What you'll learn
This revision guide covers all CSEC-testable content on polymers and plastics. You will understand how small molecules combine to form large polymer chains, distinguish between natural and synthetic polymers, and explain the properties that make plastics suitable for specific applications. The guide also addresses environmental concerns relevant to Caribbean nations and the global community.
Key terms and definitions
Polymer — a large molecule made up of many small repeating units (monomers) joined together by chemical bonds
Monomer — a small molecule that can join with other identical molecules to form a polymer chain
Polymerisation — the chemical reaction in which monomer molecules join together to form a polymer
Plastic — a synthetic polymer that can be shaped or moulded when heated
Thermoplastic — a polymer that softens when heated and can be remoulded multiple times without chemical change
Thermosetting plastic — a polymer that sets hard when first heated and cannot be softened or remoulded by further heating
Synthetic polymer — a polymer made by humans through chemical processes, not found in nature
Biodegradable — capable of being broken down by living organisms such as bacteria
Core concepts
Natural vs. Synthetic polymers
Polymers exist in both natural and manufactured forms.
Natural polymers include:
- Proteins (found in meat, fish, beans)
- Starch (in cassava, yam, breadfruit, dasheen)
- Cellulose (in plant cell walls)
- Natural rubber (from Hevea brasiliensis rubber trees, historically important in Caribbean economies)
- DNA (genetic material in all living things)
Synthetic polymers are manufactured from petroleum products and include:
- Polyethene (plastic bags, bottles)
- Polystyrene (food containers, packaging)
- PVC/polyvinyl chloride (water pipes, electrical insulation)
- Nylon (fishing nets, ropes, clothing)
- Perspex (windows, signs)
The key difference is origin: natural polymers are produced by living organisms, while synthetic polymers are manufactured in factories through controlled chemical reactions.
The polymerisation process
Polymerisation occurs when many small monomer molecules join together through chemical bonds to form long chain molecules.
Addition polymerisation:
- Monomers containing carbon-carbon double bonds (C=C) join together
- No other product is formed besides the polymer
- The double bond opens up, allowing monomers to link
Example: Ethene monomers polymerise to form polyethene (polythene)
n(CH₂=CH₂) → -(CH₂-CH₂)ₙ-
ethene monomer polyethene polymer
The "n" represents a very large number (often thousands) of repeating units.
Common addition polymers and their monomers:
| Monomer | Polymer | Uses |
|---|---|---|
| Ethene | Polyethene | Plastic bags, bottles, containers |
| Propene | Polypropene | Ropes, crates, bottle caps |
| Chloroethene (vinyl chloride) | PVC | Water pipes, window frames, electrical cables |
| Styrene | Polystyrene | Food packaging, insulation, disposable cups |
Structure and properties of polymers
The physical properties of polymers depend on their molecular structure.
Chain length:
- Longer polymer chains create stronger materials
- More intermolecular forces exist between longer chains
- Longer chains increase melting point and tensile strength
Cross-linking:
- Chemical bonds form between adjacent polymer chains
- Creates a rigid three-dimensional network
- Prevents chains from sliding past each other
- Thermosetting plastics have extensive cross-links
Crystallinity:
- Regular arrangement of polymer chains (crystalline regions)
- Irregular arrangement (amorphous regions)
- More crystalline = stronger, less flexible, higher melting point
- Less crystalline = more flexible, easier to mould
Properties of thermoplastics:
- Soften when heated
- Can be remoulded repeatedly
- Chains held together by weak intermolecular forces only
- Examples: polyethene, polystyrene, PVC, nylon
- Recyclable through melting and reshaping
Properties of thermosetting plastics:
- Do not soften when heated
- Cannot be remoulded once set
- Strong covalent cross-links between chains
- Heating causes charring and decomposition
- Examples: melamine (kitchen worktops), bakelite (electrical plugs), epoxy resins (adhesives)
- Cannot be recycled by melting
Uses of plastics in the Caribbean context
Plastics have become essential in Caribbean life due to their versatile properties.
Polyethene (PE):
- Low-density polyethene (LDPE): flexible, used for plastic bags, wrapping film, squeeze bottles
- High-density polyethene (HDPE): stronger, used for water pipes, jerry cans, crates for transporting agricultural produce (bananas, mangoes)
Polypropene (PP):
- Ropes for fishing industry
- Crates for market vendors
- Bottle caps
- Higher melting point than polyethene
Polyvinyl chloride (PVC):
- Water pipes throughout Caribbean homes
- Electrical cable insulation (important in tropical humidity)
- Rainwater guttering
- Window frames resistant to salt air corrosion
Polystyrene (PS):
- Food containers for street vendors
- Disposable cups at festivals and sporting events
- Packaging material
- Insulation in coolers for fish transport
Nylon:
- Fishing nets and lines for Caribbean fishing industry
- Clothing and sportswear
- Toothbrush bristles
Advantages and disadvantages of plastics
Advantages:
- Lightweight: reduces transportation costs for Caribbean imports
- Waterproof: essential for tropical climates with heavy rainfall
- Durable: resistant to corrosion in salt-air coastal environments
- Versatile: can be moulded into countless shapes
- Electrical insulators: safe for wiring in humid conditions
- Cheap to manufacture and purchase
- Chemical resistance: unaffected by many household chemicals
Disadvantages:
- Most are non-biodegradable: persist in environment for centuries
- Contribute to litter and pollution of Caribbean beaches and marine environments
- Marine animals (sea turtles, fish) can ingest or become entangled in plastic waste
- Burning releases toxic gases including carbon monoxide and dioxins
- Made from petroleum: non-renewable resource
- Microplastics enter food chains, affecting seafood industries
- Clogged drainage systems contribute to flooding during hurricane season
Environmental impact and solutions
Caribbean nations face particular challenges with plastic pollution due to:
- Coastal populations
- Tourism industry generating significant plastic waste
- Limited landfill space on small islands
- Ocean currents depositing international plastic waste on Caribbean shores
Problems:
- Plastic waste accumulates in oceans, forming Caribbean sections of ocean gyres
- Damages coral reefs when plastic debris settles on reefs
- Threatens endangered species like hawksbill and leatherback turtles
- Reduces aesthetic appeal of beaches, affecting tourism revenue
- Blocks waterways, worsening flood damage during storms
Solutions being implemented:
Reduction strategies:
- Bans on single-use plastic bags (implemented in Jamaica, Barbados, other territories)
- Use of biodegradable alternatives made from plant materials
- Reusable shopping bags from fabric or jute
- Deposit-refund schemes for plastic bottles
Recycling programs:
- Collection centres in major Caribbean cities
- Thermoplastics can be melted and remoulded
- Separation by plastic type using recycling codes (numbers 1-7)
- Creation of new products from recycled plastic
Disposal improvements:
- Proper landfill management to prevent ocean pollution
- Investment in waste-to-energy incineration plants (with proper emission controls)
- Beach cleanup initiatives involving schools and communities
Development of alternatives:
- Biodegradable plastics from starch, cellulose, or bacterial polymers
- These break down through microbial action
- More expensive but environmentally safer
- Research into plastics from Caribbean agricultural waste (bagasse, coconut husks)
Worked examples
Example 1: Ethene can undergo polymerisation to form polyethene.
(a) State what is meant by the term polymerisation. [2 marks]
(b) Draw the displayed structure of: (i) an ethene molecule [1 mark] (ii) a section of polyethene showing three repeating units [2 marks]
(c) Explain why polyethene is described as an addition polymer. [2 marks]
Model answer:
(a) Polymerisation is the chemical reaction [1] in which many small monomer molecules join together to form a large polymer molecule. [1]
(b) (i)
H H
| |
C = C
| |
H H
[1 mark for correct displayed structure showing C=C double bond and all four hydrogen atoms]
(ii)
H H H H H H
| | | | | |
- C - C - C - C - C - C -
| | | | | |
H H H H H H
[1 mark for correct carbon backbone with single bonds; 1 mark for correct hydrogen atoms and continuation bonds]
(c) It is called an addition polymer because the monomer molecules add together [1] without any other substance being produced/no small molecules are eliminated. [1]
Example 2: A Caribbean fishing company needs to choose between nylon rope and natural fibre rope for their nets.
(a) State whether nylon is a natural or synthetic polymer. [1 mark]
(b) Give two advantages of using nylon rope instead of natural fibre rope for fishing nets. [2 marks]
(c) Describe one environmental problem caused by discarded nylon fishing nets in the Caribbean Sea. [2 marks]
Model answer:
(a) Synthetic polymer [1]
(b) Any two from:
- Nylon is stronger/more durable than natural fibres [1]
- Nylon does not rot in seawater [1]
- Nylon is lightweight/easier to handle [1]
- Nylon is water-resistant/does not absorb water [1] [Maximum 2 marks]
(c) Discarded nylon nets remain in the ocean for many years because nylon is non-biodegradable [1]. Marine animals such as turtles, dolphins, or fish can become entangled in the nets and may be injured or die [1]. OR The nets continue to trap and kill marine life (ghost fishing) [1], reducing fish populations and affecting the fishing industry [1].
Example 3: The table shows properties of two types of plastic.
| Property | Plastic A | Plastic B |
|---|---|---|
| Effect of heating | Softens | Chars/burns |
| Can be remoulded | Yes | No |
| Molecular structure | Long chains | Cross-linked chains |
(a) Identify whether Plastic A is a thermoplastic or thermosetting plastic. [1 mark]
(b) Explain, in terms of molecular structure, why Plastic B cannot be remoulded. [3 marks]
(c) State which plastic type would be more suitable for making electrical plug casings. Give a reason for your answer. [2 marks]
Model answer:
(a) Thermoplastic [1]
(b) Plastic B has covalent cross-links/bonds between adjacent polymer chains [1]. These cross-links form a rigid three-dimensional network [1]. The cross-links cannot be broken by heating, so the chains cannot slide past each other to allow remoulding [1].
(c) Plastic B/thermosetting plastic [1] because it will not soften or deform when heated by electrical current flowing through the plug [1].
Common mistakes and how to avoid them
Confusing monomers and polymers: Remember that monomers are small molecules that join together; polymers are the large molecules formed. Think "mono" = one, "poly" = many.
Incorrect polymerisation equations: Always include the "n" to show many monomers joining, and use continuation bonds (the lines extending beyond the bracketed structure) in the polymer structure. Don't forget to open the double bond in the monomer.
Mixing up thermoplastics and thermosetting plastics: Create a memory aid: "THERMOplastics can be reshaped with THERMAL energy (heat); THERMOsetting plastics SET permanently and cannot be reshaped."
Stating all plastics are bad for the environment: Be balanced in your answers. Mention both advantages (durability, low cost, versatility) and disadvantages (non-biodegradable, pollution problems). Examiners reward balanced responses.
Writing "plastic" when the question asks about "polymers": All plastics are polymers, but not all polymers are plastics (proteins and starch are polymers too). Read the question carefully and use the correct term.
Claiming biodegradable plastics solve all environmental problems: While they help, biodegradable plastics still require specific conditions to break down and may not decompose in ocean water. Show critical thinking in your answers.
Exam technique for "Polymers and Plastics"
Command word precision: "State" requires a short answer without explanation (1 mark). "Explain" requires reasoning or mechanism (usually 2-3 marks). "Describe" requires characteristics or steps in sequence. "Compare" requires similarities AND differences.
Drawing polymer structures: Use a ruler for straight bonds. Show at least two or three repeating units in brackets with the subscript "n" outside. Include continuation bonds (horizontal lines) extending beyond the structure to show the chain continues. Label all atoms clearly.
Environmental questions: Structure answers as Problem → Effect → Solution. For example: "Plastic is non-biodegradable [problem] which causes accumulation in oceans and harms marine life [effect]. Recycling programmes and bans on single-use plastics help reduce waste [solution]."
Mark allocation guides detail: If a question is worth 3 marks, you must make three distinct points. Don't repeat the same idea in different words. Read the mark scheme expectation from the question value.
Quick revision summary
Polymers are large molecules formed when many monomers join through polymerisation. Addition polymerisation creates polymers like polyethene, PVC, and polystyrene from monomers with C=C double bonds. Thermoplastics soften when heated and can be remoulded; thermosetting plastics set permanently with cross-linked structures. Plastics offer advantages including durability, low cost, and waterproofing, but cause environmental problems because most are non-biodegradable. Solutions include recycling, reducing single-use plastics, and developing biodegradable alternatives—particularly important for Caribbean nations facing marine pollution and limited disposal options.