AS & A-Level Chemistry 10 — AS synthesis, polymers and spectroscopy

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Independent Deckloop AS Chemistry study material aligned with Cambridge International 9701 (2025–2027). Deck 10 of 18: AS synthesis, polymers and spectroscopy. Original explanations, worked applications and practice. Not affiliated with or endorsed by Cambridge International Education.

Chemistry EN A-Level

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Addition Polymerisation

Addition polymerisation occurs when many small unsaturated monomer molecules, in the examples here, alkenes with one carbon–carbon double bond, join together to form a very long, saturated chain called a polymer, without the loss of any small molecules. This process involves the breaking of the

\text{C=C}

\pi

bond in the monomer to form new

\text{C-C}

\sigma

bonds that connect the repeat units. Important examples include the polymerisation of ethene to form poly(ethene), which is widely used for packaging, and chloroethene to form poly(chloroethene), commonly known as

\mathrm{PVC}

, used in pipes and insulation. The repeat unit is the simplest part of the polymer chain that, when repeated, generates the entire macromolecule. It is drawn with trailing single bonds passing through brackets, often with a subscript

n

indicating a large number of repeats. To deduce the repeat unit from a given alkene monomer, open the double bond to a single bond and draw horizontal bonds extending from these two carbon atoms, keeping all attached side groups vertically aligned. Conversely, to identify the monomer from a section of an addition polymer, locate a two-carbon backbone segment that represents a single repeat unit, remove the trailing bonds, and restore the

\text{C=C}

double bond between these two carbons.

Key points

Addition polymerisation involves joining unsaturated monomers to form a single macromolecule with no by-products.
Poly(ethene) is formed from ethene monomers, and poly(chloroethene) $(\mathrm{PVC})$ from chloroethene.
A repeat unit shows the simplest repeating section of the polymer chain, indicated by brackets with extending single bonds.
To find a repeat unit, convert the monomer's $\text{C=C}$ double bond to a $\text{C-C}$ single bond and draw trailing bonds from these backbone carbons.
To identify the monomer from a polymer chain, isolate two adjacent backbone carbons of a repeat unit and place a double bond between them.

Worked example

Question

Deduce the structure of the monomer used to make a polymer with the repeating backbone segment

\text{-CH}_2\text{-C(CH}_3\text{)(CN)-}

Solution

1. Identify the two carbon atoms that form the backbone of the repeat unit. Here they are

\text{CH}_2

and

\text{C(CH}_3\text{)(CN)}

.

2. Remove the trailing bonds that connect this unit to adjacent units.

3. Insert a double bond between the two backbone carbon atoms to form the original alkene monomer.

The monomer is 2-methylpropenenitrile,

\text{CH}_2\text{=C(CH}_3\text{)(CN)}

Common pitfalls

Drawing the repeat unit with a double bond instead of a single bond. (The polymer backbone consists of $\text{C-C}$ single bonds because the $\pi$ bond opens up to link the units.)
Including more than two backbone carbon atoms in the repeat unit of a simple addition polymer. (An addition polymer from a simple alkene always has exactly two backbone carbons in its fundamental repeat unit.)

Prerequisites

Study Organic foundations and hydrocarbons first.
Study Halogenoalkanes and alcohols first.
Study Carbonyls, acids, esters and nitrogen compounds first.