Ch 8 · Polymers · IGCSE Chemistry 0620
Topic 8 of 8 in Organic Chemistry — IGCSE Chemistry 0620 — 48 lessons.
Introduction
Polymers are the giant molecules in plastics, fibres and, as proteins, in every living cell. The chemistry is simple, joining many small molecules into one long chain, but the consequences range from cheap packaging to oceans full of waste that will not rot. Learn the definitions, how to draw poly(ethene), the arguments about disposal, and (Extended) how to move between monomer and repeat unit and how nylon and PET differ from addition polymers.
Key ideas — Monomers, polymers and addition polymerisation
Polymers are large molecules built up from many smaller molecules called monomers. In addition polymerisation the monomers are alkenes: the mathrmC=C double bond in each monomer opens and the carbons join to their neighbours, so thousands of monomers link into one chain with no other product.
🧑🏫 Teacher's explanation
Imagine a long chain made of paperclips. Each individual paperclip is a 'monomer'. When you hook hundreds of them together, you get a long, continuous chain called a 'polymer'. In chemistry, these 'paperclips' are small molecules, and the 'chain' is a massive molecule.
📖 Definitions
🔍 Worked example — Easy
Identify the repeat unit of the polymer formed from the monomer ceCH2=CH2.
🔍 Worked example — Standard
A polymer is formed from the monomer ceCH2=CHCl. Draw the repeat unit.
🔍 Worked example — Exam-style
Calculate the molar mass of a polymer chain consisting of 1000 ethene (ceC2H4) units. (Use C=12, H=1)
⚠️ Common errors
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🎯 Exam question
(a) Define the term 'polymer'. [1] (b) Draw the repeat unit for the polymer formed from the monomer ceCH2=CH-CH2-CH3. [1] (c) Explain why non-biodegradable plastics, such as poly(ethene), pose an environmental challenge. [2] [4 marks]
Key ideas — Plastics and their disposal
Plastics are made from polymers. They are cheap, light, strong, waterproof and unreactive, which makes them useful and also a problem: most are non-biodegradable, so microorganisms cannot break them down and they persist for centuries.
🧑🏫 Teacher's explanation
Imagine a long necklace made of hundreds of identical beads. Each individual bead is a 'monomer', and the entire long chain is the 'polymer'. In chemistry, polymers are giant molecules formed by linking many small, repeating units together.
📖 Definitions
🔍 Worked example — Easy
Draw the repeat unit of poly(ethene) formed from ethene (C_2H_4).
🔍 Worked example — Standard
A plastic is made from 500 g of ethene. If the M_r of ethene is 28 and the M_r of the repeat unit in poly(ethene) is 28, calculate the mass of the polymer produced, assuming 100\% yield.
🔍 Worked example — Exam-style
Compare the environmental impact of incineration versus recycling for a plastic like PVC.
⚠️ Common errors
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🎯 Exam question
(a) Define the term 'polymer'. [1] (b) Explain why the use of non-biodegradable plastics in the ocean is harmful to marine life. [2] (c) A plastic is made from the monomer CH_2=CH_2. Draw the structure of the repeat unit of the resulting polymer. [1] [4 marks]
Key ideas — Repeat units, and addition versus condensation
🧑🏫 Teacher's explanation
Imagine a long chain of paperclips. Each individual paperclip is a 'monomer'. When you hook them all together, you get a long chain called a 'polymer'. In addition polymerisation, it is like taking a long strip of tape and simply sticking it end-to-end; nothing is lost, the chain just gets longer.
📖 Definitions
🔍 Worked example — Easy
Draw the repeat unit for the addition polymer formed from ethene, CH_2=CH_2.
🔍 Worked example — Standard
A condensation polymer is formed from a dicarboxylic acid and a diamine. If 10 moles of each monomer react, how many moles of water are produced?
🔍 Worked example — Exam-style
Compare addition and condensation polymerisation regarding the number of products formed.
⚠️ Common errors
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🎯 Exam question
(a) Name the type of polymerisation that occurs. [1] (b) Draw the structure of the repeat unit for poly(chloroethene). [1] (c) State one environmental problem associated with the disposal of non-biodegradable plastics like poly(chloroethene). [1] [3 marks]
Key ideas — Nylon, PET and proteins
🧑🏫 Teacher's explanation
Imagine a long necklace made of many identical beads. If the beads are the 'monomers', the whole necklace is the 'polymer'. In addition polymerisation (like making poly(ethene)), you just link identical units together without losing anything. But in condensation polymerisation, it is like building a LEGO structure where every time two pieces click together, a tiny bit of 'debris' (a small molecule like water) is squeezed out and discarded.
📖 Definitions
🔍 Worked example — Easy
Identify the type of polymer formed when ethene (C_2H_4) undergoes addition polymerisation.
🔍 Worked example — Standard
A monomer has the formula HOOC-CH_2-CH_2-COOH and another has H_2N-CH_2-CH_2-NH_2. When they react to form a polyamide, how many water molecules are produced for every 2 monomers joined?
🔍 Worked example — Exam-style
Draw the repeat unit of the polyester PET formed from terephthalic acid (C_6H_4(COOH)_2) and ethane-1,2-diol (HOCH_2CH_2OH).
⚠️ Common errors
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🎯 Exam question
Worked example
Question. (a) Define the term polymer. [1] (b) (Supplement) Chloroethene, mathrmCH_2=CHCl, forms the polymer PVC. Draw the repeat unit of PVC and name the type of polymerisation. [2] (c) (Supplement) Nylon is formed from two monomers. Explain why its formation is described as condensation polymerisation and name the linkage in nylon. [3]
🔍 Worked example — Easy
Define a polymer and identify the monomer used to make poly(ethene).
🔍 Worked example — Standard
Draw the repeat unit for the addition polymer formed from propene (CH_2=CHCH_3).
🔍 Worked example — Exam-style
A polymer is formed by the reaction between a dicarboxylic acid and a diamine. Identify the type of polymerisation and the small molecule eliminated.
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Practice
Answers Many ethene monomers join together [B1]; the double bond in each opens so the carbons bond to neighbouring monomers with no other product [B1]; n C_2H_4 arrow -(CH_2-CH_2)_n- [B1]. Advantage: energy released can be used / reduces the volume going to landfill [B1]. Disadvantage: produces carbon dioxide, a greenhouse gas, and possibly toxic gases [B1]. They are not broken down by microorganisms so persist for hundreds of years [B1]; they accumulate as litter and microplastics that harm wildlife [B1]. mathrmCH_2=CHCH_3 (double bond restored between the two backbone carbons) [B1]; propene [B1]. Proteins are formed from amino acid monomers [B1] joined by the same amide linkage, -CO-NH-, as nylon [B1].