Organic Chemistry – Polymers (IGCSE / A‑Level)
Learning Objectives
- Define a polymer and a repeat unit.
- Deduce the repeat unit of an addition polymer from a given alkene monomer, and vice‑versa.
- Identify the repeat unit and linkage of a condensation (step‑growth) polymer and write the two monomers that give rise to it.
- Distinguish addition (chain‑growth) polymerisation from condensation (step‑growth) polymerisation, including the role of initiators, growth rate and by‑products.
- Explain briefly why most everyday plastics are polymers and outline the main environmental problems associated with their disposal.
Key Definitions
- Polymer: a macromolecule formed by the repeated linking of small units (monomers).
- Repeat unit: the smallest structural fragment that repeats indefinitely along the polymer chain.
- Addition (chain‑growth) polymerisation: polymerisation of unsaturated monomers (alkenes) in which the C=C double bond is broken and each carbon forms a new C–C single bond to the neighbouring repeat unit. No small molecule is lost.
- Condensation (step‑growth) polymerisation: polymerisation of two (or more) monomers that each bear complementary functional groups (e.g. –OH/–COOH, –NH₂/–COOH). Each step joins two monomer units with the simultaneous loss of a small molecule (usually H₂O or CH₃OH). Growth occurs simultaneously on many chains (step‑growth).
Addition (Chain‑Growth) Polymerisation
- Typical monomers: alkenes (e.g. ethene, propene, vinyl chloride).
- Initiation: generated by free‑radical initiators (peroxides, azo compounds), by heat, UV light or by ionic initiators. The initiator creates an active centre that adds the first monomer.
- Propagation: the active centre adds successive monomer units rapidly; the chain grows at one (or both) ends.
- Termination: combination or disproportionation of two active chains, or reaction with a chain‑transfer agent.
- Features:
- No small molecule is eliminated.
- Side groups attached to the double‑bond carbons become pendant groups on the polymer chain.
- Growth is fast once initiation has occurred (rapid chain‑growth).
Condensation (Step‑Growth) Polymerisation
- Typical monomers: a di‑acid with a di‑alcohol, a di‑acid with a diamine, a di‑halide with a di‑phenol, etc.
- Reaction: each condensation step creates a new bond and eliminates a small molecule (H₂O or CH₃OH). The reaction is usually catalysed by heat, acid or base.
- Growth: all chains can grow at the same time; high molecular weight is only reached when a large proportion of functional groups have reacted (slower overall rate compared with chain‑growth).
- Linkage types:
- Esters –CO‑O‑ (polyesters)
- Amides –CO‑NH‑ (polyamides)
- Carbonates –O‑CO‑O‑ (polycarbonates)
Comparison – Addition vs. Condensation (Step‑Growth)
| Feature |
Addition (Chain‑Growth) |
Condensation (Step‑Growth) |
| Typical monomers |
Unsaturated molecules (alkenes) |
Two different monomers each bearing complementary functional groups (e.g. –OH/–COOH, –NH₂/–COOH) |
| By‑product |
None (no small molecule lost) |
Small molecule eliminated (H₂O or CH₃OH) |
| Initiation / catalyst |
Free‑radical, ionic or coordination initiators; often heat or UV |
Usually heat, acid or base catalyst; no separate initiator needed |
| Polymer growth rate |
Very rapid once active centre formed (chain‑growth) |
Slower; all chains grow simultaneously (step‑growth) |
| Typical repeat‑unit linkage |
Carbon‑carbon single bonds |
–CO‑O‑ (esters), –CO‑NH‑ (amides), –O‑CO‑O‑ (carbonates) |
| Common examples |
Polyethylene, polypropylene, PVC, polystyrene, poly‑isobutene |
Nylon‑6,6 (polyamide), PET (polyester), polycarbonate |
General Procedure – From Alkene Monomer to Addition Polymer
- Write the structural formula of the alkene.
- Identify the two carbon atoms of the C=C double bond.
- Break the double bond; each carbon now forms a single bond to the next repeat unit.
- Retain any substituents on the former double‑bond carbons as pendant groups.
- Enclose the resulting fragment in brackets and indicate repetition, e.g.
[-CH₂‑CHCl-]ₙ.
General Procedure – From Addition‑Polymer Repeat Unit to Alkene Monomer
- Isolate a single repeat unit.
- Insert a C=C double bond between the two carbons that were originally double‑bonded.
- Re‑attach the pendant groups to the appropriate carbon of the double bond.
- The resulting structure is the original alkene monomer.
General Procedure – From Condensation‑Polymer Repeat Unit to the Two Monomers
- Identify the linkage formed in the repeat unit (e.g. –CO‑O‑, –CO‑NH‑, –O‑CO‑O‑).
- Insert the eliminated small molecule (H₂O or CH₃OH) between the two halves of the repeat unit.
- Separate the structure into the two original di‑functional monomers (usually a di‑acid and a di‑alcohol or diamine).
Common Addition Polymers
| Alkene Monomer |
Repeat Unit |
Typical Uses |
Ethene, CH₂=CH₂ |
[-CH₂‑CH₂-]ₙ (Polyethylene) |
Shopping bags, bottles, film |
Propene, CH₂=CHCH₃ |
[-CH₂‑CH(CH₃)-]ₙ (Polypropylene) |
Textiles, automotive parts, food containers |
Vinyl chloride, CH₂=CHCl |
[-CH₂‑CHCl-]ₙ (PVC) |
Pipes, window frames, flooring |
Styrene, CH₂=CHC₆H₅ |
[-CH₂‑CH(C₆H₅)-]ₙ (Polystyrene) |
Disposable cups, insulation, CD cases |
Vinyl acetate, CH₂=CHOCOCH₃ |
[-CH₂‑CH(OCOCH₃)-]ₙ (Polyvinyl acetate) |
Adhesives, paints |
Isobutene, (CH₃)₂C=CH₂ |
[-CH₂‑C(CH₃)₂-]ₙ (Poly‑isobutene) |
Sealants, rubber‑like elastomers |
Condensation Polymers – Representative Examples
1. Nylon‑6,6 (polyamide)
- Monomers: hexamethylenediamine
H₂N‑(CH₂)₆‑NH₂ and adipic acid HOOC‑(CH₂)₄‑COOH.
- Reaction (step‑growth):
HOOC‑(CH₂)₄‑COOH + H₂N‑(CH₂)₆‑NH₂ → [-CO‑(CH₂)₄‑CO‑NH‑(CH₂)₆-]ₙ + 2 H₂O.
- Repeat unit:
[-CO‑(CH₂)₄‑CO‑NH‑(CH₂)₆-]ₙ.
2. Polyethylene terephthalate (PET, polyester)
- Monomers: terephthalic acid
HOOC‑C₆H₄‑COOH and ethylene glycol HO‑CH₂‑CH₂‑OH.
- Reaction:
HOOC‑C₆H₄‑COOH + HO‑CH₂‑CH₂‑OH → [-O‑CH₂‑CH₂‑O‑CO‑C₆H₄‑CO-]ₙ + 2 H₂O.
- Repeat unit:
[-O‑CH₂‑CH₂‑O‑CO‑C₆H₄‑CO-]ₙ.
3. Polycarbonate (PC)
- Monomers: bisphenol‑A
(HO‑C₆H₄‑C(CH₃)₂‑C₆H₄‑OH) and phosgene Cl‑CO‑Cl.
- Reaction:
(HO‑C₆H₄‑C(CH₃)₂‑C₆H₄‑OH) + Cl‑CO‑Cl → [-O‑C(=O)‑O‑C₆H₄‑C(CH₃)₂‑C₆H₄-]ₙ + 2 HCl.
- Repeat unit:
[-O‑C(=O)‑O‑C₆H₄‑C(CH₃)₂‑C₆H₄-]ₙ (carbonate linkage).
Worked Examples
Example 1 – Alkene → Addition Polymer (Vinyl Acetate)
Monomer: CH₂=CHOCOCH₃
- Identify the double‑bond carbons: the
CH₂ and the CH bearing the acetate group.
- Break the C=C bond; each carbon now bonds to the next repeat unit.
- Retain the acetate pendant group (
OCOCH₃) on the second carbon.
Repeat unit: [-CH₂‑CH(OCOCH₃)-]ₙ (polyvinyl acetate).
Example 2 – Branched Alkene → Polymer (Isobutene)
Monomer: (CH₃)₂C=CH₂ (isobutene)
- Double‑bond carbons are the terminal
CH₂ and the quaternary carbon C(CH₃)₂.
- After polymerisation the quaternary carbon becomes part of the backbone; the two methyl groups remain pendant.
Repeat unit: [-CH₂‑C(CH₃)₂-]ₙ (poly‑isobutene).
Example 3 – Polymer → Alkene (PVC)
Repeat unit: [-CH₂‑CH(Cl)-]ₙ
- Isolate a single unit:
CH₂‑CH(Cl).
- Insert a double bond between the two carbons.
Monomer: CH₂=CHCl (vinyl chloride).
Example 4 – Condensation Repeat Unit → Monomers (Nylon‑6,6)
Repeat unit: [-CO‑(CH₂)₄‑CO‑NH‑(CH₂)₆-]ₙ
- Recognise the –CO‑NH‑ amide linkage formed by loss of water.
- Insert
H₂O between the carbonyl carbon and the nitrogen.
- Separate into the two di‑functional monomers:
- Di‑acid:
HOOC‑(CH₂)₄‑COOH (adipic acid)
- Di‑amine:
H₂N‑(CH₂)₆‑NH₂ (hexamethylenediamine)
Example 5 – Condensation Repeat Unit → Monomers (Polycarbonate)
Repeat unit: [-O‑C(=O)‑O‑C₆H₄‑C(CH₃)₂‑C₆H₄-]ₙ
- Identify the carbonate linkage –O‑C(=O)‑O‑.
- Insert the eliminated small molecule
HCl (from phosgene) to reconstruct the original monomers.
- Resulting monomers:
- Bisphenol‑A:
HO‑C₆H₄‑C(CH₃)₂‑C₆H₄‑OH
- Phosgene:
Cl‑CO‑Cl
Why Are Most Everyday Plastics Addition Polymers?
Petro‑chemical feedstocks (ethene, propene, etc.) are cheap, abundant and easily polymerised by simple radical initiators. Addition polymerisation gives high‑molecular‑weight polymers in a single step, producing strong, durable materials at low cost. Consequently, the majority of consumer plastics – bags, bottles, packaging, automotive parts – are addition polymers such as polyethylene, polypropylene and PVC.
Environmental Issues of Polymer Disposal
- Land‑fill persistence: addition polymers are resistant to biodegradation; they can persist for centuries, fragmenting into micro‑plastics.
- Incineration: releases energy but also toxic gases (e.g., HCl from PVC, dioxins from chlorinated polymers) and CO₂.
- Recycling challenges:
- Mechanical recycling degrades polymer quality.
- Chemical recycling is limited to a few polymer types.
- Mixed‑polymer items (e.g., multilayer packaging) are difficult to separate.
- Additives: plasticisers (phthalates), stabilisers (lead, cadmium) and flame‑retardants can leach into soil and water, posing health risks.
- Carbon footprint: production relies on fossil fuels; the energy‑intensive polymerisation and transport contribute substantially to greenhouse‑gas emissions.
Practice Questions
- Write the repeat unit for the polymer formed from the monomer
CH₂=CHCH₂CH₃ (1‑butene).
- Identify the alkene monomer that would give the repeat unit
[-CH₂‑CH(C₂H₅)-]ₙ.
- Given the polymer repeat unit
[-CH₂‑CH(OCH₃)-]ₙ, draw the structure of the original alkene monomer.
- State the two monomers that combine to give the repeat unit
[-CO‑(CH₂)₄‑CO‑NH‑(CH₂)₆‑]ₙ and name the polymer.
- List three environmental problems associated with the disposal of addition‑polymer plastics.
Answers to Practice Questions
- Repeat unit:
[-CH₂‑CH(CH₂CH₃)-]ₙ (poly‑1‑butene).
- Monomer:
CH₂=CHC₂H₅ (1‑pentene).
- Monomer:
CH₂=CHOCH₃ (vinyl methyl ether).
- Monomers: adipic acid
HOOC‑(CH₂)₄‑COOH and hexamethylenediamine H₂N‑(CH₂)₆‑NH₂; polymer name: Nylon‑6,6 (polyamide).
- Environmental problems:
- Long‑term persistence in land‑fills and oceans, leading to micro‑plastic formation.
- Release of toxic gases (e.g., HCl, dioxins) during incineration.
- Leaching of plasticisers, stabilisers and other additives into the environment.