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Lesson: Chapter 14 — Hydrocarbons and their derivatives

14.3. Polymers 2 of 4

Polythene

Polythene (polyethene) is made by the polymerisation of ethene molecules, the alkene you met in the last section. What happens here?

Ethene, the monomer: two carbons joined by a double bond, each with two hydrogens.

One bond of each double bond breaks, and thousands of ethene molecules join to one another, as drawn below for four of them.

Four ethene units joined into a chain of eight carbons, each carbon with two hydrogens, and the chain continuing at both ends.

The polymerisation can be written in short like this:

n
Ethene, the monomer: two carbons joined by a double bond, each with two hydrogens.
Polythene: the repeating unit –CH2–CH2– in square brackets, repeated n times.

This means that n ethene molecules have joined to one another to form a polythene molecule with n repeating units –CH2–CH2–. So polythene is a large molecule formed when many ethene molecules join in a definite pattern, and it has a high molecular mass.

Assignment 14.3

Make models of several ethene molecules. Join them suitably to build a molecule of the polymer polythene.

Why the double bond has to go

Each carbon can hold only four bonds. In ethene, two are used by hydrogens and two by the double bond, so there is no room to join anything else. When one bond of the double bond opens, each carbon has one bond free — one reaches out to the molecule on the left and one to the molecule on the right. Every ethene becomes a –CH2–CH2– link, with only single bonds left. That is why a polymer drawn with its repeating unit has no double bond in it, and why an alkane such as ethane, with no double bond to open, cannot polymerise like this.

n ethene → polythene

The double bonds of n ethene molecules open and link into polythene, with n repeating units –CH2–CH2–.