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B3.4 · Distinguish addition from condensation polymerization

Learn to distinguish addition from condensation polymerization through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Organic Chemistry

Two ways small molecules build a polymer

A polymer is a long molecule assembled from repeating building blocks called monomers. In SCH4U, the useful first question is what happens to the monomer during joining: does its carbon–carbon double bond open, or do two functional groups react and release a small molecule? That distinction helps you classify the process before memorizing a product name.

What you will learn

  • Recognize the starting feature of each polymerization type.
  • Show the repeating unit of a simple addition polymer.
  • Explain why a small by-product appears in a condensation reaction.

Addition polymerization

An alkene monomer contains a carbon–carbon double bond. During addition polymerization, one bond of that double bond opens and new single bonds connect adjacent monomers. The atoms of each monomer remain in the polymer chain; this idealized joining step does not eliminate a small molecule. For example, ethene units can form poly(ethene). Draw the repeating unit in brackets and place the repeat count outside the brackets.
A structural drawing matters more than the word “addition”: check that the double bond has become a single bond in the backbone. The repeated unit still contains the same two carbon atoms and four hydrogen atoms as the ethene monomer.
n CH2=CH2⟶[−CH2−CH2−]n\mathrm{CH_2{=}CH_2}\longrightarrow [\mathrm{-CH_2-CH_2-}]_n
  • Look for an alkene double bond in the monomer.
  • No small-molecule by-product is removed in this simplified addition step.

Condensation polymerization

Condensation polymerization joins molecules through reactive functional groups, typically with the elimination of a small molecule such as water. An alcohol group and a carboxylic-acid group can form an ester link. A diol and a dicarboxylic acid each have two reactive ends, so repeated link formation can build a polyester chain.
Do not assume every real polymerization releases water: the leaving molecule depends on the reactants and process. For the SCH4U comparison, identify the functional groups, the new link in the chain, and the by-product for the particular reaction shown.
  • Name the functional groups on both monomers.
  • Show the bond that connects repeating units and account for the eliminated molecule.

How to tell them apart

FeatureAdditionCondensation
Typical starting featureCarbon–carbon double bondTwo complementary reactive functional groups
Small by-product in the simple modelNoneOften water or another small molecule
ExampleEthene → poly(ethene)Diol + dicarboxylic acid → polyester

Worked example

Classify the polymerization of ethene

Ethene molecules react to make poly(ethene). Is this addition or condensation polymerization, and what happens to the double bond?
  1. Inspect the monomer
    Ethene has a carbon–carbon double bond, which is the diagnostic feature of this simple addition example.
    CH2=CH2\mathrm{CH_2{=}CH_2}
  2. Track the change
    One bond of each double bond opens so the units can connect through carbon–carbon single bonds.
    n CH2=CH2⟶[−CH2−CH2−]n\mathrm{CH_2{=}CH_2}\longrightarrow [\mathrm{-CH_2-CH_2-}]_n
  3. Check the products
    The polymer chain contains the monomer atoms; the simplified reaction shows no water or other small by-product.
Answer: It is addition polymerization. The double bonds open to make single-bond links in the polymer backbone.
Check: Each repeating unit still has two carbon atoms and four hydrogen atoms.

Common mistakes and how to avoid them

Calling the ethene reaction condensation because two molecules combine.
Correction: Combining molecules alone is not enough. Classify it by the opened alkene bond and absence of an eliminated small molecule.

Lesson summary

  • Addition polymerization usually starts from an alkene and incorporates its atoms into a chain.
  • Condensation polymerization links functional groups and commonly releases a small molecule.
  • Use a structural change and the product list, not just the polymer name, to justify your classification.

Check your understanding

Question 1

A diol reacts with a dicarboxylic acid to form a polyester and water. Which type is illustrated?
  1. Addition polymerization
  2. Condensation polymerization
  3. A redox half-reaction
Show answer and explanation
Condensation polymerization
Functional groups form ester links and a small molecule, water, is released.

Key terms

Monomer
A building-block molecule that can join into a polymer.
Repeating unit
The structural fragment repeated along the polymer chain.

Continue through SCH4U

View the complete SCH4U Ontario Grade 12 Chemistry curriculum and lessons

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation B3.4. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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