11.8 Polymers
- Syllabus
- 0620–2026–2027
- Topic
- 11.8
- Level
- —
Learning objectives
11.8.1Polymers as large molecules built up• Define polymers as large molecules built up from many smaller molecules called monomers11.8.2Formation of poly(ethene) as an• Describe formation of poly(ethene) as an example of addition polymerisation using ethene monomers11.8.3Plastics are made from polymers• State: plastics are made from polymers11.8.4Properties of plastics have• Describe how the properties of plastics have implications for their disposal11.8.5Environmental challenges caused by• Describe environmental challenges caused by plastics, (a) disposal in landfill sites (b) accumulation in oceans (c) formation of toxic gases from burning11.8.6Repeat units and/or linkages in• Identify the repeat units and/or linkages in addition polymers and in condensation polymers11.8.7Deduce the structure or repeat unit of• Deduce the structure or repeat unit of an addition polymer from a given alkene and vice versa11.8.8Deduce the structure or repeat unit of• Deduce the structure or repeat unit of a condensation polymer from given monomers and vice versa, (a) polyamides from a dicarboxylic acid and a diamine (b) polyesters from a dicarboxylic acid and a diol11.8.9Differences between addition and• Describe differences between addition and condensation polymerisation11.8.10And draw the structure of: (a) nylon• Describe and draw the structure of: (a) nylon, a polyamide C O C O C C OO N H N H N H N H (b) PET, a polyester C O C O C C OO O OO O The full name for PET, polyethylene terephthalate, is not required11.8.11PET can be converted back into• State: PET can be converted back into monomers and re-polymerised11.8.12Proteins as natural polyamides and• Describe proteins as natural polyamides and that they are formed from amino acid monomers with the general structure: H O H OH N H C C R where R represents different types of side- chain11.8.13And draw the structure of proteins as• Describe and draw the structure of proteins as: N H C O N H C O N H C O
Define polymers and monomers
A polymer is a large molecule built up from many smaller molecules called monomers.
Polymerisation joins many monomer molecules into a long chain. The repeating structural pattern in the chain is called the repeat unit.
| Term | Scale and role |
|---|---|
| monomer | small molecule capable of joining to others |
| polymer | one very large chain molecule |
| repeat unit | smallest structural pattern repeated along the chain |
A polymer is one macromolecule, not merely a mixture of many separate small molecules. A monomer is the starting molecule, while the repeat unit is the pattern inside the polymer.
Form poly(ethene) by addition polymerisation
Poly(ethene) forms when many ethene monomers join by addition polymerisation.
n CH₂=CH₂ → [–CH₂–CH₂–]ₙ
The C=C bond in each ethene opens to a C–C single bond. The carbon atoms link into a long chain, and no small molecule is removed.
Draw the repeat unit with two backbone carbon atoms, single bonds, continuation bonds through brackets and n outside the brackets.
Do not leave C=C inside poly(ethene), and do not add water or another by-product: the polymer is the only product.
Connect plastics with polymers
Plastics are made from polymers.
A plastic material contains long polymer molecules and may also contain additives that adjust colour, flexibility, strength or durability.
The long-chain structure gives plastics useful material properties, while those same properties affect how plastic waste behaves after use.
Plastic is the material; polymer describes the large molecules from which the material is made. The terms are related but not always interchangeable.
Relate plastic properties to disposal
| Useful plastic property | Disposal implication |
|---|---|
| durable / chemically resistant | persists for a long time after disposal |
| non-biodegradable | microorganisms do not break it down quickly |
| low density / lightweight | easily transported by wind and water |
| combustible | burning may reduce volume but can release harmful gases |
A disposal explanation must connect a property to a consequence. For example: chemical resistance makes a container durable in use, but also slows degradation in landfill.
The problem is not that the properties fail; the problem is that useful lifetime properties remain after the item is discarded.
Do not claim every plastic decomposes harmlessly or that burning is automatically safe. Disposal depends on polymer composition and controlled treatment.
Explain three environmental challenges from plastics
| Challenge | Environmental consequence |
|---|---|
| landfill disposal | non-biodegradable waste occupies land and persists |
| accumulation in oceans | animals may ingest plastic or become entangled; fragments remain in ecosystems |
| burning | some plastics form toxic gases that pollute air and harm health |
Name the disposal route or location, then state a specific consequence rather than only saying 'pollution'.
Because many plastics are non-biodegradable, the problems can accumulate as more waste is added over time.
Landfill, ocean accumulation and toxic gases from burning are three separate challenges; do not substitute one generic statement for all three.
Identify repeat units and polymer linkages
| Polymer type | Structural signature | Linkage to identify |
|---|---|---|
| addition polymer | C–C backbone carrying substituents | no new ester/amide linkage; repeat comes from alkene |
| polyester | alternating monomer residues | ester link, –COO– |
| polyamide / protein | alternating or amino-acid residues | amide link, –CONH– |
| complex carbohydrate | sugar residues | –O– linkage |
A repeat unit must reproduce the chain when copied end-to-end. Include the exact atoms between equivalent continuation points, not an arbitrary visual segment.
First locate the repeating pattern; then identify any characteristic linkage crossing between monomer residues.
The repeat unit is not necessarily the same drawing as the monomer. In addition polymerisation C=C becomes C–C; in condensation polymerisation atoms are lost in a small molecule.
Convert between an alkene and its addition polymer
| Direction | Operation |
|---|---|
| alkene → repeat unit | replace C=C by C–C, keep every substituent on the same carbon, add continuation bonds and brackets |
| repeat unit → alkene | select the two-carbon backbone repeat, remove continuation bonds, change the backbone C–C to C=C, keep substituents |
Propene CH₂=CHCH₃ gives [–CH₂–CH(CH₃)–]ₙ. Chloroethene CH₂=CHCl gives [–CH₂–CH(Cl)–]ₙ.
Each carbon must have four bonds. Side groups stay attached to the same backbone carbon throughout the conversion.
Do not place the double bond inside the polymer repeat unit, and do not move or duplicate a substituent when reconstructing the monomer.
Build condensation polymers from bifunctional monomers
| Polymer | Monomers | Link formed | Small molecule removed |
|---|---|---|---|
| polyamide | dicarboxylic acid + diamine | –CONH– | water |
| polyester | dicarboxylic acid + diol | –COO– | water |
Connect one functional group at each end of a monomer to a functional group on the next monomer. Remove OH from –COOH and H from –NH₂ or –OH to form water, then draw the new linkage.
To deduce monomers from a polymer, cut each ester or amide linkage and restore –COOH plus –OH (polyester) or –COOH plus –NH₂ (polyamide).
The monomers must be bifunctional so chains can continue at both ends. A monocarboxylic acid or monohydric alcohol alone cannot build the required long condensation chain.
Distinguish addition and condensation polymerisation
| Feature | Addition polymerisation | Condensation polymerisation |
|---|---|---|
| monomer | usually contains C=C | has two reactive functional groups |
| bond change | C=C opens to C–C chain | ester or amide link forms |
| products | polymer only | polymer plus small molecule, usually water |
| atom accounting | all monomer atoms enter polymer | atoms are removed into small molecule |
| examples | poly(ethene), poly(propene) | polyesters, polyamides, proteins |
Look first for C=C versus paired functional groups, then check whether a small molecule is produced.
Do not call every polymerisation addition merely because monomers join. Condensation is identified by link formation with loss of a small molecule.
Recognise and draw nylon and PET
| Polymer | Type | Characteristic repeat representation |
|---|---|---|
| nylon | polyamide | [–NH–(CH₂)₆–NH–CO–(CH₂)₄–CO–]ₙ |
| PET | polyester | [–O–CH₂–CH₂–O–CO–C₆H₄–CO–]ₙ |
Nylon contains repeating amide links, –CONH–, between diamine and dicarboxylic-acid residues.
PET contains repeating ester links, –COO–, between diol and dicarboxylic-acid residues. The full name of PET is not required.
Show continuation bonds through a complete repeat unit and include every atom and bond in each amide or ester linkage.
Nylon is a polyamide and PET is a polyester. Do not swap their linkages or identify either as an addition polymer.
Explain how PET can be re-polymerised
PET can be converted back into its monomers and then re-polymerised.
PET polymer → chemical breakdown to monomers → purification of monomers → condensation polymerisation → PET
This is chemical recycling: the ester links are broken to recover starting molecules, which can form new ester links.
Re-polymerisation is more than melting and reshaping PET. The polymer is first converted back into monomers.
Describe proteins as natural polyamides
Proteins are natural polyamides formed from amino acid monomers.
General amino acid structure: H₂N–CH(R)–COOH. Every amino acid has an amino group, a carboxylic acid group and an R side-chain attached to the central carbon.
R represents different side-chains. Different sequences of amino acids therefore give different proteins.
The –NH₂ group of one amino acid reacts with the –COOH group of another, forming an amide (peptide) link and water.
Amino acids are monomers; proteins are the resulting natural polyamides. R is a variable side-chain, not a fixed element symbol.
Draw the repeating structure of a protein
A protein chain contains the repeating backbone pattern: –NH–CH(R)–CO–NH–CH(R′)–CO–.
The amide/peptide linkage is –CO–NH–. It joins the carbonyl carbon of one amino acid residue to the nitrogen of the next.
When drawing a section, keep the backbone order N–C–C, show each C=O and N–H bond, place the appropriate R side-chain on each central carbon and add continuation bonds.
Do not draw free –NH₂ and –COOH groups at every internal residue. Those groups have reacted; the chain contains –CONH– links.