Addition polymerisation

1. Overview

Addition polymerisation joins thousands of small molecules that contain a C=C bond into one very long chain. This section covers how poly(ethene) and PVC are made, how to work out a polymer's repeat unit from its monomer and the monomer from a piece of polymer, and why poly(alkene)s are so hard to get rid of once they are thrown away. Topic 14.2 met the reaction for ethene and propene; here you learn to apply it to any alkene.

Key Definitions

• Monomer: a small molecule that can join to many other molecules to form a polymer; a monomer for addition polymerisation contains a C=C bond.
• Polymer: a very large molecule made of a long chain of repeat units, formed by joining together many monomer molecules.
• Addition polymerisation: a reaction in which many monomer molecules that contain a C=C bond join together to form a polymer and no other product; the π bond of each C=C breaks and new C-C single bonds link the monomers into a chain.
• Repeat unit: the section of a polymer chain that is repeated over and over to make up the chain; for an addition polymer made from one monomer with one C=C, it is the part formed from one monomer molecule: two chain carbon atoms, their side groups and a bond at each end.
• Poly(alkene): an addition polymer made from an alkene or a substituted alkene, such as poly(ethene), poly(propene) or poly(chloroethene); when the monomer has one C=C, the polymer chain is saturated.
• Copolymer: a polymer whose chains are built from two or more different monomers.
• Biodegradable: able to be broken down into simpler substances by microorganisms, such as bacteria and fungi, in the environment; poly(alkene)s are non-biodegradable.

Content

Monomers, polymers and the backbone

A monomer is a small molecule that can link up with many others. The giant molecule that forms is a polymer. In addition polymerisation, every monomer has a C=C bond, and the monomers simply add together: nothing else is made, so every atom of every monomer ends up in the polymer.

When the monomers join, one of the two bonds in each C=C (the π bond) breaks. Each of the two carbon atoms then forms a new C-C single bond to a neighbouring monomer, one on each side. The result is the backbone: a long chain of carbon atoms joined by single bonds. Anything that was attached to the C=C carbons, such as H, CH₃ or Cl, now hangs off the backbone as a side group.

Because nothing else is made, a polymer made from one monomer has the same empirical formula as that monomer: ethene, C₂H₄, and poly(ethene) both have the empirical formula CH₂.

Poly(ethene) and poly(chloroethene), PVC

Ethene polymerises to poly(ethene), used for bags, films and bottles:

n CH₂=CH₂ → -[CH₂-CH₂]ₙ-

Chloroethene polymerises to poly(chloroethene), used for pipes, window frames and cable insulation:

n CH₂=CHCl → -[CH₂-CHCl]ₙ-

Its common name, PVC, comes from "polyvinyl chloride" (vinyl chloride is an old name for chloroethene). To name any addition polymer, write "poly" and then the monomer's name in brackets: poly(ethene), poly(propene), poly(chloroethene).

The bracket-and-n convention. A polymer is shown by drawing one repeat unit, putting square brackets around it, and writing n after the closing bracket, low down like a subscript. n is the number of repeat units in one chain, often thousands. In a displayed formula of the PVC repeat unit, two carbon atoms sit side by side, joined by a single bond. The left carbon has an H above and an H below; the right carbon has an H above and a Cl below. A single bond leaves the outer side of each carbon and passes through the bracket, to show that the chain continues. On this page, the hyphen at each end of a formula such as -CH₂-CHCl- stands for those two end bonds.

In an equation, n appears twice: n molecules of monomer give one chain of n repeat units.

What happens to the bonds

A C=C bond is one σ bond plus one π bond (13.3). Only the π bond breaks. Each carbon of the old C=C then has four single bonds, so it changes from sp² (trigonal planar, 120°) to sp³ (tetrahedral, 109.5°). The backbone is therefore a zig-zag, not a straight line, and the four bonds around each carbon point to the corners of a tetrahedron, not into one plane as in ethene.

A poly(alkene) made from a monomer with one C=C has no C=C left. Despite the "ene" in its name, poly(ethene) is saturated: it behaves like a very long alkane (14.1). With no π electrons, it does not attract electrophiles, so it does not decolourise bromine water.

The chains in a sample do not all have the same length, so the n and $M_{\text{r}}$ quoted for a sample are averages.

Deducing the repeat unit from a monomer

Follow four steps:

1. Find the C=C. Its two carbon atoms, and only those two, become the backbone.
2. List the two groups on each C=C carbon. In propene, CH₂=CHCH₃, the first carbon has H and H; the second has H and CH₃.
3. Write the two carbons joined by a single bond, keep each group on its own carbon, and add a bond at each end: -CH₂-CH(CH₃)-.
4. Check: the repeat unit has exactly two backbone carbons, no C=C, and the same molecular formula as the monomer.

Monomer	Repeat unit
ethene, CH₂=CH₂	-CH₂-CH₂-
propene, CH₂=CHCH₃	-CH₂-CH(CH₃)-
chloroethene, CH₂=CHCl	-CH₂-CHCl-
1,1-dichloroethene, CH₂=CCl₂	-CH₂-CCl₂-
1-chloroprop-1-ene, CH₃CH=CHCl	-CH(CH₃)-CHCl-

• If each C=C carbon has a group other than H, both backbone carbons carry side groups, as in the last row.
• Write two carbons even for poly(ethene), where CH₂ alone repeats: the repeat unit is the part made from one monomer.

Monomers with two C=C bonds. In 2-methylbuta-1,3-diene, CH₂=C(CH₃)CH=CH₂, all four carbons of the C=C-C=C chain can go into the backbone. Both π bonds break and a new C=C forms between the middle two of those carbons, giving the repeat unit -CH₂-C(CH₃)=CH-CH₂-. So a C=C in the backbone of a polymer usually means that the monomer had two C=C bonds.

Worked example 1: repeat units from monomers

Question: Give the repeat unit of the addition polymer formed by each monomer: (a) 3,3,3-trifluoropropene, CH₂=CHCF₃; (b) 3-methylpent-2-ene, CH₃CH=C(CH₃)CH₂CH₃; (c) tetrafluoroethene, CF₂=CF₂. (d) A sample of the polymer from (c) has an average $M_{\text{r}}$ of $4.20 \times 10^{5}$. Estimate the average number of repeat units in a chain.

(a) The C=C is between carbons 1 and 2. Carbon 1 has H and H; carbon 2 has H and CF₃. The three F atoms are on carbon 3, not on a C=C carbon, so they stay in the side group:

-CH₂-CH(CF₃)-

Check: C₃H₃F₃, the same as the monomer.

(b) The C=C is between carbons 2 and 3. Carbon 2 has H and CH₃ (carbon 1). Carbon 3 has CH₃ and CH₂CH₃. Both backbone carbons carry side groups:

-CH(CH₃)-C(CH₃)(CH₂CH₃)-

Check: C₆H₁₂, with only two carbons in the backbone.

(c) Each carbon of the C=C has two F atoms, so here the F atoms sit directly on the backbone: -CF₂-CF₂-. The polymer is poly(tetrafluoroethene), PTFE.

(d) One repeat unit has the same atoms as one monomer, C₂F₄:

$M_{\text{r}}$(C₂F₄) = 2(12.0) + 4(19.0) = 100.0

$$n \approx \frac{4.20 \times 10^{5}}{100.0} = 4200$$

Answer: (a) -CH₂-CH(CF₃)-; (b) -CH(CH₃)-C(CH₃)(CH₂CH₃)-; (c) -CF₂-CF₂-; (d) about $4.20 \times 10^{3}$ repeat units per chain (an average; the chain ends are ignored).

Finding the monomer from a section of polymer

Work backwards:

1. Find the backbone, the continuous chain of carbon atoms along the section.
2. Cut it into two-carbon pieces, starting at one end of the section (or at the bracket). Each piece is one monomer residue, the part of the chain made from one monomer molecule.
3. Rebuild each monomer: put a C=C between the two carbons of a piece and keep every side group where it was. Leave out the bonds that run off the ends of the section.
4. Compare the pieces. All the same means one monomer. Two different kinds mean a copolymer made from two monomers.

Reading a skeletal section. In a skeletal formula, each corner, and the free end of each branch line, is a carbon atom with enough H atoms to give it four bonds. So a short branch line with nothing written at its end is a methyl group. But a line that runs off the end of the section, or through a bracket, is a bond to the next unit, not a methyl group.

A repeat unit with four backbone carbons. Cut it into two pieces in the same way. If the pieces differ, the polymer comes from two monomers (Worked example 2(c)). Do not rebuild it as one four-carbon monomer: a monomer with one C=C puts only two of its carbons into the backbone, and a diene that joins through both of its C=C bonds leaves a C=C in the chain.

Worked example 2: monomers from polymer sections

Question: Identify the monomer or monomers in each case.

• (a) A section: -CF₂-CFCl-CF₂-CFCl-CF₂-CFCl-
• (b) A skeletal section: a zig-zag line with six corners, C1 to C6 from left to right, and a line running off each end. C2, C4 and C6 each carry two branches: one short line, and a zig-zag of three lines.
• (c) The repeat unit of a copolymer: -CH₂-CH₂-CF₂-CF₂-

(a) Six backbone carbons give three pieces, and each is CF₂-CFCl. Rebuilding the C=C gives CF₂=CFCl, chlorotrifluoroethene.

(b) C1 has two bonds to carbon (C2 and the end line), so it carries 2 H: it is CH₂. C2 has four bonds to carbon (C1, C3 and the two branches), so it carries no H. The short branch is CH₃; the three-line branch (two corners and a free end) is CH₂CH₂CH₃. So each piece is CH₂-C(CH₃)(CH₂CH₂CH₃). The lines off the two ends are links to the rest of the chain, not methyl groups. The monomer is CH₂=C(CH₃)CH₂CH₂CH₃, 2-methylpent-1-ene (C₆H₁₂: six carbons in total, two of them in the backbone).

(c) Four backbone carbons, so two pieces: CH₂-CH₂ and CF₂-CF₂. Rebuilding gives CH₂=CH₂ and CF₂=CF₂.

Answer: (a) chlorotrifluoroethene, CF₂=CFCl; (b) 2-methylpent-1-ene, CH₂=C(CH₃)CH₂CH₂CH₃; (c) two monomers, ethene, CH₂=CH₂, and tetrafluoroethene, CF₂=CF₂.

Why poly(alkene)s are hard to dispose of

They are non-biodegradable. Microorganisms break materials down using enzymes. Polyesters and polyamides have ester and amide links that can be hydrolysed (A Level, 35.3), so they can be broken down. A poly(alkene) chain has no such link. Its backbone is a saturated chain of strong C-C bonds (350 kJ mol⁻¹, a Data Booklet average), and the C-H bonds along it (410 kJ mol⁻¹) are strong and almost non-polar: the same reasons that alkanes are unreactive (14.1). PVC also has C-Cl bonds, but its saturated carbon backbone resists microorganisms in the same way. So poly(alkene)s are chemically inert, and discarded items last for a very long time: they fill landfill sites, and as litter they spread into rivers and the sea, where animals can swallow them or be trapped by them.

Burning them gives harmful products. Incineration removes the bulk and releases energy that can generate electricity, but the products cause problems:

Product	Formed when	Harm
CO₂	burning is complete	greenhouse gas
CO	oxygen is limited	toxic
soot	oxygen is limited	damages lungs
HCl	PVC burns	toxic, acidic

CO binds to haemoglobin more strongly than oxygen does, so the blood carries less oxygen. HCl forms from any chlorine-containing polymer; it is a corrosive gas that adds to acid rain. Burning such polymers at too low a temperature can also form small amounts of very toxic chlorinated organic compounds, such as dioxins.

For complete combustion, use one repeat unit as the formula of the polymer. Water leaves an incinerator as steam:

• poly(ethene): C₂H₄(s) + 3O₂(g) → 2CO₂(g) + 2H₂O(g)
• PVC: C₂H₃Cl(s) + 2½O₂(g) → 2CO₂(g) + H₂O(g) + HCl(g)

So burning PVC releases one mole of HCl for every mole of repeat units.

Reducing the harm. Incinerators burn waste hot, with plenty of air, to cut CO, soot and dioxins, and pass the waste gases through a base, such as calcium hydroxide, to neutralise HCl. Reuse and recycling reduce both problems, but the polymers must first be sorted.

Worked example 3: disposing of a chlorinated plastic film

Question: Poly(1,1-dichloroethene), -[CH₂-CCl₂]ₙ-, is used in some food-packaging films. (a) Explain why the film does not biodegrade in landfill. (b) Write an equation for its complete combustion, using one repeat unit. (c) Calculate the mass of HCl formed when 0.500 kg of film, assumed to be pure polymer, burns completely. (d) State why the waste gases must be treated, and suggest how.

(a) The chain is a saturated carbon backbone of strong C-C bonds. It has no ester or amide link that enzymes could hydrolyse, so microorganisms cannot break it down at any significant rate.

(b) One repeat unit is C₂H₂Cl₂. Its two H atoms and two Cl atoms make 2HCl, so no water forms, and its two C atoms make 2CO₂:

C₂H₂Cl₂(s) + 2O₂(g) → 2CO₂(g) + 2HCl(g)

(c) Step 1: $M_{\text{r}}$(C₂H₂Cl₂) = 2(12.0) + 2(1.0) + 2(35.5) = 97.0

Step 2: 0.500 kg = 500 g, so the amount of repeat units = $\frac{500}{97.0}$ = 5.155 mol

Step 3: amount of HCl = 2 × 5.155 = 10.31 mol

Step 4: $M_{\text{r}}$(HCl) = 1.0 + 35.5 = 36.5, so the mass of HCl = 10.31 × 36.5 = 376 g

(d) HCl is toxic, corrosive and acidic, and adds to acid rain. Passing the gases through a base, such as calcium hydroxide, neutralises it.

Answer: (a) the saturated chain has no link that the enzymes of microorganisms can break; (b) the equation above; (c) 376 g of HCl (3 s.f.), about three-quarters of the mass of the film; (d) HCl is toxic and acidic, so neutralise it with a base.

Key Equations

X is H for ethene, CH₃ for propene and Cl for chloroethene; n is the number of repeat units in a chain.

Addition polymerisation of a monomer CH₂=CHX: n CH₂=CHX → -[CH₂-CHX]ₙ-

Poly(ethene): n CH₂=CH₂ → -[CH₂-CH₂]ₙ-

Poly(chloroethene), PVC: n CH₂=CHCl → -[CH₂-CHCl]ₙ-

$M_{\text{r}}$ of a chain (one monomer; chain ends ignored): $$M_{\text{r}}(\text{chain}) \approx n \times M_{\text{r}}(\text{monomer})$$

Complete combustion, per repeat unit (water forms as steam in an incinerator):

• poly(ethene): C₂H₄(s) + 3O₂(g) → 2CO₂(g) + 2H₂O(g)
• PVC: C₂H₃Cl(s) + 2½O₂(g) → 2CO₂(g) + H₂O(g) + HCl(g)

Common Mistakes to Avoid

• Mistake: putting side-group carbons into the backbone. Fix: only the two C=C carbons form the backbone. Poly(propene) is -CH₂-CH(CH₃)-, not -CH₂-CH₂-CH₂-; CH₃CH=CHCl gives -CH(CH₃)-CHCl-, with two backbone carbons, not three.
• Mistake: keeping the C=C in the repeat unit, or drawing a monomer without one. Fix: for a monomer with one C=C, the backbone of the repeat unit has only single bonds, and the monomer you deduce must have its C=C: -CH₂-CH(CF₃)- comes from CH₂=CHCF₃.
• Mistake: drawing two or three repeat units, or leaving off the end bonds, when asked for one repeat unit. Fix: show exactly two backbone carbons with a bond sticking out of each end, such as -CF₂-CF₂-.
• Mistake: reading the bonds at the ends of a section as methyl groups. Fix: they are links to the rest of the chain. In Worked example 2(b), the monomer is just CH₂=C(CH₃)CH₂CH₂CH₃, with no extra CH₃ at either end.
• Mistake: treating a four-carbon repeat unit as one monomer. Fix: cut it into two-carbon pieces. -CH₂-CH₂-CF₂-CF₂- comes from ethene and tetrafluoroethene.
• Mistake: giving the name of a polymer when asked for the type of polymerisation or polymer. Fix: the type is "addition". Poly(propene) is the name of one polymer, not a type.
• Mistake: calling poly(ethene) unsaturated because its name ends in "ene". Fix: the C=C bonds open during polymerisation, so poly(ethene) is saturated and does not decolourise bromine water.
• Mistake: matching a combustion product with the wrong harm, such as linking CO to acid rain or to the ozone layer. Fix: CO is toxic because it reduces the oxygen the blood carries, CO₂ is a greenhouse gas, and the HCl from burning PVC is toxic and acidic, so it adds to acid rain.

Exam Tips

• Draw one repeat unit: two backbone carbons joined by a single bond, every side group in place, and a continuation bond at each end. If you add square brackets and n, the end bonds must pass through the brackets.
• Draw the monomer: show the C=C and every side group, then check that its molecular formula matches the repeat unit.
• Identify the monomer(s) by cutting the backbone into two-carbon pieces from one end; two kinds of piece means two monomers.
• State or suggest difficulties of disposal as separate, specific points: "non-biodegradable, so it stays in landfill for a very long time" and "burning releases harmful gases", naming a gas and its harm, such as HCl from PVC, which is toxic and acidic.
• Explain why a poly(alkene) such as poly(ethene) is unreactive in terms of its bonds: the chain is saturated, with strong, almost non-polar C-C and C-H bonds, so it has no π electrons to attract electrophiles and no link that enzymes can break.