Addition Polymers

AS · 10 min

Polythene bags, PVC pipes, polystyrene cups and non-stick pan coatings are all addition polymers: enormous molecules made by joining thousands of small alkene molecules end to end. This is the most commercially important reaction of alkenes. This note explains how addition polymerisation works, how to draw the repeat unit of a polymer from its monomer and how to work back from a section of polymer to its monomer or monomers, and why poly(alkene)s are so difficult to dispose of. Typical exam questions ask you to draw a repeat unit or a section of a polymer, identify a monomer, and comment on the environmental problems of disposal.

Monomers, polymers and repeat units

Definition

A monomer is a small molecule that can join to many others to form a polymer.

A polymer is a very large molecule (a macromolecule) made of many repeating units joined by covalent bonds.

Addition polymerisation is the joining of many unsaturated monomer molecules (alkenes) to form a polymer with no other product. The polymer has the same empirical formula as the monomer.

The repeat unit is the smallest section of the polymer chain that, repeated over and over, gives the whole polymer.

In addition polymerisation, the $\pi$ bond of each C=C breaks. Each carbon atom uses the electron released to form a new $\sigma$ bond to a carbon of the next monomer. The double bond becomes a single bond, and a long saturated chain forms:

\ce{nCH2=CH2 -> [-CH2-CH2-]_{n}}

Here $n$ is a large number, typically thousands to hundreds of thousands. Nothing is lost: all the atoms of every monomer end up in the polymer.

Drawing the repeat unit

Addition polymerisation of chloroethene. The π bond of each monomer breaks and the two carbons bond to neighbouring monomers. The repeat unit has two main-chain carbons, keeps every substituent (three H and one Cl), and has bonds extending through the brackets.

Method

Drawing the repeat unit of an addition polymer from its monomer

Draw the monomer with the C=C in the middle and its four substituents above and below, as in a displayed formula.
Change the C=C to a C–C single bond.
Keep every substituent exactly where it was (H, $\ce{CH3}$ , Cl, and so on).
Add a bond sticking out on each side of the two carbons (the "continuation bonds"). These must pass through the brackets.
Draw square brackets and write $n$ after the closing bracket.

The repeat unit of an addition polymer always has two carbon atoms in the main chain (from the two carbons of one C=C), and all other groups hang off the chain as side groups.

Key result

monomer	monomer formula	polymer	repeat unit	uses
ethene	$\ce{CH2=CH2}$	poly(ethene), "polythene"	$\ce{[-CH2-CH2-]_{n}}$	bags, bottles, film
propene	$\ce{CH2=CHCH3}$	poly(propene)	$\ce{[-CH2-CH(CH3)-]_{n}}$	ropes, crates, carpets
chloroethene	$\ce{CH2=CHCl}$	poly(chloroethene), PVC	$\ce{[-CH2-CHCl-]_{n}}$	pipes, window frames, cable insulation
tetrafluoroethene	$\ce{CF2=CF2}$	poly(tetrafluoroethene), PTFE	$\ce{[-CF2-CF2-]_{n}}$	non-stick coatings
phenylethene	$\ce{CH2=CHC6H5}$	poly(phenylethene), polystyrene	$\ce{[-CH2-CH(C6H5)-]_{n}}$	packaging, insulation

The name of the polymer is "poly" followed by the name of the monomer in brackets.

Drawing a section of polymer

A question may ask for a section of polymer made from, say, three monomer molecules. Draw a six-carbon chain with each pair of carbons carrying the substituents of one monomer, and a continuation bond at each end. For propene, three monomers give:

\ce{-CH2-CH(CH3)-CH2-CH(CH3)-CH2-CH(CH3)-}

In a regular chain the side groups appear on every second carbon.

Identifying the monomer from a polymer

Method

Finding the monomer(s) from a section of an addition polymer

Look along the main carbon chain and find the pattern of side groups. Split the chain into two-carbon units so that each unit has the same pattern.
Take one two-carbon unit (the repeat unit), remove the continuation bonds, and put a double bond between the two carbons.
If the two-carbon units are not all the same, the polymer is a copolymer: each different unit comes from a different monomer.

For example, the section $\ce{-CH2-CH(CN)-CH2-CH(CN)-}$ splits into $\ce{-CH2-CH(CN)-}$ units, so the monomer is $\ce{CH2=CHCN}$ , propenenitrile (used to make acrylic fibres).

Properties: why poly(alkene)s last so long

A poly(alkene) chain is saturated: once the C=C bonds have gone, the molecule contains only strong, almost non-polar C–C and C–H bonds (plus C–Cl in PVC or C–F in PTFE). Chemically, poly(alkene)s behave like very large alkanes: they are unreactive, which makes them excellent for containers, pipes and insulation.

That same unreactivity is the problem when they are thrown away.

Disposal of poly(alkene)s

Key result

Poly(alkene)s are non-biodegradable: microorganisms have no enzymes to break down their long saturated C–C chains, so they persist in landfill and in the environment (rivers, oceans) for hundreds of years.
Burning them releases energy, but produces harmful combustion products:
- carbon dioxide, a greenhouse gas;
- carbon monoxide (toxic) and soot from incomplete combustion;
- hydrogen chloride from PVC (toxic, corrosive, forms acidic solutions), and other toxic chlorinated compounds such as dioxins if the combustion temperature is not high enough.

The combustion of PVC, written per repeat unit:

\ce{2C2H3Cl + 5O2 -> 4CO2 + 2H2O + 2HCl}

Ways of reducing the problem (background, useful for "suggest" questions):

Recycling: sorting polymers by type, melting and remoulding them.
Incineration with energy recovery, with the acidic gases (such as HCl) removed from the waste gases by scrubbing with a base.
Feedstock recycling: cracking waste polymers back into small molecules for the chemical industry.
Using biodegradable or photodegradable polymers for short-life items.

Tip

Making poly(ethene) needs specific conditions, which you do not need to recall at AS: either very high pressure and about $200\ ^\circ\text{C}$ with a trace of oxygen as initiator (low-density poly(ethene), by a free-radical mechanism), or a Ziegler–Natta catalyst at low pressure (high-density poly(ethene)).

Worked examples

Routine: from monomer to repeat unit

Write an equation for the formation of poly(propene) from propene and draw its repeat unit in words.

Solution

\ce{nCH2=CH(CH3) -> [-CH2-CH(CH3)-]_{n}}

Repeat unit: two main-chain carbons joined by a single bond. The first carries two H atoms; the second carries one H and one $\ce{CH3}$ group. A continuation bond leaves each end through the brackets, and $n$ is written after the closing bracket. The $\ce{CH3}$ is a side group: it is not part of the main chain.

Routine: from repeat unit to monomer

Perspex has the repeat unit $\ce{[-CH2-C(CH3)(COOCH3)-]_{n}}$ . Give the structural formula of the monomer and name the functional groups in it.

Solution

Remove the continuation bonds and make the C–C a C=C: $\ce{CH2=C(CH3)COOCH3}$ (methyl 2-methylpropenoate).

Functional groups: alkene (C=C) and ester ( $\ce{-COO-}$ ).

Standard: reading a polymer section

A section of a polymer chain is $\ce{-CH2-CHCl-CH2-CHCl-CH2-CHCl-}$ . (a) How many monomer units does it contain? (b) Draw the repeat unit. (c) Name the monomer and the polymer.

Solution

(a) Six main-chain carbons, two per monomer: three monomer units.

(b) $\ce{[-CH2-CHCl-]_{n}}$ .

Standard: a copolymer

A section of a copolymer is $\ce{-CH2-CH2-CH2-CH(CH3)-CH2-CH2-}$ . Identify the two monomers.

Solution

Split into two-carbon units from one end: $\ce{-CH2-CH2-}$ , $\ce{-CH2-CH(CH3)-}$ , $\ce{-CH2-CH2-}$ .

Two different units, so two monomers: ethene, $\ce{CH2=CH2}$ , and propene, $\ce{CH2=CHCH3}$ .

Exam-hard: burning PVC

(a) Write an equation for the complete combustion of PVC, using one repeat unit $\ce{C2H3Cl}$ . (b) Calculate the mass of hydrogen chloride released when $1.00\ \text{kg}$ of PVC is burnt completely. (c) Explain why PVC should not be disposed of by simple burning, and suggest how an incinerator could reduce the problem. ( $A_r$ : H 1.0, C 12.0, Cl 35.5)

Solution

(a) $\ce{2C2H3Cl + 5O2 -> 4CO2 + 2H2O + 2HCl}$ . Check: C 4, H 6, Cl 2, O 10 on each side.

(b) $M_r(\ce{C2H3Cl}) = 24.0 + 3.0 + 35.5 = 62.5$ . Amount of repeat units $= 1000 / 62.5 = 16.0\ \text{mol}$ . One HCl per repeat unit: $16.0\ \text{mol}$ HCl, mass $= 16.0 \times 36.5 = 584\ \text{g}$ .

(c) Burning releases toxic, corrosive hydrogen chloride (which forms acid rain), toxic chlorinated organic compounds if combustion is incomplete, carbon monoxide, and the greenhouse gas $\ce{CO2}$ . An incinerator can burn the waste at a high temperature to destroy chlorinated organics, and pass the waste gases through a base (such as calcium hydroxide or calcium carbonate) to neutralise the HCl before release.

Exam-hard: chain length

A sample of poly(propene) has an average relative molecular mass of 210 000. (a) Calculate the average number of monomer units per chain. (b) State and explain how the empirical formula of poly(propene) compares with that of propene. (c) Explain why poly(propene) does not decolourise bromine water, although propene does.

Solution

(a) $M_r(\ce{C3H6}) = 42.0$ . $n = 210\,000 / 42.0 = 5000$ .

(b) The same, $\ce{CH2}$ . Addition polymerisation forms no other product, so the polymer contains exactly the same atoms in the same ratio as the monomer.

(c) In the polymer, every C=C has been converted to C–C during polymerisation; the polymer is saturated, so there is no $\pi$ bond to react with bromine by electrophilic addition. Propene has a C=C and decolourises bromine water.

Watch out

Leaving the double bond in the repeat unit. The polymer chain contains only single bonds: $\ce{-CH2-CH2-}$ , not $\ce{-CH2=CH2-}$ .
Putting side groups into the main chain. In poly(propene), the $\ce{CH3}$ is a side group. Drawing $\ce{-CH2-CH2-CH2-}$ as the repeat unit is wrong.
Continuation bonds. They must cross the brackets. Repeat units without them, or with $n$ inside the bracket, lose the mark.
More than two main-chain carbons. The repeat unit of a single-monomer addition polymer has two main-chain carbons, not three or four.
"Polymers are non-biodegradable because they are strong". The reason is that they are saturated, unreactive, with strong non-polar C–C and C–H bonds, and microorganisms cannot break them down.

Exam tip

"Draw the repeat unit" questions expect the displayed form: two carbons, all substituents, continuation bonds through brackets, and $n$ .
"Identify the monomer" questions: split the chain into two-carbon units and restore the C=C. Look carefully for copolymers.
"Explain the problems of disposal" questions usually need two points: non-biodegradable (persists in landfill) and harmful combustion products (name one: HCl from PVC, CO, $\ce{CO2}$ as greenhouse gas).
Addition polymerisation has no by-product. Do not include water or any other small molecule (that is condensation polymerisation, an A Level topic).

Summary

Addition polymerisation: many alkene monomers join, the $\pi$ bonds break, and a saturated chain forms with no other product.
Repeat unit: two main-chain carbons, all substituents kept, continuation bonds through square brackets, $n$ outside.
Examples: poly(ethene) $\ce{[-CH2-CH2-]_{n}}$ ; poly(chloroethene) (PVC) $\ce{[-CH2-CHCl-]_{n}}$ ; poly(propene) $\ce{[-CH2-CH(CH3)-]_{n}}$ .
To find the monomer: split the chain into two-carbon units and restore the C=C; different units mean a copolymer.
Poly(alkene)s are unreactive and non-biodegradable; burning gives $\ce{CO2}$ , CO and, from PVC, toxic HCl.

Practice

Question

Define addition polymerisation.
Write an equation for the polymerisation of ethene.
Draw the repeat unit of the polymer formed from but-2-ene, $\ce{CH3CH=CHCH3}$ .
Give the repeat unit of poly(tetrafluoroethene) and explain why this polymer is extremely unreactive.
Give the monomer of the polymer with the repeat unit $\ce{[-CH2-CH(CN)-]_{n}}$ , and name it.
A section of polymer is $\ce{-CH2-CH(OOCCH3)-CH2-CH(OOCCH3)-}$ . Give the structural formula of the monomer and name its two functional groups.
Explain why poly(alkene)s are non-biodegradable.
Give two environmental problems caused by the disposal of poly(alkene)s, and one way of reducing each.
A sample of PVC has an average $M_r$ of 87 500. Calculate the average number of chloroethene units per chain. ( $A_r$ : H 1.0, C 12.0, Cl 35.5)
A copolymer is made from equal numbers of chloroethene, $\ce{CH2=CHCl}$ , and 1,1-dichloroethene, $\ce{CH2=CCl2}$ , monomers. (a) Draw a section of the copolymer containing one unit of each monomer. (b) Calculate the percentage by mass of chlorine in the copolymer. (c) Write an equation for the complete combustion of this section ( $\ce{C4H5Cl3}$ ), showing that all the chlorine is released as HCl. ( $A_r$ : H 1.0, C 12.0, Cl 35.5)

Answers

The joining together of many unsaturated monomer molecules (with C=C bonds) to form a polymer, with no other product.
$\ce{nCH2=CH2 -> [-CH2-CH2-]_{n}}$
$\ce{[-CH(CH3)-CH(CH3)-]_{n}}$ : two main-chain carbons, each carrying one H and one $\ce{CH3}$ , with continuation bonds through the brackets.
$\ce{[-CF2-CF2-]_{n}}$ . It is saturated, and the C–F bonds are very strong (and C–C bonds are strong), so there is no reactive site; fluorine atoms also shield the carbon chain.
$\ce{CH2=CHCN}$ , propenenitrile.
$\ce{CH2=CHOOCCH3}$ (ethenyl ethanoate). Functional groups: alkene (C=C) and ester ( $\ce{-OOC-}$ ).
Poly(alkene)s are saturated, with only strong, non-polar C–C and C–H bonds; they are chemically unreactive, and microorganisms do not have enzymes that can break these long chains down, so they are not decomposed by natural processes.
Non-biodegradable, so they persist in landfill sites and in the environment for very long times: reduce by recycling (or using biodegradable polymers). Burning releases toxic gases (HCl from PVC, CO) and greenhouse $\ce{CO2}$ : reduce by high-temperature incineration with scrubbing of acidic gases (and energy recovery).
$M_r(\ce{C2H3Cl}) = 62.5$ ; $n = 87\,500 / 62.5 = 1400$ .
(a) $\ce{-CH2-CHCl-CH2-CCl2-}$ (with continuation bonds at both ends). (b) $\ce{C4H5Cl3}$ : $M_r = 48.0 + 5.0 + 106.5 = 159.5$ ; $\%\ce{Cl} = 106.5 / 159.5 \times 100 = 66.8\%$ . (c) $\ce{C4H5Cl3 + 4.5O2 -> 4CO2 + H2O + 3HCl}$ , or doubled, $\ce{2C4H5Cl3 + 9O2 -> 8CO2 + 2H2O + 6HCl}$ . Check: H $10 = 4 + 6$ ; Cl $6 = 6$ ; O $18 = 16 + 2$ .

Monomers, polymers and repeat units

Drawing the repeat unit

Drawing a section of polymer

Identifying the monomer from a polymer

Properties: why poly(alkene)s last so long

Disposal of poly(alkene)s

Worked examples

Practice

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