Addition and condensation polymers (SACE Stage 2 Chemistry)

Syllabus dot point

“Distinguish addition and condensation polymerisation, identify monomers and repeating units, and relate polymer structure to properties.”

9 min readUpdated 2 Jun 2026

Quick answer

Polymers are long chains built from many small repeating units (monomers). In addition polymerisation, monomers with a $\text{C}=\text{C}$ double bond join as the bond opens, with no by-product (e.g. poly(ethene)). In condensation polymerisation, monomers each carrying two reactive groups join with the loss of a small molecule (usually water) at every bond, forming ester (polyester) or amide (polyamide) linkages. You must identify the monomer, draw the repeating unit, and link structure to properties.

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What this dot point is asking
Lead worked calculation
Addition polymerisation
Condensation polymerisation
Comparing the two
Structure determines properties
Recycling and degradation
Why it matters
Exam-style questions

What this dot point is asking

SACE expects you to distinguish the two mechanisms, identify monomers and repeating units, and explain how chain structure determines polymer properties.

Lead worked calculation

Finding the repeating unit and degree of polymerisation

A sample of poly(ethene) has an average molar mass of $2.80 \times 10^{4}\ \text{g mol}^{-1}$ . Identify the monomer, draw the repeating unit, and find the average number of monomers per chain. ( $M(\text{C}_2\text{H}_4) = 28.0\ \text{g mol}^{-1}$ .)

Identify the monomer and repeating unit

The monomer is ethene, $\text{CH}_2=\text{CH}_2$ . In addition polymerisation the double bond opens, so the repeating unit is $-[\text{CH}_2-\text{CH}_2]-$ (drawn with bonds extending out on each side and an $n$ subscript).

Find the degree of polymerisation

Each repeating unit has the same mass as the monomer ( $28.0\ \text{g mol}^{-1}$ , since nothing is lost), so:

\text{degree of polymerisation} = \frac{M(\text{polymer})}{M(\text{repeating unit})} = \frac{2.80 \times 10^{4}}{28.0} = 1.00 \times 10^{3}

Conclude

The chain contains about $1000$ ethene units. For an addition polymer the repeating-unit mass equals the monomer mass; for a condensation polymer you would subtract the mass of the small molecule lost.

Addition polymerisation

Addition polymerisation joins many unsaturated monomers (each containing a $\text{C}=\text{C}$ double bond) into a long saturated chain. The double bonds open and the monomers add directly together, so no small molecule is eliminated and the repeating unit has the same atoms as the monomer. Examples: poly(ethene) from ethene, poly(propene) from propene, PVC from chloroethene.

To draw the repeating unit, take the monomer, open the $\text{C}=\text{C}$ into a single bond, and show bonds extending from each end into the rest of the chain, all inside brackets with subscript $n$ .

Condensation polymerisation

Condensation polymerisation joins monomers that each carry two reactive functional groups, eliminating a small molecule (usually water) at each new bond. A diol plus a dicarboxylic acid gives a polyester (ester linkages, $\text{-COO-}$ ); a diamine plus a dicarboxylic acid gives a polyamide (amide linkages, $\text{-CONH-}$ , as in nylon). Each monomer must be difunctional so the chain can grow at both ends.

The linkage chemistry is the same as esterification and amide formation: condensation polymers form ester or amide bonds exactly as a single ester or amide would, just repeated thousands of times.

Comparing the two

Addition versus condensation at a glance

Addition: monomers contain $\text{C}=\text{C}$ ; the double bond opens; no by-product; repeating unit mass equals monomer mass. Condensation: monomers contain two functional groups; a small molecule (water) is lost at each bond; ester or amide linkages form. The quickest test: if the monomer has a double bond and nothing is released, it is addition; if a small molecule is eliminated, it is condensation.

Structure determines properties

Polymer properties follow from chain structure and the forces between chains.

Chain length: longer chains have stronger total intermolecular forces, raising melting point and strength.
Branching: unbranched chains (HDPE) pack closely, giving a denser, stronger, more crystalline material; branched chains (LDPE) pack loosely, giving a softer, more flexible plastic.
Cross-linking: covalent bridges between chains give rigid, heat-resistant thermosets that do not melt.
Polar groups: ester and amide linkages allow hydrogen bonding between chains (nylon, polyester fibres), increasing strength and raising melting point compared with non-polar addition polymers.

Recycling and degradation

Condensation polymers contain ester or amide linkages that can be hydrolysed, so polyesters and nylons can be chemically broken back into monomers and are more biodegradable. Addition polymers have inert carbon-carbon backbones with no easily hydrolysed linkage, making them very durable but slow to break down, a key environmental drawback.

Confusing the mechanisms and the repeating unit

The most common SACE error is calling a reaction "addition" when a small molecule is released, or drawing the repeating unit with the double bond still present. In addition polymers the double bond has opened to a single bond. For condensation polymers, remember the monomers each need two functional groups and that water (or another small molecule) leaves at each link, so the repeating-unit mass is less than the combined monomer masses.

Why it matters

Polymers are everywhere, from packaging and fibres to biological macromolecules. Understanding the two mechanisms explains why some plastics are inert and persistent while others can be hydrolysed and recycled, and it connects directly to the condensation chemistry of proteins, carbohydrates and lipids.

Exam-style questions

Questions in the style of SACE Board exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

SACE 2022-style5 marks

Propene (

\text{CH}_2=\text{CHCH}_3

) polymerises to poly(propene). (a) Draw the repeating unit and write the type of polymerisation. (b) Explain why no small molecule is lost. (c) A sample of poly(propene) has an average molar mass of

1.26 \times 10^{5}\ \text{g mol}^{-1}

; calculate the average number of monomer units per chain. (

M(\text{C}_3\text{H}_6) = 42.0\ \text{g mol}^{-1}

Show worked answer →

(a) The repeating unit is $-[\text{CH}_2-\text{CH}(\text{CH}_3)]-$ shown with continuation bonds; this is addition polymerisation. (2 marks)

(b) In addition polymerisation the $\text{C}=\text{C}$ double bonds simply open and the monomers add together; no atoms are removed, so no small molecule (such as water) is lost. (1 mark)

(c) Degree of polymerisation $= \dfrac{1.26 \times 10^{5}}{42.0} = 3.00 \times 10^{3}$ monomer units per chain. (2 marks)

SACE 2020-style5 marks

Nylon-6,6 is formed from a diamine and a dicarboxylic acid. (a) State the type of polymerisation and name the small molecule released. (b) Name the type of linkage formed between monomers. (c) Compare condensation polymerisation with addition polymerisation in terms of the monomers required and whether a by-product forms.

Show worked answer →

(a) Condensation polymerisation; the small molecule released is water. (2 marks)

(b) The linkage formed is an amide (peptide) linkage, $\text{-CONH-}$ . (1 mark)

(c) Condensation polymerisation needs monomers with two reactive functional groups each (e.g. two $\text{-COOH}$ or two $\text{-NH}_2$ / $\text{-OH}$ ) and releases a small molecule (water) each time a bond forms. Addition polymerisation needs monomers with a $\text{C}=\text{C}$ double bond and releases no by-product, the double bond simply opens. (2 marks)

Practise this

Sources & how we know this

Stage 2 Chemistry subject outline (2025 onwards) — SACE Board
Stage 2 Chemistry: external assessment — SACE Board