Why Addition Polymerisation Does Not Usually Produce Water

Take ethene, \(\ce{CH2=CH2}\). It contains two carbon atoms and four hydrogen atoms, but no oxygen. If thousands of ethene molecules join to form polyethylene, where would the oxygen needed to make water come from?

It cannot come from nowhere. Addition polymerisation does not usually produce water because the monomer molecules join together without losing atoms. Their carbon-carbon double bonds open, allowing new carbon-carbon bonds to form between neighbouring monomers. All the atoms from the original monomers become part of the polymer chain.

This is the key difference between addition polymerisation and condensation polymerisation.

Before looking at the equations, make one prediction. When ethene molecules join together, do their carbon atoms separate, or do they stay connected?

They stay connected. The double bond becomes a single bond, and new bonds connect the monomer units into a long chain.

01What actually happens during addition polymerisation?

Ethene contains a carbon-carbon double bond:

\[
\ce{CH2=CH2}
\]

A double bond consists of:

• One sigma (\(\sigma\)) bond, which holds the two carbon atoms together.
• One pi (\(\pi\)) bond, which is consumed during an addition reaction.

When ethene undergoes addition polymerisation, the pi bond is broken. The original carbon-carbon connection remains as a single bond, and each carbon can form a new bond with a neighbouring monomer.

This process repeats, producing a long chain of carbon atoms.

The overall reaction is:

\[
\ce{n CH2=CH2 -> [-CH2-CH2-]_{n}}
\]

The product is poly(ethene), also called polyethylene.

The brackets show the repeating section of the polymer. The subscript \(n\) indicates that this section is repeated many times.

Notice what has happened to the atoms:

Nothing has been removed from the ethene molecules.

There is no elimination of hydrogen, oxygen, or any other small molecule. Therefore, no water is produced.

This is what the word addition means in this context: monomer units are added to the growing polymer chain without eliminating a small molecule.

02How different monomers form addition polymers

The same principle applies to several commercially important polymers.

The important thing is to recognise what happens to the double bond and what happens to the groups attached to it.

Look closely at the repeat units.

Something important happens to the groups attached to the carbon atoms:

• In polypropylene, the methyl group (\(\ce{-CH3}\)) remains attached to the polymer backbone.
• In PVC, the chlorine atom remains attached to the carbon chain.
• In polystyrene, the benzene ring remains attached to the backbone.
• In PTFE, the fluorine atoms remain bonded to the carbon atoms.

These groups are not expelled as small molecules during addition polymerisation.

For example, a common misconception is that the chlorine in chloroethene must leave as hydrogen chloride (\(\ce{HCl}\)) when PVC forms.

It doesn’t. The chlorine remains in the repeating unit:

\[
\ce{n CH2=CHCl -> [-CH2-CH(Cl)-]_{n}}
\]

The same atom-retention principle applies to all these examples.

An important exception to a common shortcut

You might now be thinking:

Addition polymerisation produces no water because its monomers contain no oxygen.

That explanation works for ethene, propene, and many other hydrocarbon monomers. However, it is not the complete rule.

Consider vinyl acetate, an oxygen-containing monomer:

\[
\ce{CH2=CH-O-C(=O)-CH3}
\]

It forms poly(vinyl acetate), a polymer commonly used in adhesives.

Its repeat unit is:

\[
\ce{[-CH2-CH(O-C(=O)-CH3)-]_{n}}
\]

The oxygen-containing acetate group remains attached to the polymer backbone.

No water is eliminated.

This example exposes the more important rule:

Whether water is produced depends on the bonds being formed and broken, not simply on whether the monomer contains oxygen.

In addition polymerisation, the monomer atoms are retained in the polymer rather than eliminated as a small molecule.

03Why condensation polymerisation can produce water

Now compare addition polymerisation with condensation polymerisation.

In condensation polymerisation, monomers react through suitable functional groups. Forming a new link between the monomers involves removing a small molecule.

For many HSC examples, that small molecule is water.

Two important examples are nylon and polyester.

Nylon: forming an amide link

Nylon-6,6 is made from two monomers:

• Hexane-1,6-diamine, containing amine groups (\(\ce{-NH2}\)).
• Hexanedioic acid, containing carboxylic acid groups (\(\ce{-COOH}\)).

When an amine group reacts with a carboxylic acid group, an amide link forms:

\[
\ce{-NH2 + HOOC- -> -NH-C(=O)- + H2O}
\]

The hydrogen from the amine group and the hydroxyl group from the carboxylic acid form water.

The remaining atoms form the new amide link in the polymer.

Polyester: forming an ester link

Poly(ethylene terephthalate), or PET, is another example.

It is commonly made using:

• Ethane-1,2-diol, containing hydroxyl groups (\(\ce{-OH}\)).
• Benzene-1,4-dicarboxylic acid, containing carboxylic acid groups (\(\ce{-COOH}\)).

An alcohol group reacts with a carboxylic acid group to form an ester link:

\[
\ce{R-OH + HOOC-R’ -> R-O-C(=O)-R’ + H2O}
\]

Here, \(R\) and \(R’\) represent the remaining parts of the molecules.

Again, hydrogen and hydroxyl groups are removed to form water.

The difference is now clear.

One detail is worth remembering: condensation polymerisation does not always produce water. Other small molecules can be eliminated, depending on the reacting functional groups.

Water is common, but the defining idea is the elimination of a small molecule.

For a more detailed comparison of the structures and reactions involved, see Condensation Polymerisation: Nylon and Polyester Explained for HSC Chemistry.

04Don’t confuse addition polymerisation with hydration

There is another reaction that can cause confusion.

Ethene can react with water to form ethanol:

\[
\ce{CH2=CH2 + H2O -> CH3CH2OH}
\]

This reaction is called hydration.

It is an addition reaction because the components of water, hydrogen and hydroxyl, add across the carbon-carbon double bond.

However, it is not addition polymerisation.

The difference is what reacts with the alkene:

• During hydration, water reacts with one alkene molecule and is consumed.
• During addition polymerisation, many alkene molecules join together to form a polymer chain.

So water can participate in an addition reaction without being produced by that reaction.

This distinction is particularly useful when studying alkene reactions. You can review the different types in Addition Reactions of Unsaturated Hydrocarbons Explained.

05A reliable way to identify whether water is produced

When you are given a polymerisation reaction in an exam, don’t guess based on the name of the polymer.

Instead, examine the structures.

Use this decision process:

Step 1: Identify the reactive groups in the monomers.

Look for a carbon-carbon double bond or functional groups such as hydroxyl, carboxylic acid, and amine groups.

Step 2: Examine the repeating unit.

Has the double bond become a single bond, with the original monomer atoms retained?

If so, you are looking at the standard alkene addition-polymerisation pattern.

Step 3: Look for atoms that have been removed.

If functional groups react and atoms are eliminated as a small molecule, you are looking at condensation chemistry.

Step 4: Check the reaction equation.

If water appears on the product side, it is being produced. If it appears on the reactant side, it is being consumed.

The key is to follow the atoms rather than rely on memorised labels.

06Worked examples

07Questions and solutions

Question 1

Ethene undergoes addition polymerisation to form poly(ethene).

Write the overall structural equation and explain why water is not produced.

Solution 1

The reaction produces poly(ethene), with the repeating unit \(\ce{[-CH2-CH2-]_{n}}\), and no water is formed.

The overall equation is:

\[
\ce{n CH2=CH2 -> [-CH2-CH2-]_{n}}
\]

The carbon-carbon double bond in each ethene molecule becomes a single bond, allowing the monomers to connect into a long chain.

All carbon and hydrogen atoms from the original ethene molecules are retained in the polymer.

Because no atoms are eliminated as a small molecule, water is not produced.

Question 2

Chloroethene, \(\ce{CH2=CHCl}\), undergoes addition polymerisation to form PVC.

A student proposes the following reaction:

\[
\ce{n CH2=CHCl -> [-CH=CH-]_{n} + nHCl}
\]

The student argues that chlorine must leave the monomer because the polymer backbone needs to form new bonds.

Explain why this is not the correct equation for addition polymerisation. Write the correct repeating unit and describe what happens to the chlorine atoms.

Solution 2

The correct repeat unit is \(\ce{[-CH2-CH(Cl)-]_{n}}\), and chlorine remains attached to the polymer backbone.

The correct overall equation is:

\[
\ce{n CH2=CHCl -> [-CH2-CH(Cl)-]_{n}}
\]

During addition polymerisation, the carbon-carbon double bond becomes a single bond. New carbon-carbon bonds connect neighbouring monomer units.

The chlorine atom does not need to leave for this to happen. It remains attached to the second carbon atom of each repeating unit.

The student’s proposed equation represents a different type of reaction: elimination of hydrogen chloride to form an unsaturated carbon chain.

Although that proposed equation can be atom-balanced, it does not describe the addition polymerisation of chloroethene.

The important distinction is that atom balance alone does not identify the reaction mechanism. You must also consider the actual structural changes taking place.

Question 3

A chemical manufacturer feeds 25.0 g of propene into a reactor.

The reaction conditions produce the following results:

• 72.0% of the propene undergoes addition polymerisation.
• 90.0% of the polypropylene formed is successfully recovered during isolation.

Assume that all reacted propene forms polypropylene and no reaction by-products are produced.

Calculate:

(a) The mass of propene that reacts.

(b) The theoretical mass of polypropylene formed in the reactor.

(c) The mass of polypropylene recovered.

(d) Explain why the difference between the original feed mass and the recovered polymer mass does not represent water produced during polymerisation.

Solution 3

(a) The mass of propene that reacts is 18.0 g.

Conversion is the fraction of the original monomer that undergoes reaction.

\[
m_{\text{reacted}}=25.0\times\frac{72.0}{100}
\]

\[
\boxed{m_{\text{reacted}}=18.0\text{ g}}
\]

(b) The theoretical mass of polypropylene is 18.0 g.

All reacted propene molecules become part of the polymer.

Therefore:

\[
m_{\text{polymer}}=m_{\text{reacted propene}}
\]

\[
\boxed{m_{\text{polymer}}=18.0\text{ g}}
\]

No water is eliminated, so the mass of polymer formed equals the mass of propene consumed.

(c) The recovered mass of polypropylene is 16.2 g.

Only 90.0% of the polymer formed is recovered.

\[
m_{\text{recovered}}=18.0\times\frac{90.0}{100}
\]

\[
\boxed{m_{\text{recovered}}=16.2\text{ g}}
\]

(d) The missing mass is not water.

First, calculate the mass of unreacted propene:

\[
m_{\text{unreacted}}=25.0-18.0
\]

\[
m_{\text{unreacted}}=7.0\text{ g}
\]

Next, calculate the mass of polymer that was formed but not recovered:

\[
m_{\text{not recovered}}=18.0-16.2
\]

\[
m_{\text{not recovered}}=1.8\text{ g}
\]

The complete mass balance is:

The 8.8 g difference between the original feed and recovered polymer consists of unreacted monomer and polymer lost during isolation.

Neither represents water produced by the chemical reaction.

This question highlights an important distinction between reaction conversion, polymer formation, and product recovery.

Question 4

Consider the following three reactions.

Reaction A:

\[
\ce{CH2=CH2 + H2O -> CH3CH2OH}
\]

Reaction B:

\[
\ce{n CH2=CH2 -> [-CH2-CH2-]_{n}}
\]

Reaction C:

\[
\ce{R-OH + HOOC-R’ -> R-O-C(=O)-R’ + H2O}
\]

Reaction C represents the formation of an ester link during polyester synthesis.

For each reaction:

(a) Identify whether it is an addition reaction, addition polymerisation, or condensation link-forming reaction.

(b) State whether water is a reactant, a product, or absent.

(c) Explain why the presence of the word addition in reactions A and B does not mean they are the same process.

Solution 4

Reaction A is hydration, reaction B is addition polymerisation, and reaction C is a condensation reaction.

Their water relationships are different.

Reaction A: Hydration

\[
\ce{CH2=CH2 + H2O -> CH3CH2OH}
\]

Water is a reactant.

The components of water, hydrogen and hydroxyl, add across the carbon-carbon double bond to form ethanol.

This is an addition reaction, but it does not produce a polymer.

Reaction B: Addition polymerisation

\[
\ce{n CH2=CH2 -> [-CH2-CH2-]_{n}}
\]

Water is neither a reactant nor a product.

The ethene molecules connect through their carbon-carbon double bonds to form a polymer chain.

All atoms from the monomers are retained in the polymer.

Reaction C: Condensation link formation

\[
\ce{R-OH + HOOC-R’ -> R-O-C(=O)-R’ + H2O}
\]

Water is a product.

The hydroxyl group from the alcohol and the hydrogen from the carboxylic acid are eliminated as water when the ester link forms.

The important distinction is the substance being added or removed.

Reaction A adds the components of water to one alkene molecule. Reaction B adds many alkene monomers to a growing polymer chain. Reaction C forms a new link while eliminating water.

The word addition alone does not tell you whether a reaction is polymerisation or whether water is produced.

Question 5

A chemist investigates the addition polymerisation of vinyl acetate.

The monomer has the structural formula:

\[
\ce{CH2=CH-O-C(=O)-CH3}
\]

The polymer has the repeating unit:

\[
\ce{[-CH2-CH(O-C(=O)-CH3)-]_{n}}
\]

A sample contains 8.61 g of vinyl acetate, corresponding to 0.100 mol. The monomer polymerises completely.

A student argues:

“Because vinyl acetate contains oxygen and hydrogen, one water molecule must be eliminated for every monomer molecule that joins the polymer. Therefore, the polymer must have a lower mass than the original monomer.”

Evaluate this argument.

Calculate the theoretical mass of dry polymer produced, and explain why the student’s reasoning is incorrect.

Assume:

• All monomer molecules are incorporated into the polymer.
• The repeat unit contains all atoms from the original monomer.
• Chain-end masses, initiator fragments, and side reactions can be ignored.
• No other substances are incorporated into the polymer.

Solution 5

The student is incorrect. The theoretical mass of poly(vinyl acetate) is 8.61 g, and no water is produced during the addition polymerisation.

This example is important because it shows why the absence of oxygen is not the fundamental reason addition polymerisation does not produce water.

Step 1: Identify the part of the monomer that reacts.

Vinyl acetate contains a carbon-carbon double bond:

\[
\ce{CH2=CH-}
\]

It also contains an oxygen-containing acetate group:

\[
\ce{-O-C(=O)-CH3}
\]

The carbon-carbon double bond is used to connect the monomers into the polymer chain.

Step 2: Compare the monomer with the repeating unit.

The repeating unit is:

\[
\ce{[-CH2-CH(O-C(=O)-CH3)-]_{n}}
\]

The acetate group remains attached to the carbon backbone.

All the original carbon, hydrogen, and oxygen atoms are retained.

There is no removal of hydrogen and hydroxyl groups to form water.

Step 3: Calculate the theoretical polymer mass.

For this addition polymerisation, the mass of the polymer equals the mass of the monomer consumed.

\[
m_{\text{polymer}}=m_{\text{monomer reacted}}
\]

Since all 8.61 g of monomer reacts:

\[
\boxed{m_{\text{polymer}}=8.61\text{ g}}
\]

No mass is lost as water.

Step 4: Identify the flaw in the student’s reasoning.

The student assumes that the presence of oxygen and hydrogen automatically means water must be eliminated.

However, these atoms are already part of the acetate side group. They do not need to be removed for the carbon-carbon double bond to form the polymer backbone.

The reaction is classified by the structural changes taking place, not simply by the elements present in the monomer.

The decisive evidence is the repeating unit: it retains all the atoms from vinyl acetate.

This is why the polymerisation is addition polymerisation rather than condensation polymerisation.

07The key idea to remember

Addition polymerisation of alkene monomers does not usually produce water because the monomers join through their carbon-carbon double bonds without eliminating atoms as a small molecule.

Condensation polymerisation is different. Its monomers react through functional groups, often eliminating water as new covalent bonds form.

Remember these three points:

1. Addition polymerisation: the C=C bond becomes a C-C bond, and the monomer atoms are retained in the polymer.
2. Condensation polymerisation: functional groups react, and a small molecule such as water is eliminated.
3. Hydration: water is added to an alkene, so it is a reactant rather than a polymerisation by-product.

Once you can follow the atoms through each reaction, identifying the polymerisation type becomes much easier.

The next useful step is applying this atom-retention principle to calculate polymer masses, repeat-unit quantities, and the theoretical yield of addition polymerisation reactions.

HSC syllabus note (November 2026): This resource uses the 2017 Chemistry Stage 6 course context. NESA schedules implementation of the 2025 Chemistry 11-12 syllabus from 2028, with the first HSC examination under the new syllabus in 2029. Check the syllabus applicable to your cohort: NESA Chemistry syllabus implementation information.