation
- Monomer requirement: two (or more) complementary functional groups (e.g. –OH, –COOH, –NH₂, –NCO).
- Mechanism: any two reactive chain ends can combine; there is no separate initiation step. Polymer chains grow stepwise, from monomer → dimer → trimer … → polymer.
- By‑product: a small molecule is released each time a bond forms (commonly H₂O, CH₃OH, HCl, CO₂).
- Typical polymers: polyesters (PET), polyamides (nylon‑6,6), polyurethanes, polycarbonates.
Comparison of Addition and Condensation Polymerisation
| Feature |
Addition (Chain‑Growth) |
Condensation (Step‑Growth) |
| Typical monomer structure |
Contains a C=C double bond (e.g. CH₂=CHX) |
Contains two complementary functional groups (e.g. HO–R–COOH, NH₂–R–COOH, NCO–R–OH) |
| Terminology used in syllabus |
“Addition polymerisation” (also called *chain‑growth*) |
“Condensation polymerisation” (also called *step‑growth*) |
| Mechanistic steps |
Initiation → Propagation → Termination |
Stepwise linking of any two reactive ends; no distinct initiation |
| By‑product |
None |
Small molecule (H₂O, CH₃OH, HCl, CO₂, etc.) |
| Rate of molecular‑weight build‑up |
Very rapid once initiation occurs; high molecular weight reached early. |
Gradual; high molecular weight only after long reaction times and removal of the by‑product. |
| Typical polymers |
PE, PVC, PS, PP |
PET, nylon‑6,6, polyurethane, polycarbonate |
| Reaction conditions |
Radical initiator (peroxide, azo) ± heat or UV; inert atmosphere often required. |
Heat and/or catalyst; continuous removal of the by‑product (distillation, reduced pressure) to drive the reaction forward. |
Identifying Repeat Units & Linkages
Addition Polymers
Delete the C=C double bond and join the carbon atoms directly.
- Monomer: CH₂=CH₂ (ethene) → Repeat unit: –CH₂–CH₂– (polyethylene)
- Monomer: CH₂=CHCl (vinyl chloride) → Repeat unit: –CH₂–CHCl– (PVC)
- Linkage: C–C single bond between repeat units.
Condensation Polymers
Join the functional groups, inserting the characteristic linkage.
- Ester linkage: –O–C(=O)– (formed from –OH and –COOH)
- Amide linkage: –NH–C(=O)– (formed from –NH₂ and –COOH)
- Urethane linkage: –NH–C(=O)–O– (formed from an isocyanate –NCO and an alcohol –OH)
Worked‑out Exercise (Answer Key)
- Ethene (CH₂=CH₂) →
–CH₂–CH₂–
- Vinyl chloride (CH₂=CHCl) →
–CH₂–CHCl–
- Ethylene glycol + terephthalic acid (PET) →
–O–CH₂–CH₂–O–C(=O)–C₆H₄–C(=O)–
- Hexamethylenediamine + adipic acid (Nylon‑6,6) →
–NH–(CH₂)₆–NH–C(=O)–(CH₂)₄–C(=O)–
- Di‑isocyanate (e.g. toluene‑2,4‑diisocyanate) + polyol (e.g. trimethylolpropane) →
–NH–C(=O)–O– urethane linkage (polyurethane).
Deduction of Polymer Structures from Monomers (and Vice‑versa)
Example 1 – Addition polymer
Monomer: CH₂=CHCH₃ (propene)
Repeat unit: –CH₂–CH(CH₃)– (isotactic polypropylene if stereochemistry is controlled).
Example 2 – Condensation polymer (non‑water by‑product)
Monomers: Toluene‑2,4‑diisocyanate (OCN‑C₆H₄‑NCO) and 1,4‑butanediol (HO‑(CH₂)₄‑OH).
Polymerisation: each –NCO reacts with an –OH to give a urethane linkage and releases CO₂? (actually releases no small molecule; the reaction is a direct addition, but in practice a catalyst is used). The repeat unit is:
–NH–C(=O)–O–(CH₂)₄–O–C(=O)–NH–
Plastics, Their Uses and Environmental Implications
- Most everyday plastics (e.g. bottles, films, packaging) are synthetic polymers produced by the two polymerisation methods described.
- Advantages: lightweight, durable, mouldable, inexpensive.
- Environmental concerns:
- Non‑biodegradable nature leads to long‑term accumulation in landfills and oceans.
- Production relies on fossil‑fuel feedstocks, contributing to carbon emissions.
- Release of micro‑plastics through degradation and abrasion.
- Improper disposal can cause wildlife ingestion and habitat disruption.
- Mitigation strategies taught at IGCSE level:
- Recycling (e.g. PET bottles → depolymerisation or mechanical re‑processing).
- Design for biodegradability (e.g. polylactic acid, PLA, a condensation polymer from lactic acid).
- Reduction of single‑use plastics and promotion of re‑usable alternatives.
Safety & Practical Considerations
- Free‑radical addition: use peroxide initiators with caution (shock‑sensitive). Carry out reactions under inert gas (N₂ or Ar), wear gloves, goggles and lab coat.
- Condensation step‑growth: high temperatures and catalysts (acid, metal salts) are common; ensure good ventilation and use a fume hood. Continuous removal of water or other by‑product (distillation, reduced pressure) is essential for high molecular weight.
- All waste must be collected in labelled containers and disposed of according to school/ local regulations.
Summary
- Addition (chain‑growth) polymerisation uses monomers with C=C bonds, proceeds via initiation‑propagation‑termination, gives no small‑molecule by‑product, and attains high molecular weight rapidly.
- Condensation (step‑growth) polymerisation uses bifunctional monomers, proceeds by stepwise linking of any two reactive ends, releases a small molecule (often water), and requires removal of that by‑product to reach high molecular weight.
- Both types produce the plastics that dominate modern life; understanding their mechanisms helps explain properties, recycling possibilities, and environmental impact.
- Students should be able to:
- Define polymer, monomer, repeat unit and linkage.
- Distinguish addition from condensation polymerisation.
- Write repeat units and draw the main linkage for common polymers.
- Explain why plastics are polymers and discuss at least two environmental issues.