Addition Polymerisation and Polymer Disposal

The final reaction of alkenes you must know is addition polymerisation: the joining of very large numbers of alkene monomers into a single long-chain molecule, a polymer. This reaction underpins the whole modern plastics industry — polyethene, polypropene, PVC, polystyrene and more are all produced this way. But the same properties that make polymers useful (strength, durability, inertness) also make disposing of them a major environmental problem. This lesson covers OCR A-Level Chemistry A (H432) specification 4.1.3 (g)–(h).

1. What is Addition Polymerisation?

Key Definition — Addition polymerisation: A reaction in which many unsaturated monomer molecules (typically alkenes) join together to form a single long-chain molecule (the polymer) with no other product.

Key features:

The C=C double bond in each monomer opens up, allowing a new C–C bond to form between adjacent monomers.
No atoms are lost — the polymer contains all the atoms of the monomers (unlike condensation polymerisation, which releases water or HCl).
Many monomers (usually thousands) join together.
The reaction proceeds by a chain mechanism, typically initiated by a radical, carbocation, or transition metal catalyst (beyond the scope of A-Level).

1.1 The Generic Scheme

n CH₂=CHR → –(CH₂–CHR)– n

The double bond is replaced by two single bonds — one within the repeat unit and one joining it to the next. The polymer chain can extend for thousands of repeat units in each direction.

Monomer:       H   R            Repeat unit:    | H   R |
                \ /                              | |   | |
                 C = C                           | C - C |
                / \                              | |   | |
               H   H                             | H   H |
                                                        n

2. Common Addition Polymers

Monomer	Polymer (common name)	Uses
Ethene (CH₂=CH₂)	Poly(ethene) / polythene	Plastic bags, bottles, film
Propene (CH₂=CHCH₃)	Poly(propene)	Food containers, rope, carpet
Chloroethene (CH₂=CHCl, vinyl chloride)	Poly(chloroethene) / PVC	Window frames, pipes, cable insulation
Phenylethene (CH₂=CHC₆H₅, styrene)	Poly(phenylethene) / polystyrene	Packaging, insulation, disposable cups
Tetrafluoroethene (CF₂=CF₂)	Poly(tetrafluoroethene) / PTFE	Non-stick coatings, bearings

3. Writing Polymer Equations

3.1 Drawing the Repeat Unit

The repeat unit is the smallest unit that, when repeated, generates the entire polymer. For an alkene CH₂=CHR, the repeat unit is:

  |  H   R  |
  |  |   |  |
  |  C - C  |
  |  |   |  |
  |  H   H  |

Key drawing rules (OCR expects these exactly):

Open the C=C — draw two single bonds between the two carbons and a single bond in the repeat unit.
Use square brackets (or at least a clear bracket) around the repeat unit.
Put two extension bonds (short lines) coming out of the left and right ends to show where the polymer continues.
Write n outside the bracket on the right.

3.2 Worked Example — Poly(propene)

Propene: CH₂=CHCH₃

Reaction:

n CH₂=CHCH₃ → ( CH₂–CH(CH₃) )ₙ

The repeat unit is:

      | H   H  |
      | |   |  |
    — | C - C  | —
      | |   |  |
      | H  CH3 |
                n

Note that the CH₃ branch dangles off every other carbon.

3.3 Worked Example — PVC

Chloroethene: CH₂=CHCl

Reaction:

n CH₂=CHCl → ( CH₂–CHCl )ₙ

The repeat unit contains one Cl on every other carbon.

3.4 Finding the Monomer from the Polymer

If given the polymer repeat unit and asked for the monomer:

Draw the repeat unit.
Remove the side extension bonds.
Redraw the two carbons of the repeat unit as a C=C double bond.
Name the resulting alkene.

Example: the repeat unit –(CH₂–CF₂)– gives the monomer CH₂=CF₂ (1,1-difluoroethene).

4. Properties That Make Polymers So Useful

Chemically inert — do not react with acids, bases, air, water, or most chemicals at room temperature. This is because the backbone consists only of strong C–C and C–H σ bonds.
Strong and flexible — long entangled chains resist breaking.
Lightweight — organic polymers are much less dense than metals.
Cheap to make — from crude oil via cracking of long-chain alkanes to produce alkene monomers.
Easily moulded — thermoplastics can be melted and reshaped.
Electrically insulating — useful for cable sheathing.

5. The Disposal Problem

The same chemical inertness that makes polymers useful also makes them an environmental problem:

Most addition polymers are non-biodegradable — microorganisms cannot break down C–C bonds in the backbone. An unprotected plastic bag can persist for 500+ years in landfill.
Polymers do not rot or rust.
Over 380 million tonnes of plastic are produced globally each year, of which only a small fraction is recycled.
Improperly discarded plastic ends up in rivers and oceans, contributing to the "plastic soup" and microplastics crisis.

Lesson 10: Addition Polymerisation and Polymer Disposal