Polymerisation

Organic Chemistry Weightage: AS · Papers 1 and 2 (assumed in Paper 4) Topic 20

“Welcome! At AS, polymerisation means addition polymerisation: alkene monomers join end to end to form a saturated chain, and the polymer is the only product. The skills are all about the repeat unit: find it from a monomer, find the monomer from a section of chain, and say why these plastics are a disposal problem. Practise drawing them until the open bonds and brackets are automatic.”

— SCORECHEM ACADEMIC TEAM

0. What Examiners Want

How marks are earned in Polymerisation

Definitions and description: monomer, polymer, repeat unit; addition polymerisation needs a C=C and gives the polymer as the only product.
Repeat unit from a monomer: change C=C to C–C, add open bonds, keep every substituent on its own carbon. Show brackets and n when asked for the polymer formula.
Monomer from a section: pick out the repeat unit (two carbons in the main chain) and restore the C=C; deal with more than one monomer if the pattern alternates.
Equations and calculations: nCH₂=CHCl → [–CH₂–CHCl–]_n; the number of repeat units from an average M_r, using A_r values from the Data Booklet.
Disposal: non-biodegradability and the harmful products of combustion (CO, HCl from PVC, dioxins), with the use of energy recovery.

Where this topic appears. Recent AS papers have asked for: the monomer that gives a drawn section of a chlorine-containing polymer; the name of the monomer for a given repeat unit written in condensed form (–CH(CH₃)CH(CH₂CH₃)–); one repeat unit of the addition polymer from a drawn alkene; and one repeat unit of Perspex® made from methyl 2-methylpropenoate, with how the infrared spectra of the monomer and polymer differ (the C=C absorption disappears; Topic 22).

1. Monomers, Polymers and Repeat Units

Addition polymerisation of ethene. In each monomer the π bond of the C=C breaks and the carbon atoms join to those of neighbouring monomers, so the polymer is the only product. The section in brackets is the repeat unit, and n is the number of repeat units.

A polymer is a very long molecule built from a large number of repeating units. The small molecules that join to make it are monomers. Addition polymerisation is the reaction of unsaturated monomers, which contain a C=C bond, to form a polymer.

The π bond of each C=C breaks and the carbon atoms bond to those of neighbouring monomers, as in other addition reactions of alkenes.
The polymer is the only product; no small molecule (such as water) is lost.
The chain contains only C–C single bonds in its backbone, so the polymer is saturated.

The section of the chain that repeats is the repeat unit. For ethene:

n CH₂=CH₂ → [–CH₂–CH₂–]_n

Industrially n can be several thousand. In an equation write the monomer with n in front, the repeat unit in brackets, and n as a subscript after the bracket.

2. Poly(ethene), PVC and Other Addition Polymers

Four common addition polymers. Each repeat unit has the same atoms as its monomer; only the C=C has become a C–C single bond. Always keep substituents (CH₃, Cl, F) on the carbon they started on.

Monomer	Polymer	Repeat unit
ethene, CH₂=CH₂	poly(ethene)	–CH₂–CH₂–
propene, CH₃CH=CH₂	poly(propene)	–CH(CH₃)–CH₂–
chloroethene, CH₂=CHCl	poly(chloroethene), PVC	–CH₂–CHCl–
tetrafluoroethene, CF₂=CF₂	PTFE	–CF₂–CF₂–

PVC is short for polyvinyl chloride, the old name of chloroethene being vinyl chloride. Poly(ethene) is used for bags and bottles, PVC for pipes and cable insulation, and PTFE for non-stick coatings.

A polymer made from an alkene has no C=C bond, so it does not decolourise bromine water. Its properties are like those of a very large alkane (unreactive, non-polar). It is not an alkene, even though its name begins poly(alkene).

⚠️ Examiner Trap: Poly(ethene) is saturated. Do not draw a double bond in the repeat unit, and do not say poly(ethene) reacts with bromine water. The monomer ethene decolourises bromine water; the polymer does not.

3. Deducing the Repeat Unit

Deducing the repeat unit of an addition polymer from its monomer, here propene. The main chain of the repeat unit always contains the two carbons of the original double bond; the CH₃ group is a side group.

Method. (1) Draw the monomer with the C=C in the centre of the page and all substituents shown. (2) Change the C=C to a single C–C bond. (3) Add an open (continuation) bond at the left and right of the two carbons. (4) Enclose the unit in brackets with n. The two carbons of the original double bond form the main chain; everything else is a side group.

Worked examples.

2-methylpropene, (CH₃)₂C=CH₂ → repeat unit –C(CH₃)₂–CH₂–.
but-2-ene, CH₃CH=CHCH₃ → repeat unit –CH(CH₃)–CH(CH₃)–. The main chain still has only two carbons; each CH₃ is a side group.
methyl 2-methylpropenoate, CH₂=C(CH₃)COOCH₃ (the monomer for Perspex®) → repeat unit –CH₂–C(CH₃)(COOCH₃)–. The ester group is untouched by the polymerisation.
propenenitrile, CH₂=CHCN → repeat unit –CH₂–CH(CN)–.

The relative molecular mass of an addition polymer is n times the relative molecular mass of the monomer, because the repeat unit has the same atoms as the monomer. For example, a sample of poly(chloroethene) with an average M_r of 93 750 contains 93 750 / 62.5 = 1500 repeat units, where M_r(C₂H₃Cl) = 2 × 12.0 + 3 × 1.0 + 35.5 = 62.5 (A_r values from the Periodic Table in the Data Booklet). Because the polymer is the only product, the atom economy is 100%.

Polymer size and atom economy. Because the repeat unit has the same atoms as the monomer, the relative molecular mass of an addition polymer is n times that of the monomer, and all of the monomer atoms end up in the polymer.

⚠️ Examiner Trap: Side groups stay put. The repeat unit of poly(propene) is –CH(CH₃)–CH₂–, not –CH₂–CH₂–CH₂– (that would put the CH₃ into the chain). The main chain contains only the two carbons that were joined by the double bond. Do not forget the open bonds at the ends.

4. Identifying the Monomer(s)

Identifying the monomer(s) in a section of addition polymer. Divide the chain into units of two main-chain carbons, then restore the C=C in each unit. If all the units are identical there is one monomer; if two kinds alternate, there are two monomers.

One monomer. Look for the pattern that repeats along the chain. Divide the chain into units of two main-chain carbons, check that all the units are identical, and put the C=C back between the two carbons of one unit.

Worked example. A section of polymer has the chain –CH₂–CHCl–CH₂–CHCl–CH₂–CHCl–. Split it into two-carbon units: each unit is –CH₂–CHCl–, so the monomer is chloroethene, CH₂=CHCl. When the section is drawn out with every atom shown, count the Cl atoms: one Cl on every second carbon means one Cl per unit, which rules out 1,2-dichloroethene, CHCl=CHCl, as the monomer.

Naming the monomer from a condensed repeat unit. The repeat unit –CH(CH₃)CH(CH₂CH₃)– becomes CH₃CH=CHCH₂CH₃. The longest chain through the double bond has five carbons with the C=C between carbons 2 and 3, so the monomer is pent-2-ene.

Two monomers. If two different units alternate, the polymer was made from two monomers. A chain with the pattern –CH₂–CH₂–CH₂–CHCl–CH₂–CH₂–CH₂–CHCl– splits into –CH₂CH₂– and –CH₂CHCl– units, so the monomers are ethene and chloroethene.

⚠️ Examiner Trap: Two carbons, not one. Always take a unit of two carbons in the chain, because each monomer contributes the two carbons of its C=C. Do not choose a single CH₂ or a CHCl as the monomer, and do not put the C=C back at the ends of the section: the open bonds at the ends belong to neighbouring units.

5. Disposal of Poly(alkene)s

Disposal of poly(alkene)s. They are chemically inert and non-biodegradable, so they persist in the environment. Burning them has harmful combustion products, including carbon monoxide, and hydrogen chloride and dioxins from PVC.

The unreactivity that makes poly(alkene)s so useful is also their disposal problem.

Non-biodegradable. They are saturated, non-polar and held together by strong C–C and C–H bonds, so they are not broken down by microorganisms, water, acids or alkalis. Waste plastics last for hundreds of years in landfill, use up land and pollute rivers and seas.
Harmful combustion products. Burning in plenty of oxygen gives carbon dioxide and water (CO₂ is a greenhouse gas); burning in limited oxygen also gives toxic carbon monoxide. Burning PVC also gives hydrogen chloride, an acidic gas, and toxic dioxins, so the waste gases must be neutralised and the incinerator must run at a very high temperature.
Using the waste. Burning waste plastic releases energy that can be used to generate electricity, which conserves fossil fuels, but it still produces CO₂. Recycling is difficult because mixed plastics must first be sorted.

⚠️ Examiner Trap: Name the products, say why. ‘Pollution’ scores nothing. Name carbon monoxide, hydrogen chloride (from PVC) or dioxins, and say why a poly(alkene) is non-biodegradable: it is a saturated, unreactive, non-polar hydrocarbon chain.

6. Quick Sheet and Checklist

Topic	Key point
Addition polymerisation	unsaturated monomers (C=C) join; polymer is the only product; 100% atom economy
Repeat unit	monomer with C=C → C–C, open bonds at both ends, in brackets with n
Poly(ethene)	nCH₂=CH₂ → [–CH₂–CH₂–]_n
PVC	nCH₂=CHCl → [–CH₂–CHCl–]_n
Monomer from a section	unit of two main-chain carbons; restore the C=C; alternating units mean two monomers
Properties	no C=C, so no reaction with bromine water; inert like a large alkane
Mass	M_r(polymer) = n × M_r(repeat unit)
Disposal	non-biodegradable; burning gives CO₂, CO, and HCl and dioxins from PVC
Not at AS	condensation polymers (polyesters, polyamides), proteins (A Level)

Before you leave the question, check:

The repeat unit has brackets, open bonds and n, and no C=C.
Only the two carbons from the C=C are in the main chain; the substituents are side groups on the right carbons.
The monomer from a section has the C=C restored and is named from the longest chain through it.
If the units alternate, give both monomers.
The M_r or n calculation uses A_r values from the Data Booklet.
Disposal answers name a product (CO, HCl, dioxins) and give the reason for non-biodegradability.