A polymer is a very large molecule built from many small repeating units called monomers, joined together by covalent bonds. Natural polymers such as natural rubber, protein, starch and cellulose come from living things; synthetic polymers such as polythene, PVC and nylon are made by people from petrochemical monomers.
The number of monomer units in one polymer molecule is its degree of polymerisation.
This page covers content standard 12.1 of the Form 5 Polymers chapter: natural and synthetic polymers. It is the foundation of the whole chapter, because once you understand what a polymer is and how monomers join, everything that follows, the types of polymerisation, natural rubber and its vulcanisation, and the uses and environmental issues of plastics, becomes far easier to reason about. Learn the vocabulary carefully here and you will read the rest of the chapter with confidence.
What is a polymer?
A polymer is a very large molecule (a macromolecule) built from many small, repeating units joined together by covalent bonds. Each small unit that joins up is called a monomer, and the joining process is called polymerisation. Because thousands of monomers can link into one chain, a single polymer molecule has a very large relative molecular mass, often in the tens or hundreds of thousands.
Key terms. A monomer is the small starting molecule. A polymer is the long-chain product. The repeating unit is the part of the chain that is repeated over and over; you write it inside brackets with an n outside to show it repeats many times. The degree of polymerisation is the number of monomer units, n, in one polymer molecule.
For example, the monomer ethene (CH₂=CH₂) polymerises into polythene (also called polyethene), whose repeating unit is, CH₂, CH₂, . The whole polymer is written (, CH₂, CH₂, )ₙ, where n is a very large number.
Natural polymers
Natural polymers are polymers made by living things or found in nature. You should be able to name common examples and say where each comes from:
- Natural rubber, a polymer of isoprene, obtained as latex from the rubber tree (Hevea brasiliensis).
- Proteins, polymers of amino acids; found in meat, egg, hair and enzymes.
- Starch and cellulose, polymers of glucose; starch stores energy in plants, cellulose forms plant cell walls and is the main part of cotton and wood.
- Nucleic acids (DNA), polymers that carry genetic information.
These form inside living cells through the chemistry of life, not in a factory.
Synthetic polymers
Synthetic polymers are man-made polymers, manufactured from monomers that usually come from petroleum (crude oil) and natural gas. Common examples you must know include:
| Synthetic polymer | Common use |
|---|---|
| Polythene (polyethene) | Plastic bags, bottles, films |
| Polypropene (polypropylene) | Containers, ropes, crates |
| Poly(vinyl chloride), PVC | Pipes, cable insulation, raincoats |
| Perspex (poly(methyl methacrylate)) | Clear sheets, aircraft windows |
| Nylon | Fishing line, textiles, ropes |
| Terylene (a polyester) | Clothing fibre, bottles |
Synthetic polymers are designed to have useful properties, strength, flexibility, resistance to water and chemicals, which is why plastics have replaced so many traditional materials.
Telling the two apart
The test is simply the origin of the polymer. If it is made by a living organism or grows in nature (rubber, protein, starch, cellulose, silk, wool), it is natural. If people manufacture it from small monomer molecules, usually from petrochemicals (polythene, PVC, nylon, terylene, Perspex), it is synthetic. Both are still true polymers, long chains of repeating units held by covalent bonds, so the classification is about how the polymer is produced, not about its structure.
Worked example
Question. A molecule of polythene, (, CH₂, CH₂, )ₙ, has a relative molecular mass of 42 000. The monomer is ethene, C₂H₄. Calculate the degree of polymerisation, n. [Relative atomic masses: H = 1, C = 12]
Step 1, Find the relative molecular mass of the repeating unit. The repeating unit is, CH₂, CH₂, which has the same formula as one ethene molecule, C₂H₄.
Mr of C₂H₄ = (2 × 12) + (4 × 1) = 24 + 4 = 28Step 2, Set up the relationship. One polymer molecule is made of n repeating units, so:
Mr of polymer = n × Mr of repeating unitStep 3, Solve for n.
n = Mr of polymer ÷ Mr of repeating unit
n = 42 000 ÷ 28
n = 1500Answer. The degree of polymerisation is n = 1500, that is, 1500 ethene monomers joined to make one polythene molecule.
Practice question
Question. Classify each of the following as a natural polymer or a synthetic polymer, and state the monomer or building unit of each: (a) starch, (b) polythene, (c) natural rubber, (d) nylon.
Answer. (a) Starch, natural polymer; building unit is glucose. (b) Polythene, synthetic polymer; monomer is ethene. (c) Natural rubber, natural polymer; monomer is isoprene. (d) Nylon, synthetic polymer; made from two monomers (a diamine and a dicarboxylic acid).
Exam tip
The most reliable marks in this standard come from precise vocabulary. Never write “small molecules join to form a big molecule”, say the monomers join by covalent bonds to form a polymer, and if asked, name the repeating unit. When you classify a polymer, justify it by origin: “natural, because it is obtained from a living thing” or “synthetic, because it is manufactured from petrochemical monomers.” For a degree-of-polymerisation calculation, always find the Mr of the repeating unit first, then divide the polymer’s Mr by it, a common slip is to divide by the wrong mass. Remember that the repeating unit of an addition polymer has the same molecular formula as its monomer, so its Mr equals the monomer’s Mr.
Where this fits
This is content standard 12.1 of the Polymers chapter, and it sets up everything that follows. The next standard, types of polymerisation, explains exactly how monomers join; natural rubber and vulcanisation applies these ideas to one important natural polymer; and the final standard looks at the uses and environmental issues of the synthetic polymers listed above. Practise the degree-of-polymerisation calculations on the chapter worked examples page, and check the vocabulary traps on the common mistakes page. In our online 1-to-1 SPM Chemistry lessons, taught in English from RM50/hr, our teachers make sure you can draw and name a repeating unit and never confuse a monomer with a polymer.
Quick recap
- A polymer is a large molecule of many monomers joined by covalent bonds; the repeated part is the repeating unit.
- Natural polymers: rubber, protein, starch, cellulose, made by living things.
- Synthetic polymers: polythene, polypropene, PVC, Perspex, nylon, terylene, man-made from petrochemicals.
- Degree of polymerisation n = Mr of polymer ÷ Mr of repeating unit.
- Classify by origin, not by structure, both types are true polymers.
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