C26: Polymers
How addition and condensation polymers are formed from monomers, the structure and uses of common polymers, natural polymers, and the environmental problems of polymer disposal.
What Are Polymers?
Polymers are very large molecules (macromolecules) made from many small molecules called monomers joined together in a repeating chain. The process of making polymers from monomers is called polymerisation.
Polymers can be natural (e.g. proteins, starch, DNA) or synthetic (e.g. polyethene, nylon, PET). Synthetic polymers are made from chemicals derived from crude oil.
Addition Polymers
Addition polymers are formed from alkene monomers. The C=C double bond in each monomer opens up and the monomers join together in a long chain. No other products are formed — this is an addition reaction.
Poly(ethene) — Polythene
Poly(ethene) is made from ethene monomers. It is a tough, flexible polymer used in plastic bags, bottles and packaging.
n C₂H₄ → (−CH₂−CH₂−)ₙ
The "n" represents a very large number — typically thousands of monomer units join together.
Poly(propene) — Polypropylene
Poly(propene) is made from propene monomers. It is stronger and more heat-resistant than poly(ethene). Used in car bumpers, crates and rope.
n C₃H₆ → (−CH₂−CH(CH₃)−)ₙ
Other Addition Polymers
| Polymer | Monomer | Properties | Uses |
|---|---|---|---|
| Poly(chloroethene) (PVC) | Chloroethene | Tough, rigid or flexible | Window frames, pipes, cable insulation |
| Poly(tetrafluoroethene) (PTFE) | Tetrafluoroethene | Very unreactive, non-stick | Non-stick coatings, Gore-Tex |
| Poly(styrene) | Styrene (phenylethene) | Light, good insulator | Expanded polystyrene packaging, insulation |
When drawing addition polymer structures from a monomer: open the C=C double bond, draw the repeating unit with single bonds on either side, and add subscript "n" outside brackets.
Worked Example: Identifying monomers from polymers
Question: A polymer has the repeating unit (−CH₂−CHCl−)ₙ. What is the monomer?
Answer: Close the single bonds on either side of the repeating unit to reform the C=C double bond. The monomer is chloroethene: CH₂=CHCl
Worked Example: Writing the polymer equation
Question: Write the equation for the formation of poly(propene) from propene.
Answer: n CH₂=CHCH₃ → (−CH₂−CH(CH₃)−)ₙ
The C=C double bond in propene opens and the monomers join in a chain. No other product is formed.
Condensation Polymers
Condensation polymers are formed when two different types of monomer join together. Each time a bond forms between monomers, a small molecule (usually water) is eliminated. Two functional groups are needed — one on each monomer type.
Polyesters
Polyesters are formed from a diol (an alcohol with two −OH groups) and a dicarboxylic acid (a carboxylic acid with two −COOH groups). Water is eliminated each time an ester link forms.
Diol + dicarboxylic acid → polyester + water
Ester link: −COO−
Worked Example: Formation of a polyester
When ethanediol (HO−CH₂−CH₂−OH) reacts with hexanedioic acid (HOOC−(CH₂)₄−COOH):
The −OH of the diol reacts with the −COOH of the dicarboxylic acid, eliminating water and forming an ester link (−COO−).
This repeats at both ends of each monomer, building a long polymer chain with the repeating unit containing the ester link.
PET (polyethylene terephthalate) is a common polyester used in plastic bottles and clothing fibres (fleece).
Polyamides
Polyamides are formed from a diamine (an amine with two −NH₂ groups) and a dicarboxylic acid. Water is eliminated each time an amide link forms.
Diamine + dicarboxylic acid → polyamide + water
Amide link: −CO−NH−
Worked Example: Formation of a polyamide (nylon)
When hexanedioic acid (HOOC−(CH₂)₄−COOH) reacts with 1,6-diaminohexane (H₂N−(CH₂)₆−NH₂):
The −NH₂ of the diamine reacts with the −COOH of the dicarboxylic acid, eliminating water and forming an amide link (−CO−NH−).
The polymer formed is nylon-6,6, used in clothing, ropes and carpets.
Kevlar is another polyamide made from benzene-1,4-dicarboxylic acid and benzene-1,4-diamine. It is extremely strong and used in bulletproof vests.
| Property | Addition Polymers | Condensation Polymers |
|---|---|---|
| Monomer types | One type (alkene) | Two types (each with two functional groups) |
| Other product | None (only polymer formed) | Small molecule eliminated (e.g. water) |
| Linkage type | C−C bonds from opened C=C | Ester link (−COO−) or amide link (−CO−NH−) |
| Monomer requirement | C=C double bond | Two functional groups per monomer |
| Examples | Poly(ethene), PVC, PTFE | PET, nylon, Kevlar |
Natural Polymers
DNA
DNA (deoxyribonucleic acid) is a natural polymer made from four different nucleotide monomers. It carries the genetic code for all living organisms. The nucleotides contain a sugar, a phosphate group and a base.
DNA is a condensation polymer — nucleotides join together with the elimination of water. Two polymer chains wind together to form the famous double helix structure.
Proteins
Proteins are natural polymers made from amino acid monomers. There are about 20 different amino acids that can join in various sequences to make different proteins.
Amino acids contain both an amine group (−NH₂) and a carboxylic acid group (−COOH). They join by condensation polymerisation, forming amide links (−CO−NH−) and eliminating water.
Amino acid + amino acid → dipeptide + water
Many amino acids → polypeptide (protein) + water
Proteins are essentially natural polyamides. Examples include enzymes, haemoglobin, keratin (hair, nails) and collagen (skin, tendons).
Starch and Cellulose
Starch and cellulose are natural polymers made from glucose monomers. They are both carbohydrates but have different structures because the glucose units are linked differently.
- Starch — energy storage polymer in plants; glucose units joined by α-glycosidic links
- Cellulose — structural polymer in plant cell walls; glucose units joined by β-glycosidic links, forming straight chains that pack together tightly
Worked Example: Identifying natural polymer types
Question: What type of polymer is a protein — addition or condensation?
Answer: A protein is a condensation polymer. It is made from amino acid monomers that join together with the elimination of water. The amino acids have two functional groups (−NH₂ and −COOH), forming amide links (−CO−NH−), so proteins are natural polyamides.
Problems with Polymer Disposal
Most synthetic addition polymers are non-biodegradable — they are not broken down by microorganisms. This causes significant environmental problems.
Problems caused by non-biodegradable polymers:
- Landfill sites fill up with waste plastic that does not decompose
- Plastic waste in oceans harms marine life
- Burning plastics can release toxic gases
- Microplastics form as large plastics slowly fragment
Methods of Polymer Disposal
| Method | Advantages | Disadvantages |
|---|---|---|
| Landfill | Simple, cheap | Wastes land, polymers do not decompose, produces methane |
| Incineration | Reduces waste volume, produces energy | Releases CO₂ and possibly toxic gases, wastes resources |
| Recycling | Conserves resources, reduces landfill | Difficult to separate different polymers, polymers must be sorted by type |
| Reusing | No processing needed, reduces waste | Limited applications |
Recycling is the most sustainable option, but it requires sorting polymers by type because different polymers have different properties and cannot be mixed. Many plastic products are labelled with recycling codes to help sorting.
Biodegradable Polymers
Biodegradable polymers can be broken down by microorganisms over time. They are made from renewable raw materials such as starch or from polymers designed to degrade under certain conditions.
Advantages of biodegradable polymers:
- Reduce the amount of waste going to landfill
- Made from renewable resources (e.g. plant starch)
- Break down naturally in the environment
Disadvantages of biodegradable polymers:
- Often more expensive to produce than conventional polymers
- May need specific conditions to break down (e.g. industrial composting)
- May not be as strong or durable as conventional polymers
- Could release methane if broken down anaerobically in landfill
When discussing polymer disposal, always consider the environmental impact of each method and explain why recycling is preferable. Mention that the best solution is to reduce polymer use in the first place (reduce, reuse, recycle).
Drawing Polymer Repeating Units
For addition polymers: take the monomer, open the C=C double bond to single bonds, and draw the repeating unit in brackets with subscript n.
For condensation polymers: identify the two functional groups that react, show the link formed (ester or amide), and indicate that water is eliminated.
Worked Example: Drawing a repeating unit
Monomer: propene (CH₂=CH−CH₃)
Step 1: Open the C=C double bond — change the double bond to two single bonds
Step 2: Draw the repeating unit: −CH₂−CH(CH₃)−
Step 3: Place in brackets with subscript n: (−CH₂−CH(CH₃)−)ₙ
This represents poly(propene).
Worked Example: Identifying monomer from repeating unit
Repeating unit: (−CH₂−C(CN)−)ₙ
Step 1: Close the single bonds on either end to form a C=C double bond
Step 2: The monomer is CH₂=C(CN) — this is acrylonitrile
Step 3: The polymer is poly(acrylonitrile), used to make acrylic fibres
Practice Questions
1. Describe how poly(ethene) is formed from ethene in terms of the bonds that change. (3 marks)
The C=C double bond in each ethene monomer opens up to form single C−C bonds. The monomers then join together in a long chain by these single bonds. This is an addition reaction because no other products are formed — only the polymer.
2. Give two differences between addition polymerisation and condensation polymerisation. (2 marks)
Addition polymerisation uses one type of monomer with a C=C double bond, whereas condensation polymerisation uses two types of monomer each with two functional groups. Addition polymerisation produces only the polymer, whereas condensation polymerisation also produces a small molecule such as water.
3. A polymer has the repeating unit (−CH₂−CHCl−)ₙ. Give the name and structure of the monomer. (2 marks)
The monomer is chloroethene, with structure CH₂=CHCl. The C=C double bond is reformed by closing the single bonds at each end of the repeating unit.
4. Explain why most synthetic polymers cause environmental problems when disposed of in landfill. (3 marks)
Most synthetic polymers are non-biodegradable, meaning they cannot be broken down by microorganisms. They persist in the environment for hundreds of years. This means landfill sites fill up with waste plastic that does not decompose, taking up valuable land and potentially releasing harmful chemicals.
5. Proteins are natural polymers made from amino acids. Explain why proteins are classified as condensation polymers. (3 marks)
Amino acids have two functional groups (−NH₂ and −COOH). When amino acids join together, the amine group of one reacts with the carboxylic acid group of another, forming an amide link and eliminating a molecule of water. This is condensation polymerisation because a small molecule (water) is lost each time a bond forms between monomers.
Maths Skills
Repeating Unit Calculations
A polymer is made from many monomer units joined together. The repeating unit is the smallest section of the polymer chain that, when repeated, generates the full polymer structure.
From monomer to repeating unit (addition polymerisation):
- Open the C=C double bond in the monomer to form single bonds on each carbon.
- Draw the unit in brackets with a subscript 'n' to show it repeats.
Example: Ethene (CH₂=CH₂) becomes the repeating unit (−CH₂−CH₂−)ₙ, which is polyethene.
Calculating the mass of a polymer: If n monomers join, the relative mass of the polymer chain is n × M_r(monomer). For addition polymers, the mass of the polymer equals the total mass of monomers (no atoms are lost).
Condensation polymers: For each bond formed between monomers, a small molecule (usually water) is eliminated. So if n monomers react, (n−1) water molecules are lost. The mass of the polymer = n × M_r(monomer) − (n−1) × 18.
Example: 100 glycine monomers (M_r = 75) form a polypeptide: Mass = 100 × 75 − 99 × 18 = 7500 − 1782 = 5718.
Common Misconceptions
Misconceptions About Polymers
All plastics are the same type of polymer.
There are many different types of polymers with very different properties. Thermosoftening polymers (e.g. polyethene) soften when heated and can be remoulded. Thermosetting polymers (e.g. melamine resin) have strong cross-links and do not soften when heated — they char instead. Addition polymers (e.g. polyethene, PVC) and condensation polymers (e.g. nylon, PET) are formed by completely different processes.
The monomer and the repeating unit are the same thing.
The monomer has a C=C double bond, but the repeating unit has the double bond opened to form single bonds. For example, propene (CH₂=CHCH₃) has a double bond, but the repeating unit of polypropene is (−CH₂−CH(CH₃)−)ₙ with no double bond.
Polymers can easily be broken back down into their monomers.
Addition polymerisation is very difficult to reverse. Most polymers cannot be easily depolymerised back into their original monomers, which is one reason why recycling polymers is challenging.
6-Mark Question
Compare addition and condensation polymerisation. Explain the problems with disposing of polymers.
Addition polymerisation involves monomers with C=C double bonds joining together without losing any atoms — the polymer has the same atoms as the monomers. Examples include polyethene from ethene and PVC from chloroethene. Condensation polymerisation involves two different types of monomer joining together with the loss of a small molecule (usually water). Each monomer must have two functional groups. Examples include nylon from a diamine and a dicarboxylic acid, and PET from a diol and a dicarboxylic acid. Disposal problems include: landfill — polymers are non-biodegradable so they persist for hundreds of years, taking up space and potentially releasing toxic substances; incineration — burning polymers releases CO₂ (contributing to climate change) and can release toxic gases such as HCl from PVC; recycling — sorting different types of polymers is difficult and expensive, and recycled polymer is often lower quality than virgin polymer. Biodegradable polymers and bioplastics offer some solutions but are not yet widely used.
AO3: Analysis and Evaluation
Evaluating Polymer Disposal Methods
A local council needs to decide how to dispose of waste polymers. The options are: landfill, incineration with energy recovery, or recycling.
Evaluate each method, giving advantages and disadvantages, and justify which method is most sustainable.
Answer: Landfill is cheapest in the short term but polymers do not biodegrade, taking up land for centuries, and toxic additives can leach into soil and groundwater. Incineration with energy recovery reduces waste volume and generates electricity, but releases CO₂ and potentially toxic gases (e.g. dioxins from PVC), requiring expensive gas cleaning. Recycling conserves crude oil resources, reduces landfill, and uses less energy than making new polymer, but sorting is costly and labour-intensive, contamination reduces quality, and recycled polymer often cannot be used for food packaging. Recycling is the most sustainable long-term option as it reduces resource use and waste, though it requires investment in sorting infrastructure and public education on recycling correctly.
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