B. Organic Chemistry · Organic ChemistryPolymers: Addition and Condensation Polymerization
B3.4 B2.1 B2.4 B2.2 B1.1 A1.4About 75 minutes
Learning goals- I can define polymer, monomer and repeating unit, and give natural and synthetic examples.
- I can draw an addition polymer from its alkene monomer, and identify the monomer from a polymer.
- I can draw a condensation polymer (polyester or polyamide) from its monomers and identify the small molecule released.
- I can explain the difference between addition and condensation polymerization.
Words to know
polymer
A very large molecule made of many small repeating units (monomers) bonded together.
monomer
A small molecule that can bond to others like it to form a polymer.
repeating unit
The smallest part of a polymer chain that repeats over and over; written in brackets with a subscript n.
addition polymerization
Polymerization in which monomers with a C=C bond join without losing any atoms; the polymer is the only product.
condensation polymerization
Polymerization in which monomers with two functional groups each join and release a small molecule, usually water, at every link.
polyester
A condensation polymer whose units are joined by ester links, such as PET.
polyamide
A condensation polymer whose units are joined by amide links, such as nylon, Kevlar and proteins.
thermoplastic and thermoset
A thermoplastic softens when heated and can be remoulded; a thermoset has cross-links between chains and does not soften.
Giant molecules
Your winter jacket's fleece, the jug of milk in the fridge, a hockey helmet, a non-stick pan, the protein in your muscles and the cellulose in a maple leaf are all polymers: giant molecules made of thousands of small units, called monomers, bonded in long chains.
- Natural polymers: proteins (from amino acids), starch and cellulose (from glucose), DNA (from nucleotides) and natural rubber.
- Synthetic polymers: plastics and fibres such as polyethene, PVC, nylon and polyester.
Polymers form in two main ways, and the difference between them is the focus of this lesson (B3.4): addition and condensation polymerization.
Addition polymerization
The monomer is an alkene (or another compound with a C=C bond). Under the right conditions (heat, pressure and a catalyst or an initiator), the C=C bond of each monomer opens and the monomers link end to end, like people joining hands in a long line.
- No atoms are lost: the polymer is the only product (100% atom economy).
- The polymer's backbone is a chain of carbon atoms only.
- The repeating unit has the same atoms as the monomer, but the C=C has become C–C, with a bond reaching out at each end.
n CH₂=CH₂ → –(CH₂–CH₂)ₙ–
To name an addition polymer, put poly in front of the monomer's name: polyethene (polyethylene), polypropene (polypropylene), polychloroethene (polyvinyl chloride, PVC), polyphenylethene (polystyrene) and polytetrafluoroethene (PTFE, Teflon).
| Monomer | Polymer | Repeating unit | Resin code | Uses |
|---|
| ethene, CH₂=CH₂ | polyethene (PE) | –CH₂–CH₂– | 2 (HDPE), 4 (LDPE) | milk jugs, bags, squeeze bottles |
| propene, CH₂=CHCH₃ | polypropene (PP) | –CH₂–CH(CH₃)– | 5 | yogurt tubs, bottle caps, rope |
| chloroethene, CH₂=CHCl | polychloroethene (PVC) | –CH₂–CHCl– | 3 | pipes, window frames, flooring |
| phenylethene, CH₂=CHC₆H₅ | polystyrene (PS) | –CH₂–CH(C₆H₅)– | 6 | foam cups, packaging, CD cases |
| tetrafluoroethene, CF₂=CF₂ | PTFE (Teflon) | –CF₂–CF₂– | 7 (other) | non-stick coatings |
Worked exampleExample 1: Drawing an addition polymer
Draw three repeating units of polypropene and write the repeating unit.
- Monomer: CH₂=CH–CH₃. The C=C carbons become the backbone; the CH₃ becomes a branch.
- Three units: –CH₂–CH(CH₃)–CH₂–CH(CH₃)–CH₂–CH(CH₃)–.
- Repeating unit in brackets: –[CH₂–CH(CH₃)]ₙ–.
–CH₂–CH(CH₃)–CH₂–CH(CH₃)–CH₂–CH(CH₃)–; repeating unit –[CH₂–CH(CH₃)]–. The CH₃ branches hang off every second carbon.
Worked exampleExample 2: Finding the monomer
A section of polymer is –CF₂–CF₂–CF₂–CF₂–. What is the monomer? And for –CH₂–CHCl–CH₂–CHCl–?
- Find the repeating unit: –CF₂–CF₂–. Put a double bond between its two carbons: CF₂=CF₂, tetrafluoroethene.
- Second polymer: repeating unit –CH₂–CHCl–. Monomer: CH₂=CHCl, chloroethene (vinyl chloride).
tetrafluoroethene (making PTFE) and chloroethene (making PVC).
Condensation polymerization
Here each monomer has two functional groups, one at each end. Every time two monomers join, a small molecule, usually water, is released, just like the single condensation reactions you studied in Lessons 11 and 12. Because each monomer can react at both ends, the chain keeps growing.
Polyesters form from a diacid (two –COOH) and a diol (two –OH). Each link is an ester. PET (polyethylene terephthalate, resin code 1), used for drink bottles and fleece, is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol.
Polyamides form from a diacid and a diamine (two –NH₂). Each link is an amide.
- Nylon-6,6 comes from hexanedioic acid (6 C) and hexane-1,6-diamine (6 C). It was developed by Wallace Carothers's team at DuPont in the 1930s and is used in rope, tents and clothing.
- Kevlar, invented by chemist Stephanie Kwolek at DuPont in 1965, is a polyamide with benzene rings in its chain. The rigid chains line up and hydrogen-bond strongly, so the fibre is about five times stronger than steel of the same weight. It is used in protective gear and cut-resistant gloves.
- Proteins are natural polyamides made from about 20 different amino acids; each amino acid has an –NH₂ and a –COOH.
Polysaccharides such as starch and cellulose are condensation polymers of glucose: each link forms by joining two –OH groups and releasing water.
Worked exampleExample 3: A polyester
Draw the repeating unit of the polyester made from ethane-1,2-diol (HO–CH₂CH₂–OH) and hexanedioic acid (HOOC–(CH₂)₄–COOH). What is released?
- At each link, the –OH of an acid end and the H of an alcohol end leave as water, forming an ester link –CO–O–.
- Write the acid part, then the diol part, joined by ester links: –CO–(CH₂)₄–CO–O–CH₂CH₂–O–.
- For each repeating unit formed in the chain, two water molecules are released (one at each ester link).
Repeating unit –[CO–(CH₂)₄–CO–O–CH₂CH₂–O]ₙ–; the small molecule released is water.
Worked exampleExample 4: Addition or condensation?
Classify each polymer: (a) –CH₂–CH(CN)–CH₂–CH(CN)–; (b) –NH–(CH₂)₅–CO–NH–(CH₂)₅–CO–; (c) –O–CH₂CH₂–O–CO–C₆H₄–CO–.
- (a) Backbone is carbon only; the units look like an alkene (CH₂=CHCN) with the double bond opened → addition (polyacrylonitrile, used in acrylic yarn).
- (b) Amide links –CO–NH– in the backbone → condensation (a polyamide: nylon-6, made from a single monomer with an amine at one end and an acid at the other).
- (c) Ester links –O–CO– in the backbone → condensation (a polyester: PET).
(a) addition, (b) condensation (polyamide), (c) condensation (polyester).
| Addition polymerization | Condensation polymerization |
|---|
| Monomers | contain a C=C double bond | contain two functional groups each (–OH, –COOH, –NH₂) |
| By-product | none: the polymer is the only product | a small molecule (usually H₂O) at every link |
| Backbone | carbon atoms only | contains O or N in ester or amide links |
| Repeating unit vs monomer | same atoms as the monomer | monomers minus the atoms lost as water |
| Examples | polyethene, PVC, polystyrene, PTFE | PET, nylon, Kevlar, proteins, starch |
| Breakdown | very resistant; does not hydrolyse | links can be hydrolysed (slowly), as in digestion of proteins |
Structure and properties
- Chain length and forces: longer chains tangle more and have more London forces in total, so they are stronger and melt higher.
- Branching: low-density polyethene (LDPE) has many branches that stop chains packing, so it is soft and flexible (bags). High-density polyethene (HDPE) has few branches, packs tightly and is stiffer (milk jugs).
- Polar links: polyamides hydrogen-bond between chains (N–H···O=C), which makes nylon and Kevlar strong.
- Thermoplastics (PE, PP, PVC, PS, PET) soften when heated and can be remoulded, which makes them recyclable. Thermosets, such as the resins in some countertops and electrical fittings, have cross-links (covalent bonds between chains) and do not melt; they char instead.
- Vulcanized rubber: heating natural rubber with sulfur forms S–S cross-links, making it tougher and more elastic, as in car tires.
Investigation: the nylon rope trick (described)
In many school labs, nylon is made at the boundary between two liquids that do not mix:
- A solution of hexane-1,6-diamine in water is placed in a small beaker.
- A solution of decanedioyl dichloride (a more reactive relative of a diacid) in hexane is carefully poured on top.
- A film of nylon forms instantly where the layers meet. With forceps, the film is lifted and wound onto a glass rod as a continuous "rope".
Here the small molecule released at each link is HCl instead of water, but it is still condensation polymerization.
Safety (A1.4): the diamine is corrosive and the acid chloride is corrosive and reacts with moisture to release HCl gas; hexane is flammable. The demonstration is done in a fume hood with goggles and gloves, and the nylon must be washed before anyone touches it. This is not a home experiment.
Hands-on activityMake a natural polymer: casein plastic
You need: 250 mL (1 cup) of milk, 20 mL (4 teaspoons) of white vinegar, a microwave-safe cup or a small pot, a spoon, a strainer, paper towels, a cookie cutter or mould (optional), and a grown-up. Grown-up note: an adult should heat the milk and handle it while hot. Let the curds cool before kneading them. Do not eat the product.
- Warm the milk until it is hot but not boiling (about 50 °C, steaming).
- Stir in the vinegar. Lumps (curds) form within a minute.
- Pour the mixture through the strainer once it is cool enough to handle. Press the curds between paper towels to squeeze out the liquid.
- Knead the curds into a ball, then press them into a mould or shape them. Leave them to dry for two days.
- Answer: What is the polymer? Was it made here, or was it separated from the milk? Which kind of link joins its monomers?
What to look for: the solid is casein, a protein found in milk. The acid changes the conditions so the casein molecules clump together and come out of solution. You did not make a new polymer: casein was already a long chain of amino acids joined by amide (peptide) links, made by condensation inside the cow's cells. Casein-based plastics were used to make buttons and jewellery in the early 1900s.
Just for funPTFE (Teflon) was discovered by accident in 1938, when Roy Plunkett at DuPont opened a cylinder of tetrafluoroethene gas and found nothing came out: the gas had polymerized into a slippery white powder inside. It is now so slippery that getting it to stick to the pan is the hard part.
Your turn
1What is the monomer of polystyrene?
phenylethene (styrene), CH₂=CH–C₆H₅.
2Draw two repeating units of PVC.
3What by-product forms in addition polymerization?
None: the polymer is the only product.
4Which two kinds of monomer make a polyamide?
A diacid (or a monomer with –COOH) and a diamine (or a monomer with –NH₂).
5Classify –O–(CH₂)₂–O–CO–(CH₂)₂–CO– as addition or condensation, and name the type.
Condensation; it has ester links, so it is a polyester.
6Why can't ethanol (one –OH) be used to make a polyester?
With only one functional group, ethanol can react at one end only. It would cap the chain, not extend it. Condensation polymers need monomers with two reactive groups.
7Which is a natural polymer: nylon, starch or polypropene?
More practice: warm-up
A small molecule that joins with others to make a polymer.
2What bond must a monomer have for addition polymerization?
3What small molecule is usually released in condensation polymerization?
4Name the polymer made from ethene.
polyethene (polyethylene).
Stephanie Kwolek (DuPont, 1965).
More practice: on your own
1Draw the repeating unit and name the polymer made from but-2-ene.
Monomer CH₃CH=CHCH₃. Repeating unit: –[CH(CH₃)–CH(CH₃)]–, poly(but-2-ene).
2Give the monomers of the polyamide with repeating unit –[NH–(CH₂)₆–NH–CO–(CH₂)₈–CO]–.
hexane-1,6-diamine, H₂N(CH₂)₆NH₂, and decanedioic acid, HOOC(CH₂)₈COOH (this is nylon-6,10).
3A polyethene sample has an average molar mass of 140 000 g/mol. About how many ethene units (28.05 g/mol) are in an average chain?
140 000 ÷ 28.05 ≈ 4990 units (about 5000).
4Explain why HDPE is stiffer than LDPE, even though both are polyethene.
HDPE chains have few branches, so they pack closely and have more London attraction between chains. LDPE's branches keep chains apart, so it is softer and more flexible.
5Why do proteins, nylon and polyesters break down by hydrolysis, while polyethene does not?
Proteins, nylon and polyesters contain amide or ester links that water (with acid, base or enzymes) can split. Polyethene's backbone has only C–C and C–H bonds, which water cannot attack.
Challenge
1Lactic acid, CH₃CH(OH)COOH, has both an –OH and a –COOH. Show how it can form a polymer by itself (PLA, used in some compostable cups), give the repeating unit and the type of polymerization.
The –OH of one molecule condenses with the –COOH of the next, forming an ester link and releasing water, again and again. Repeating unit: –[O–CH(CH₃)–CO]–. This is condensation polymerization, giving a polyester (polylactic acid). Because it has ester links, PLA can be broken down by hydrolysis in industrial composting conditions.
2A chemist makes nylon-6,6 from 1.00 mol of hexanedioic acid and 1.00 mol of hexane-1,6-diamine. If every monomer joins one long chain, how many moles of water are released (to a good approximation)? Explain.
Each new amide link releases one H₂O. Joining 2.00 mol of monomers into one chain makes (2.00 mol − 1 molecule) links, which is essentially 2.00 mol of water (about 36 g). The "minus one" is a single molecule out of 1.2 × 10²⁴, so it does not change the answer.
3Kevlar and nylon-6,6 are both polyamides, but Kevlar is far stronger. Use structure to explain.
Kevlar's chain contains benzene rings joined by amide links, making the chains rigid and straight. Straight chains line up side by side in fibres, so hydrogen bonds form regularly between N–H and C=O groups of neighbouring chains, and the rings stack well. Nylon-6,6 has flexible (CH₂) segments that coil, so its chains are less ordered and the hydrogen bonding is less regular.
Quiz
1. Which monomer can undergo addition polymerization?
2. What is released when nylon-6,6 forms from a diacid and a diamine?
3. What is the repeating unit of polypropene?
4. Which statement describes condensation polymerization but NOT addition polymerization?
5. Which natural polymer is a polyamide?
6. Which feature lets a thermoset keep its shape when heated?
7. Which polymer has resin identification code 1?
Big ideas (B3.4, B2.4): addition polymers form from alkene monomers with no by-product and have all-carbon backbones; condensation polymers form from monomers with two functional groups, release a small molecule (usually water) at each link, and contain ester or amide links that can be hydrolysed.
How to help: Collect a few plastic items and read the resin codes together. Ask your teen to name the polymer, draw its repeating unit and say whether it is an addition or condensation polymer.
Common difficulties:
- Leaving the double bond in the repeating unit of an addition polymer.
- Forgetting the water lost at every link in a condensation polymer.
- Thinking a resin code means an item is recyclable everywhere.
- Saying the casein activity "makes" a polymer (it separates one that already exists).
Safety: the casein activity needs an adult to heat the milk. The nylon rope trick is a supervised lab demonstration only.