What this chapter covers20

Almost every plastic you handle in a day is made by one reaction repeated thousands of times. An alkene's C=C opens, and the two carbons it held link to the next alkene, and the next, until a chain thousands of atoms long has formed. Nothing else is made. That single idea covers how the polymer forms, how to draw it, how to work backwards from it to the monomer, and why the finished material is so hard to get rid of.

The verbs set the depth. 20.1.1 says describe: say what joins to what, what happens to the double bond, and write the equation for the two named polymers. 20.1.2 and 20.1.3 say deduce and identify, which is where the marks are. The two named examples are only the start: a question can give you any monomer, however unfamiliar, and expect its repeat unit, or give you a section of any chain and expect the monomers. The method is the same every time, so learn the method rather than a list of polymers. 20.1.4 says recognise: know the two problems — the polymers do not biodegrade, and burning them makes harmful products — and be able to say why.

The page is in three parts. The first describes the reaction, using poly(ethene) and PVC and then the other common addition polymers. The second is the working core of the topic: drawing a repeat unit from any monomer, writing and balancing the equation, naming the polymer, handling copolymers, and reading monomers back out of a section of chain. The third deals with what happens to these polymers once they have been used.

Monomers, polymers and repeat units20.1.1

A polymer is a very large molecule made by joining together many small molecules. Each small molecule is a monomer. A typical poly(ethene) chain is built from between a few thousand and a few hundred thousand ethene molecules, so its relative molecular mass runs into the hundreds of thousands or more.

Two words in the addition-polymerisation definition carry the marks. Unsaturated: the monomer must have a carbon–carbon double bond, because that is the bond that opens to make the chain. And only product: all the atoms of every monomer end up in the polymer. Nothing is given off, which is what separates addition polymerisation from the condensation polymerisation you meet at A Level, where a small molecule such as water is lost at every link.

The third row is worth holding on to. Because nothing is lost, the repeat unit of an addition polymer has exactly the same molecular formula as its monomer. Chloroethene is C₂H₃Cl, and so is the repeat unit of PVC. That is the quickest check on any repeat unit you draw: count the atoms, and they must match the monomer's.

The fourth row follows from the same idea. Only the two carbons of the C=C join the chain. Everything else on the monomer — a methyl group, a chlorine atom, a benzene ring, an ester group — ends up hanging off the chain as a side group. The backbone of every addition polymer is a string of carbon atoms and nothing else.

Poly(ethene)20.1.1

Ethene, CH₂=CH₂, is the simplest alkene and the simplest monomer. When it polymerises, the double bond in each molecule opens and the molecules link into a chain of CH₂ groups:

Written in condensed form:

nCH₂=CH₂ → –[CH₂–CH₂]_n–

Three details in that drawing are what examiners look for, and each is easy to lose:

• the C=C in the monomer has become C–C in the polymer — a repeat unit drawn with a double bond is wrong;
• the bonds at the two ends of the repeat unit extend through the brackets, showing that the chain carries on in both directions;
• n appears twice: in front of the monomer and after the bracket. The same n on both sides is what makes the equation balance.

Two kinds of poly(ethene)

Poly(ethene) is made in two main forms. They are the same polymer — chains of CH₂ groups — but made under different conditions, so the chains have different shapes and the materials behave differently.

The difference comes down to intermolecular forces (topic 3.6). The only forces between poly(ethene) chains are instantaneous dipole–induced dipole forces. They act over a short range, so they are much stronger when the chains lie close together along their length. Unbranched HDPE chains can do that; the branches on LDPE keep its chains apart.

Poly(chloroethene), PVC20.1.1

The second polymer the syllabus names is poly(chloroethene). Its monomer, chloroethene, CH₂=CHCl, is ethene with one hydrogen replaced by chlorine. Its older name is vinyl chloride — CH₂=CH– is the "vinyl" group — which is why the polymer is still called PVC, polyvinyl chloride.

nCH₂=CHCl → –[CH₂–CHCl]_n–

Compare the two equations. Everything that happened to ethene happens to chloroethene: the double bond opens, the two carbons join the chain, and nothing is lost. The only difference is what hangs off the chain. That is the whole of 20.1.1 — once poly(ethene) makes sense, PVC is the same reaction with a chlorine atom in place of one hydrogen.

Rigid and flexible PVC

Pure PVC is rigid. The C–Cl bonds are polar, so as well as instantaneous dipole–induced dipole forces there are permanent dipole–permanent dipole attractions between the chains, and they hold the chains firmly in place. Rigid PVC is used for window frames, drainpipes, gutters and flooring.

Adding a plasticiser changes that. Plasticiser molecules are small and fit between the polymer chains, pushing them apart. The forces between the chains weaken, the chains can slide past one another, and the material becomes soft and flexible. Plasticised PVC is used for electrical cable insulation, hoses and synthetic leather.

What happens to the bonds20.1.1

A C=C double bond is one σ bond and one π bond (topic 13.3). The π bond is the weaker of the two, and its electrons lie above and below the plane of the molecule, where they are exposed. In addition polymerisation, it is the π bond that breaks. Its two electrons are shared out: one goes into a new σ bond on each side of the monomer, linking it to its neighbours in the chain. The σ bond between the two original carbons stays where it was.

Three things change for each monomer that joins:

The last row explains why a real polymer chain is not the straight line drawn on paper. Each backbone carbon is tetrahedral, so the chain zig-zags, and because the single bonds can rotate, a long chain coils and tangles. The straight drawing is a convention, like drawing a displayed formula flat.

A double bond has become two single bonds for every monomer that joins, so the reaction releases energy. Using CIE's average bond enthalpies, each ethene breaks one C=C and forms two C–C bonds (the one that replaces its double bond, and its new link to the next monomer):

ΔH ≈ E(C=C) − 2E(C–C) = 610 − 2(350) = −90 kJ per mole of ethene

Polymerisation is exothermic, and industrial reactors have to be cooled to keep the temperature under control.

Common addition polymers20.1.1

Poly(ethene) and PVC are the syllabus's two examples, but every alkene with something on its double bond gives a polymer by the same reaction. These are the ones you are most likely to be given, and they are worth recognising:

Read down the monomer column and every one is CH₂=CH₂ with one or more hydrogens swapped for something else. Read down the repeat-unit column and every one is –C–C– with the same things swapped. The older names follow the same pattern as PVC: phenylethene is styrene, propenenitrile is acrylonitrile.

Drawing the repeat unit from a monomer20.1.2

This is the skill 20.1.2 tests, and it works for any monomer, however unfamiliar. Three steps:

Step 1 is where most of the work is, because monomers are often printed in a way that hides the double bond. Propene is usually written CH₃CH=CH₂, which puts the methyl group in line with the double bond. Redrawn for step 1, the two carbons of the C=C become the chain and the CH₃ goes above or below one of them. It does not matter which carbon you hang it on, or whether it goes up or down: all of these are the same repeat unit, and all score.

Build any repeat unit in the model below. It draws the equation from the monomer's structure, then a section of the chain, and counts the atoms to confirm nothing has been lost.

Writing the equation20.1.2

An equation for addition polymerisation balances in the same way as any other: the same number of each kind of atom on both sides. The only unusual thing is that the numbers are written with n in them.

nCH₂=CHCH₃ → –[CH₂–CH(CH₃)]_n–

On the left, n propene molecules contain 3n carbon atoms. On the right, n repeat units, each with three carbons, also contain 3n. The equation balances.

If the bracket holds two monomer units, the coefficient has to change to match:

The general rule: the coefficient of the monomer is the number of monomer units inside the bracket, times n. Try each choice below — the model counts the carbons on both sides so a wrong coefficient shows itself.

Naming addition polymers20.1.2

An addition polymer is named by putting the name of its monomer in brackets after "poly":

The brackets matter when the monomer's name has more than one word or begins with a number — poly(1,1-dichloroethene), poly(methyl 2-methylpropenoate) — so it is simplest to use them every time.

Monomers that look less like ethene20.1.2

Because a question can use any monomer, it helps to know which molecules can polymerise and how to handle the awkward ones.

The test: is there a C=C?

A molecule can form an addition polymer if it has a carbon–carbon double bond. Other multiple bonds do not count. Propanone has C=O and ethanenitrile has C≡N, and neither forms an addition polymer by this route. And the double bonds in a benzene ring are not really double bonds at all: the ring's π electrons are delocalised (you meet this at A Level), and benzene does not undergo addition polymerisation. Phenylethene polymerises only through the C=C outside its ring.

Groups on both carbons

When both carbons of the C=C carry a group, both groups end up on the chain — one on each backbone carbon. But-2-ene, CH₃CH=CHCH₃, gives a chain with a methyl group on every carbon:

nCH₃CH=CHCH₃ → –[CH(CH₃)–CH(CH₃)]_n–

Two groups on one carbon

When both groups are on the same carbon, they stay together on the same backbone carbon. Methyl 2-methylpropenoate, CH₂=C(CH₃)COOCH₃, is the monomer for Perspex:

The methyl group and the ester group are both on the second carbon. Moving one of them to the other carbon would describe a different monomer.

Copolymers20.1.2

A polymer made from a single monomer is a homopolymer. When two different monomers polymerise together, they form a copolymer, with both kinds of unit in the same chain. Ethene and ethenyl ethanoate copolymerise to give EVA, the soft, rubbery material in sports-shoe soles and hot-melt glue.

In a real copolymer the two kinds of unit are usually spread along the chain in no fixed order. For exam purposes, draw them alternating, one of each, and put the pair in the bracket:

The carbon count still checks it: n × 2 + n × 4 = 6n on the left, and n × 6 on the right. The builder model above makes any copolymer: choose a second monomer.

Finding the monomer from a section of polymer20.1.3

20.1.3 runs 20.1.2 backwards. You are given a section of a polymer chain — often long, often with unfamiliar side groups — and asked for the monomer or monomers that made it. The method is the reverse of the one in section 7:

Work through the sections in the model. Some start part-way through a monomer unit, some are copolymers, and one is a trap. Select every monomer you think was used, then check; "show the working" draws the cuts.

Where to make the first cut20.1.3

The printed section of chain does not always start at the beginning of a monomer unit. A section of PVC could be drawn as –CHCl–CH₂–CHCl–CH₂– just as well as –CH₂–CHCl–CH₂–CHCl–. For a homopolymer this never matters: cut it either way and every piece is CH₂ joined to CHCl, which is chloroethene. The same is true of almost every copolymer you will be given.

There is one kind of section where the starting point does matter. Look at this chain:

Cut from the first carbon, the pieces are CH₂–CH₂ and CHCl–CHCl: a copolymer of ethene and 1,2-dichloroethene.

Cut from the second carbon, every piece is CH₂–CHCl, some the right way round and some reversed: chloroethene alone, with some units joined head to head.

Both readings follow the rules, and the chain by itself cannot tell them apart. Real PVC is almost entirely head to tail, so the first reading is the likely intended answer, but a question that means to test this will show the repeat unit in brackets or tell you how many monomers were used. The model in the previous section tries both starting points on every chain and reports when they disagree; the last chain in its list is this one.

Chain length and relative molecular mass20.1.2

Because nothing is lost when the monomers join, the relative molecular mass of a chain is simply the number of repeat units times the M_r of one of them. The two end groups, which depend on how the chain started and stopped, are too small to matter.

number of repeat units, n = M_r(polymer) ÷ M_r(repeat unit)

Two things are worth noticing. First, the M_r of a polymer sample is always an average: the chains in a sample stop growing at different lengths, so a polymer does not have one fixed M_r the way a simple compound does, and it softens over a range of temperatures rather than melting sharply. Second, the chains are extraordinarily long for their width, which is why they tangle, and why a polymer behaves so differently from its monomer.

Why poly(alkene)s are unreactive20.1.4

20.1.4 is about poly(alkene)s: addition polymers made from alkenes, such as poly(ethene) and poly(propene), and by extension the others on this page. Everything difficult about disposing of them comes from one property: they are chemically very unreactive. That is exactly why they are chosen for food wrapping, chemical bottles and water pipes — and exactly why they are so hard to get rid of afterwards.

The reason for the unreactivity is in the bonds that remain once the polymer has formed:

• There is no C=C left. The π bond was the reactive part of the monomer — the site of electrophilic addition in topic 14 — and every one of them was used up in making the chain. A poly(alkene) is, chemically, a very large alkane.
• The C–C and C–H bonds are almost non-polar. Carbon and hydrogen have similar electronegativities (2.55 and 2.20), so there is no δ+ carbon for a nucleophile such as water or hydroxide to attack, the way it attacks the carbon of a C–Br bond in a halogenoalkane (topic 15).
• The bonds are strong. C–C and C–H need 350 and 410 kJ mol⁻¹ to break. In PTFE the C–F bond, which is polar, is also one of the strongest single bonds in organic chemistry.

The chart below places the bonds in poly(alkene)s against bonds you have seen react at AS. The reactive ones are either weaker (C–Br, C–I, the π part of C=C) or polar enough to give a clear δ+ carbon (the C–O of an ester, which is hydrolysed in topic 16). The polymer bonds sit in the corner that neither reagents nor enzymes can do much with.

Non-biodegradability20.1.4

Microorganisms break materials down using enzymes, and enzymes work by attacking particular polar bonds — the C–O links in starch and cellulose, the C–N links in proteins. A poly(alkene) chain offers nothing of the kind: just C–C and C–H bonds, over and over. With no enzyme able to start on it, a poly(alkene) is non-biodegradable. It persists for decades, and probably much longer.

That has three consequences:

• Landfill. Buried poly(alkene)s stay essentially unchanged. Because they are also bulky and low-density, they take up a large volume of the space available.
• Litter. Discarded bags, bottles and packaging last for years in the environment. They entangle animals and are eaten by them, and plastic waste collects in rivers and oceans.
• Fragments. Sunlight and physical wear break large pieces into smaller ones without breaking the chains down to simple molecules, so the polymer ends up as tiny particles that are even harder to collect.

Burning: carbon dioxide, carbon monoxide and soot20.1.4

If a poly(alkene) cannot be left to rot, the obvious alternative is to burn it. Poly(alkene)s are hydrocarbons, so they burn well and release a lot of energy — incinerators can use that energy to generate electricity. But burning creates its own problems, and 20.1.4 names them as "harmful combustion products".

In plenty of oxygen, a hydrocarbon polymer burns completely to carbon dioxide and water. Per repeat unit of poly(ethene):

C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

or, for the whole chain, (C₂H₄)_n + 3nO₂ → 2nCO₂ + 2nH₂O. Even this "clean" case is not harmless: carbon dioxide is a greenhouse gas, and burning a polymer made from crude oil puts carbon that was underground into the atmosphere.

In a limited supply of oxygen — a bonfire, a house fire, a badly run incinerator — combustion is incomplete. Some of the carbon ends up as carbon monoxide, which is toxic because it binds to haemoglobin in the blood more strongly than oxygen does, and some as carbon: soot, fine particles that damage the lungs. These are the same products as the incomplete combustion of alkanes in topic 14.1.

The model below burns one repeat unit of each polymer with a chosen fraction of the oxygen it needs. It shows the balanced equation for complete combustion, and what comes out as the oxygen runs short.

Burning chlorinated polymers20.1.4

PVC adds a problem that poly(ethene) does not have: chlorine. When PVC burns, the chlorine leaves as hydrogen chloride, taking hydrogen from the chain:

2C₂H₃Cl + 5O₂ → 4CO₂ + 2H₂O + 2HCl

Hydrogen chloride is a toxic, corrosive gas. It dissolves in water droplets in the air to give hydrochloric acid, which contributes to acid rain, and in a fire it is one of the main dangers to anyone breathing the smoke. Incinerators that burn waste containing PVC must remove it from their flue gases before release, by passing them through a scrubber containing a base that neutralises the acid.

Two more products form when chlorinated polymers burn at too low a temperature, or with too little oxygen:

• Phosgene, COCl₂ — a highly poisonous gas, used as a chemical weapon in the First World War.
• Dioxins — a family of chlorinated organic compounds that are extremely toxic even in tiny amounts, and persist in the environment and in body fat.

To prevent them, incinerators burn waste at a high temperature, above about 850 °C, with plenty of oxygen, and clean their flue gases before release. An open fire or garden bonfire does none of these things, which is why burning PVC outside an incinerator is so dangerous.

Weighing the disposal options20.1.4

Every way of dealing with used poly(alkene)s has a drawback, and "recognise the difficulty" means being able to say what each one is.

Recycling depends on sorting because different polymers soften at different temperatures and do not mix well when melted together. That is why many plastic items carry a number from 1 to 7 inside a triangle, which identifies the polymer: 2 for HDPE, 3 for PVC, 4 for LDPE, 5 for poly(propene), 6 for polystyrene.

Self-test20.1

Twenty questions across all four outcomes. Each answer comes with the reason, so a wrong answer is worth reading as closely as a right one.

Definitions to learn20.1

The two methods, side by side

Data used on this page20.1

The models compute everything they show from the monomer structures in the first table and the values in the second. The bond enthalpies are the average values CIE use, and the electronegativities are Pauling values; they are reference values, not the official data booklet, and other sources differ by a few units.

Relative atomic masses used: C 12.0, H 1.0, N 14.0, O 16.0, F 19.0, Cl 35.5. Electronegativities: H 2.20, C 2.55, N 3.04, O 3.44, F 3.98, Cl 3.16, Br 2.96, I 2.66. The chain-length model takes a C–C bond as 0.154 nm and the backbone angle as 109.5°.